THE EVOLUTION OF NESTS AND NEST-BUILDING IN BIRDS

AM. ZOOLOCIST, 4:175-190(1964).

THE EVOLUTION OF NESTS AND NEST-BUILDING IN BIRDS

NICHOLAS E. COLLIAS

Department of Zoology, University of California at Los Angeles

External construction is a phenomenonfound throughout the animal kingdomfrom ameba to man. Such things as thenests of birds, the webs of spiders, the casesof caddisfly larvae, and the nests of ter-mites and bees, are often mentioned asexamples of highly specialized instinctivebehavior. Their study is an old subject,but one that needs to be reviewed in acritical and comprehensive way in relationto modern biology. These structures playa central role in the lives of the animalsconcerned, and help bring to a focus theirhabitat requirements.

The object of the present survey is toattempt to formulate some general prob-lems, and to stimulate renewed interest andcontinuing analysis in a promising field ofscientific endeavor that has been somewhatneglected in recent years.

My own interest in the evolution of birds'nests began years ago when I took a gradu-ate course on the principles of speciationgiven by Dr. Alfred E. Emerson at the Uni-versity of Chicago. Emerson is well knownfor studies of the evolution of termites andtermite nests (1938). He has also calledattention to the promise of the nests ofbirds for investigating the evolution of be-havior (Emerson, 1943).

A nest may be defined as an externalconstruction that aids the survival and de-velopment of the eggs and young. The fol-lowing table gives a summary of the maintypes of birds' nests. Good photographsillustrating them may be found in Strese-mann's treatise on ornithology (1927-1934)and in many other more recent books aboutbirds.

A CLASSIFICATION OF NEST-BUILDING IN BIRDS

I. Incubation by environment.Eggs generally buried in the ground.

Megapodes.

Preparation of this review was aided by Grant22236 from the National Science Foundation.

II. Incubation by parents. Most birds.1. Dig or use nest cavity2. Nest is not enclosed in a cavity

a. Open nestsb. Roofed nestsc. Compound nests

III. Eggs placed in nests of other birds.Brood parasites.

At the outset, in a discussion of nestevolution it is necessary to keep in mindcertain principles in order to maintain per-spective and orientation.

Competition between species has oftenresulted in great differences in the habi-tats and nest sites occupied by related spe-cies. In turn, differences in the nature ofthe substrate for the nest has imposed spe-cial engineering requirements on nests withregard to materials, form, structure, andplacement. Building of a nest requires aconsiderable expenditure of energy, and itis common for many birds to make 1000or more trips to gather and bring all thenecessary materials. Natural selection maytherefore be expected to generally favoranything that tends to economize on effort,so long as undue sacrifice of any crucialadvantage of the species is avoided.

Nests are so closely related to habitat andhabits in any given species that there hasbeen a tremendous amount of recurrent,convergent, and parallel evolution of dif-ferent nest types in birds, making it diffi-cult to delineate particular phylogenies.In any particular line, evolution may leadto either an increasing complexity or, con-versely, to an increasing simplification ofnests, depending on conditions. The em-phasis and object of this report is not toattempt to develop special family geneolo-gies, but rather to attempt to analyze theecological nature of the selection pressuresthat have led to the evolution of the maintypes of nests.

The primary, basic, and most generalfunctions of a nest are to help insure

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176 NICHOLAS E. COLLIAS

warmth and safety to the developing eggsand young. But growth and survival ofthe young depend on the total biology ofthe species, and to fully understand theforces in evolution of the nest of anyspecies one must often be familiar withother aspects of its life history as well.Conversely, selection of a given type ofnest-site influences other aspects of speciesbehavior, as Cullen (1957) has shown forthe Kittiwake Gull, and von Haartmann(1957) for hole-nesting birds in general.

The problems of securing warmth andsafety are most acute for small birds andtheir young, explaining why birds of smallbody size generally build nests that are asa rule more elaborate and better concealedthan are those of larger birds. Reductionin size and energy content of eggs may havehelped make possible evolution of smallbirds (Witschi, 1956; Dawson and Evans,1960), but have also required increasedparental care of the young which hatch ina helpless well-nigh embryonic state. Thereare all degrees between precocial and altri-cial young and a parallel development ofparental care (Nice, 1962), related in ageneral way to nesting habits.

ORIGIN OF NEST-BUILDING IN BIRDS

Ability of birds to maintain a high con-stant body temperature no doubt developedgradually, probably coincident with evolu-tion of the ability to fly. Even today thereare a few birds that are known to passinto a state of torpidity under certain con-ditions, and their body temperature fallsdrastically (Bartholomew, Howell, andCade, 1957). If birds passed into a torpidstate during cool nights of the breeding sea-son, they could scarcely incubate their eggseffectively. During the transitional periodof early avian evolution, when temperatureregulation mechanisms were being per-fected, some birds probably continued tobury their eggs, leaving incubation to thesun or to decaying vegetation after the rep-tilian fashion, and as megapodes do amongliving birds today. The probably low noc-turnal body temperature of ancestral birdsis an argument in favor ot the idea that

in an early stage of avian evolution, incu-bation of the eggs may have depended inpart or wholly upon some source of heatother than that furnished by the parentalbody.

NESTS OF MEGAPODES ('MEGAPODIIDAEJ

Nests of living megapodes, recently sum-marized by Frith (1962), have a tremen-dous range of variation. Within the con-fines of one genus, Megapodius (perhapsin need of revision), the nest may varyfrom a simple, small pit dug in the sand,large enough for just one egg, to a giganticmound of soil and decaying vegetation asmuch as 35 feet in diameter and 15 feethigh, and probably the largest structuremade by any one bird. Frith points outa striking parallel between bird and reptileon certain sunlit coral beaches where theturtles heave up out of the sea to dig holeson the beach to lay their eggs, and themegapodes walk out of the bush to dig pitsin the sand for their eggs. Similarly, inplaces where dark tropical forests fringethe rivers, female crocodiles build moundsof leaves in which their eggs are laid, inclose proximity to the leafy mounds ofmegapodes.

Change in climate, especially after theclose of the Mesozoic era, from humidtropical or subtropical to drier conditionswith greater extremes of temperature, wasprobably met in early avian evolution bytwo different solutions with respect to theproblem of incubation of the eggs. Somebirds developed the method of incubationby direct application of heat to the eggsfrom the body of the parent. Other birdsdeveloped into mound builders, buryingthe eggs deeply in the ground, safeguardingthem from the harsh conditions, and at thesame time evolving considerable efficiencyin regulating the temperature of the moundabout the eggs.

It is possible that modern megapodesonce had ancestors that sat on their eggsas other birds do, and evolved their nowseemingly peculiar mode of incubation bya sort of regressive evolution. But such isthe advantage of direct parental incuba-

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NEST-BUILDING IN BIRDS

tion as in most modern birds, that the op-posite theory of mound-building in mega-podes as a primitive retention of a reptilianhabit might well be true. Frith suggeststhat one thing is certain. Some megapodeshave very greatly improved in the abilityto regulate the temperature of the moundcompared with any present day reptiles.Thus, the Mallee Fowl (Leipoa ocellata)lives in arid regions of Australia wherethe temperatures may range from belowfreezing to above 38°C, and even in mid-summer the night temperature may be 17°lower than the day temperature. Neverthe-less, this bird manages to maintain the in-cubation temperature about the eggs buriedin its mound relatively constant between32° and 35°C.

ORIGIN OF DIRECTPARENTAL INCUBATION OF EGGS

The open nest may well have evolved inancient birds from a type like the simplestnest built by modern megapodes. In someparts of Australia, Megapodius merely laysa single egg in a small pit in the sand orin a crevice in a rock, covers the egg withleaves, and departs, leaving the task ofincubation to the sun (Frith, 1962). Manyprecocial birds having open nests on theground cover their eggs with either plantmaterials, downy feathers, or earth, whenaway from the nest.

Once early birds had evolved the abilityto maintain a high temperature throughthe night independently of environmentaltemperature, there would be establishd co-incidentally a tremendous selection pressurefavoring direct parental incubation of theeggs. At the same time the spread of aspecies into new cooler habitats relativelyfree from reptilian predators would befacilitated. The danger of predation onthe eggs from various nocturnal enemies,especially from contemporary mammalswhich were small and probably nocturnal,provided considerable value to the habitof staying with the eggs and defendingthem if necessary during the night. Totalprcdation on the eggs would diminish withshortening of their developmental period.

Herrick (1911) suggested that the originof incubation by sitting on the eggs prob-ably arose from the tendency of birds toconceal their eggs with the body as aprotection from potential predators.

CAVITY-NESTING BIRDS

Cavity nesting has evolved repeatedly inbirds at virtually every stage of evolution.Initial use of natural cavities may be fol-lowed by modification of the cavity, andultimately may lead to the evolution ofspecial excavated nesting cavities in theground, in banks, or in trees. About halfthe orders of birds recognized by Mayr andAmadon (1951) contain some species thatnest in cavities. Whole orders of cavitynesters are represented by the kiwis, par-rots, trogans, coraciiform, and piciformbirds.

Cavity nesting provides evident shelterfrom the elements and conserves energy(Kendeigh, 1961). In addition, there ismuch direct statistical evidence from variousstudies (see Nice, 1957) that it is saferfor altricial birds of the North TemperateZone to nest in holes than in open nests.Nice's summary showed that only about \/2of some 22,000 eggs of various species withopen nests resulted in fledglings, whereas2/3 of 94,400 eggs were successful in hole-nesting birds. Populations of small birdslike the House Wren (Troglodytes addon)(Kendeigh, 1941), have often been greatlyincreased by putting out a good supply ofnest boxes.

The shelter and safety furnished by cavi-ties has resulted in intense competition forthese cavities. Conversely, birds buildingan open nest, involving as it does a greaterchance of nest failure, are subject to rela-tively strong selection pressure to build anadequate nest or to develop markedly effi-cient concealing coloration.

The Prothonotary Warbler (Protono-taria citrea) is one of the few Americanwarblers that build their nests within na-tural cavities or birdhouses. A fledglingrate for this species of only 26% (of 413eggs) was found by Walkinshaw (1941) inMichigan. In contrast, in Tennessee the

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fledgling rate was 61% (of 163 eggs). Thisdifference in success of fledgling was largelyattributed to a high population of HouseWrens competing for nesting places withthe warblers in Michigan, whereas HouseWrens were absent from the study localityin Tennessee. The wren destroys the war-bler's eggs by puncturing them with itsbill in the absence of the warbler.

Intense aggressive competition for treeholes has been a profound force in evolu-tion of different size classes among suchbirds as woodpeckers, corresponding to dif-ferent sized entrance holes typical of eachspecies. Along with each size class of wood-pecker goes a host of other species of birdsthat compete with the woodpecker for thecorresponding size of nest cavity. TheEuropean Starling is notorious in this re-gard, and I have seen a starling in Ohioseize a flicker by the tail and cast it outof the flicker's freshly dug tree cavity inwhich a pair of starlings subsequentlynested and reared a brood. Sielmann (1959)gives a number of graphic instances ofdirect observation in Germany of birdscompeting aggressively for tree holes. Oneof his most interesting examples shows howa small bird can compete successfully witha larger one for a nest cavity in a tree.When the European Nuthatch (Sitta e.uro-p(iea) takes over a tree cavity, it forestallsits chief rivals, the starlings, by collectingmud from nearby puddles and plasteringthe mud around the entrance to the treehole, making the entrance so small andnarrow that while the nuthatch can slipthrough, the larger starling cannot.

In the classic case of the hornbills, themale is often said to provide for the safetyof his mate and young ones by imprisoningthe female in her nest cavity in a tree, wall-ing up the entrance with mud, leaving justenough room for her to put her beak outso he can feed her during the prolongedperiod of incubation and care of the young,releasing her at the end of that time. But,in the few species studied in detail, Moreau(1937) points out that it is the female whoplasters herself in, using such materials as

hiou«In h\ the male ;md "mixed with

saliva as an adhesive, and whenever she isready to leave it is the female who pecksher way out. The Ground Hornbill(Bucorvus) generally nests in depressionsin trees such as those formed at the tip ofa big broken branch, but it does not wallthem up with mud, illustrating one pos-sible step in the evolution of the veryspecialized nesting habits of other modernhornbills.

Different stages in evolution of the abilityto excavate nest cavities in trees by birdsprobably were: (1) use of natural cavi-ties; (2) modification of these cavities invarious ways by the bird; (3) excavationin decaying or very soft wood as in sometitmice (Hinde, 1952) and the tiny Oliva-ceous Piculet (Picurnnus olivaceus) ofSouth America does (Skutch, 1948); andfinally, (4) chiseling of nest cavities in hard,living trees, as the large Black Woodpecker(Dryocopus martins) of Europe does (Siel-mann, 1959).

Many birds nest in holes in the ground,and several steps in the evolution of exca-vated burrows in the ground may be sug-gested: (1) a shallow scrape such as ismade by many ground nesting birds; (2)a relatively short burrow like that of theRough-winged Swallow (Stelgidopteryxruficollis), a bird which often merely nestsin crevices; and (3) the eggs laid at theend of a burrow which may reach a lengthof over six feet in a bank, as in the BankSwallow (Riparia riparin) (Bent, 1942).Furthermore, this swallow forms its tunnelin something of an upward course, insuringprotection from driving rain.

The evolutionary climax of excavatednests is the construction of nesting cavitiesby certain birds inside the nests of socialinsects. The distribution of the Orange-fronted Parakeet (Aratinga canicularis) inMexico and Central America closely approximates that of the colonial termite,Eutermis (Xasittitermes) nigriceps, in thenests of which the parakeet breeds, appar-ently using only nests still occupied by thetermites (Hardy, 1963). Hindwood (1959)points out that 49 species of birds, Includ-ing kinglisheis, parrots, trogons, puff-birds,

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NEST-BCILDIXG IX BIRDS 179

jacamars, and a cotinga, are known to breedin either terrestrial or arboreal nests oftermites. In fact, some 25% of the speciesof kingfishers of the world nest in termites'nests. As the excavation by a bird pro-gresses, the termites seal the exposed por-tions of their nests so that there is no actualcontact between the birds and the insects.

Many birds that breed in termites' nestsno not normally eat the insects in thecolonies in which they nest (ibid.). TheRufous Woodpecker (Micropternus brachy-urus) of the Orient seems to nest almostexclusively in occupied carton-like antnests, of the genus Crematogaster, whichhave a painful bite. The woodpecker feedson the ants of the colony in which it nests(Smythies, 1953). The Gartered Trogon(Chrystrogon caligatus) of Central Amer-ica has been recorded breeding in wasps'nests, eating out the adult wasps beforedoing so (cf Hindwood, 1959, ibid.).

Birds that breed in nests of social in-sects are all from taxonomic groups charac-terized by nesting in cavities, and cavitiesin old and deserted termite nests are attimes used by birds that normally breedin earth-banks or tree-holes. These factsshow how the habit of consistently breed-ing in nests of social insects might haveevolved.

Concerning all birds that nest either innatural or modified cavities or in cavitiesexcavated by themselves or by other birds,we can make two concluding statementsregarding the general significance of thishabit. First, various specialized types ofexcavation have evolved, and secondly,nesting in a cavity goes a long way towardmeeting the essential functions of a nestfor warmth and safety, and thereby tendsto block further evolution of truly elab-orate increment nests of the type that arebuilt up from specific materials. In fact,such nests as are placed within cavities mayundergo a regressive evolution. All degreesof increasing simplification and reductionto a mere pad are seen in the case of OldWorld sparrows (Passerinae) that nest intree holes (Collias and Collias, 1964b).

EVOLUTION OF OPEN NESTS ON THE GROUND

When direct parental incubation began,it was no longer necessary to dig a pit forthe eggs. However, most birds that neston the surface of the ground today, stillbegin their nest by making a circular scrapewith the feet while crouching low and rotat-ing the body (Dixon, 1902). This hollowmay then be more or less lined with variousmaterials protecting the eggs from the cold,damp ground.

A rim of materials around the body ofthe incubating parent serves to provide in-sulation for the eggs in many ground-nesting birds with simple nests, the ma-terials being pushed to the periphery andbuilt up into a circular form by muchthe same type of movements of the feet andbody as are involved in making the initialscrape in the ground. An added feature,preventing the flattening down of the nestrim, seen in many ground-nesting birds, isthe act of repeatedly reaching out with thebill, drawing in materials to the breast orpassing them back along one side of thebody before dropping. All of these pat-terns of making a ground nest are clearlyseen, for example, in the Canada Goose(Collias and Jahn, 1959). But the CanadaGoose will not walk or fly to the nest withmaterials in its bill, as cormorants or gullsand many other birds will do.

Open nests on the surface of the groundare more exposed to the elements and topredations than are enclosed nests. For ex-ample, eggs of the Horned Lark (Otocorisalpestris) laid early in the season in thenorthern United States may sometimes be-come frozen. Horned Larks place the nestin open country in a shallow scrape pro-tected from the chill of prevailing windsby a clod or tussock of grass (cf Bent,1942).

Parental behavior may supplement orsubstitute for a nest under particularly se-vere conditions of exposure. In the Arctic,persistent close incubation by the parentbird seems to be a major adaptation andoccurs regardless of whether or not the nestis well insulated (Irving and Krog, 1956).

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For example, it has been found that theSemipalmated Sandpiper (Ereunetes pusil-lus) with no nest keeps its eggs as warmas do various other birds nesting in theArctic and having a substantial nest. TheEmperor Penguin (Aptenodytes forsteri),which breeds in the Antarctic winter, restsits single egg on the feet, covers it with afold of belly skin and incubates it againstthe body. Perhaps no other animal breedsunder such trying conditions. The fury ofrecurrent severe storms is met by the birdshuddling together in a close mass (Rivolier,1956).

Eggs or nestlings exposed to strong trop-ical or subtropical sun in open situationsare customarily shaded by the body andwings of the parent bird, as in the caseof the Sooty Tern (Sterna fuscata) of Mid-way Island, in which the nest is a merescrape in the coral sand (Howell and Bar-tholomew, 1962).

Nests on the surface of the ground areespecially liable to be flooded, and ground-nesting birds often build therr nests on anyslight elevation and may build their nestsup during a flood. For example, in theAdelie Penguin (Pygoscelis adeliae) of theAntarctic, the nest which is made of smallstones functions chiefly to raise the eggsand incubating bird above ground level,thus lessening the dangers of flooding dur-ing thaws or of being buried by snow dur-ing blizzards (Sladen, 1958). During athaw, Sladen noticed one nest had a streamof ice cold water running through it. Theincubating male, his eggs half submerged,kept reaching forward, collecting and ar-ranging stones around him. Next day theeggs were above water though the streampassed on either side of the nest. Even-tually these eggs hatched. The PaintedSnipe (Rostratula benghalensis) in Aus-tralia may lay its eggs on the bare groundwhen the ground is dry, but if water islying on the ground, a solid nest of rushesand herbage is made. (Serventy and Whit-tell, 1962).

There is evidence that it is safer for abird to nest on a platform o\er watei in

a marsh than to nest on the surface of dryland. Kiel (1955) found that 50% of 149nests of dabbling ducks, which nest on theuplands, were successful. In contrast, dur-ing the same 4-year period of study inManitoba, he found that 73% of 227 nestsof diving ducks, which nest over water,were successful. Kiel attributed the greaternesting success of the diving ducks togreater protection from terrestrial mam-malian predators and to greater safetyfrom waves and floods by virtue of theirnest sites.

The high degree of exposure to predationto which birds nesting on the surface ofthe ground are often subject is no doubtthe reason why such species of birds pro-vide some of the classical examples of con-cealing coloration, such as the incubatingptarmigan or the coloration of eggs in manyshore birds. In these instances concealmentby coloration and behavior acts as a sub-stitute for concealment by a nest and asafe nesting site. In fact, in certain caseswhere the color pattern of the eggs andyoung and parents closely matches the sur-roundings, as in the European Stone Cur-lew (Burhinus oedicnemns) (Welty, 1962,photo, p. 264) and the Whip-poor-will(Antrostomus vociferns; Bent, 1940, Plate23), the nest may virtually disappear, pre-sumably because a nest itself would be tooconspicuous on the surface of the ground.

EVOLUTION OF OPEN NESTS

ON TREES AND CLIFFS

The dangers of ground nesting and theintense competition for tree holes have ap-parently provided a strong selection pres-sure leading to the evolution of incrementnests placed in trees. The Tooth-billedPigeon (Didunculinae) of Samoa is saidto have abandoned its ground-nesting habitsand taken to nesting in trees (Austin, 1961)after cats had been introduced by whalingships. There is some direct evidence thatnesting in trees is generally safer than nest-ing on the ground. In the prairie countryof northwestern Oklahoma where there arekw ticcs. Downing (1959; found in a

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NEST-BUILDING IN BIRDS 181

Mourning Dove (Zenaidura macroura) pop-ulation 49% success of 167 tree nests butonly 29% success of 130 ground nests.

Conversely, under nesting conditions safefrom predators, certain species of birds thatnormally nest in trees elsewhere may returnto ground nesting (Preston and Norris,1947), thereby conserving the energy thatwould be required to constantly fly up intoa tree carrying nest materials or food fornestlings. On Gardiner's Island in NewYork, in the absence of mammalian preda-tors, such birds as the Osprey and the Robinhave often nested on the ground, althoughnormally nesting in trees elsewhere in theUnited States.

Species of birds with precocial young aregenerally ground nesters; species with altri-cial and therefore relatively helpless younggenerally nest either on the ground or intrees and bushes. It is probable that pas-serine birds, with their perching foot struc-ture and altricial young, evolved first inrelation to arboreal life in trees and bushes,and secondarily reinvaded ground habitatswhere they continue to construct well-rounded bowl-shaped or cup-like nests.

The nature of the materials used to helpsolve the problem of securely placing andattaching a nest in a tree vary with thebody size of the bird and its lifting power.Large birds use twigs and branches thatare not readily blown out of the tree byordinary winds. Medium-sized birds usesmall twigs or grasses or both, sometimesadding mud to help attach and bind thenest materials. A great many small birdsare known to use spider silk or insect silkas a binding material both for the attach-ment of the nest and for fastening togethervarious other materials. Some birds of agiven species may use herbaceous plant ma-terial when they nest on the ground, andtwigs when they place their nest in a treee.g. certain herons (cf Palmer, 1962).

Other problems of placing nests in treesare related to specific sites in the tree.Among the crudest twig nests are those ofdoves. In Oklahoma, Nice (1922) foundthat 39 Mourning Dove nests in crotches

were almost twice as successful in produc-ing young large enough to fly as were 59nests on branches.

Practically all birds line their nests withfiner and softer materials than are used forthe foundation and outer shell of the nest.

The platform nests of some large birdssuch as the American Bald Eagle (Hali-aeetus leucocephalus) and the EuropeanWhite Stork (Ciconia ciconia) are largelyconstructed of twigs and branches, andthese nests are added to year after year,thus providing some economy of effort.Such nests may become very large and veryold. Herrick (1932) gives an age of 36years for a Bald Eagle nest, and Haver-schmidt (1949) dates back to 1549 oneWhite Stork nest which was still in usein 1930. The limiting factor to continuednest growth is often the weakening and in-creased liability to windfall of the nest-tree.The particular eagle nest described by Her-rick, perhaps the largest eagle nest onrecord, was 12 feet tall, %\/2 feet across thetop and its weight was estimated at over2 tons. It was situated in a tall tree andin its 36th year this nest fell with the treein a storm.

The altricial nestlings of small birds,such as passerines (which comprise morethan half of the living species of birds)hatch in a blind, relatively naked, andalmost embryonic state. Compared withplatform nests, the cup nest built by smallbirds provides more adequate protectionto the young, being so constructed as toresist stresses that might cause the nest tocollapse inward or outward. According toNickell (1958), rootlets used as nest liningsare the most constant feature of the nestsof Catbird (Dumatella carolinensis) andBrown Thrasher (Toxostoma rufum).These rootlets are moist and flexible whenplaced in the nest and become like smallwire springs when dry, serving as an innerbracework which preserves the shape ofthe nest basket.

It is probable that every type of materialcharacteristic of the nest of a given speciesof bird has a definite function according tothe physical properties of the material

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rather than its taxonomy, and that the pro-portions of materials of different types thatare used vary not only with availabilitybut with the requirements of particularsubstrate and habitat situations (Horvath,1963). Horvath finds that robin nests con-tain more mud when the birds have to useshort materials, more tough flexible root-lets when the nest is in an especially windyspot, and more moss when in a relativelycold microclimate.

Cup nests of the smallest species of birds,particularly when in cool or exposed en-vironments, are likely to be heavily insu-lated, e.g., nests of most species of hum-mingbirds have a thick lining of downymaterial (Ruschi, 1949). The tightly con-structed cup provides effective insulationof the ventral surface of the incubating fe-male, i.e., from that part of her body fromwhich the greatest heat loss probably oc-curs, as Howell and Dawson (1954) havedemonstrated for the Anna Hummingbird(Calypte anna). There is some evidencethat compared with lowland species, thespecies of hummingbirds that nest in highmountains may build nests having relative-ly thick walls (Wagner, 1955), or seek theprotection of caves (Pearson, 1953).

The building movements of birds thatmake open nests in trees are similar in cer-tain basic ways to those used by birds thatmake simple ground nests, particularlywith regard to methods of shaping the nestconcavity by means of scraping movementsof the feet combined with rotation of thebody and pushing movements of the breast.Additional movements more characteristicof tree nesters than of ground nesters arethe thrusting of twigs or grasses into thenest mass with trembling movements ofthe bill, and in the case of many smallbirds the habit of wiping cobwebs from thebill in fastening materials on to the grow-ing rim of the nest, as Van Dobben (1949has described for the Icterine Warbler(Hippolaius icterina) in the Netherlands.The sticky threads of cobweb are thenstretched out and passed back and forthacross the rim of the nest.

Elevated nests attached to vertical faces

of cliffs, buildings, or caves furnish protec-tion against non-avian predators, but posespecial problems of nest attachment. Forthis purpose the swifts have generally spe-cialized on adhesive saliva (Lack, 1956;Medway, 1960), while the swallows haveevolved toward a more general use of mudprobably with some admixture of saliva(cf Bent, 1942). Different species of swiftsor swallows, respectively, can be arrangedin graded series from those swifts like Col-localia jrancica making nests of pure saliva(source of the ideal "birds' nest soup" ofthe Chinese), or from those swallows mak-ing nests of nearly pure mud, like someCliff Swallows, through various admixtureswith plant and other materials to moreconventional types of bird nests (ibid).The nest cement secreted by the cave swift-let, Collocalia fuciphaga, is sparse and soft,and the nest, which is built up from mossand other plant materials, can only beplaced on an irregularity in the cave wallwhich will take all or a good part of theweight of the nest, unlike the nest of othercave swiftlets which can be glued to verti-cal walls in a cave (Medway, 1960).

EVOLUTION OF ROOFED NESTS

Small birds in particular require theprotection from cold, rain and predationfurnished by enclosed nests. Building of aroofed increment nest is very rare amongnon-passerine birds, whereas half of some82 families and distinctive subfamilies ofpasserine birds recognized by Mayr andAmadon (1951), in their classification ofbirds of the world, build roofed nests orcontain representatives that do so. At thesame time roofed nests, aside from use ofnatural cavities, are unusual among pas-serine birds of the north temperate zone,but roofed nests are very common amongsmall tropical birds with altricial young,being typical of many tropical families andgenera (Collias and Collias, 1959, 1964b).

The roofed nest very probably evolvedfrom a nest type that was open above. Thisconclusion is suggested by the fact that inmost instances birds having non-pensileroofed nests start with the basal platform

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NEST-BUILDIING IN BIRDS 183

and then build up the sides and roofs, asfor example, in the Wattled Starling (Liv-ersidge, 1961), Australian Grassfinches (Im-melmann, 1962), European Wren (Arm-strong, 1955), Cliff Swallow (Emlen, 1954),and the tropical tanager, Chlorophoniaoccipitalis (Skutch, 1954).

Among predators of nestling birds,snakes are more numerous and varied inthe tropics than in colder regions, androofed nests perhaps help deter snakes aswell as other enemies. Pitman (1958) hasrecently summarized many instances ofsnake and lizard predation on birds. How-ever, the exact techniques used by snakesor any other predators of nesting birds donot seem to have been investigated. Prob-ably knowledge of such techniques wouldhelp explain certain nest specializations.The true weaverbirds (Ploceinae) belongto an old world family in which all of thespecies build a roofed nest. In many of thespecies that place their nests in herbaceousvegetation near the ground, the nest has aside entrance, whereas in species that placetheir nests in trees the nest generally has abottom entrance. Domed nests of weaver-birds placed in trees tend to evolve a firmpensile or pendulous attachment and longentrance tube about the ventral entrance,presumably as a protection against snakesand other non-avian predators that wouldhave to approach the nest from above(Collias and Collias, 1959, 1963, 1964b;Crook, 1963). Van Someren (1956) madean interesting observation in East Africa:"I once watched a green tree-snake tryingto get at the young in a spectacled weaver'snest. The brute negotiated the slender,pendant branch and reached the nest butcould not manage the 12-inch tubular en-trance and fell into the pond below thenest." The nest of the New World Fly-catcher, Tolmomyias flaviventris, of Suri-nam, is pensile with a ventral entrancetube (Haverschmidt, 1950) that very likelyserves the same function as does the en-trance tube in weaverbirds.

Some protection from bird and mammalpredators is aided by placement of nestsin suitable cover. Nests of some species of

birds are frequently placed in thorn treesand, in the case of the buffalo weavers, athorny covering or shell to the nest hasevolved. In fact, the Whiteheaded BuffaloWeaver (Dinemellia) even places thornytwigs along the boughs leading to its nestover a distance of several feet or more(Friedmann, 1950; Chapin, 1954; Colliasand Collias, 1963).

In the warm tropics steady incubation isnot so necessary nor so characteristic ofbirds as in colder climes, consequentlynests and eggs may be left alone for pro-longed periods. For example, Immelmann(1962) observed that Australian Grass-finches, which are estrildine finches,stopped incubation completely when thetemperature in the nest chamber exceeded100°F. It is evident that presence of a roofhelps hide the eggs from predators duringprolonged absences of the parents.

One function of the roof of domed nestsmust be to shed rain. Most small birds inthe tropics seem to nest during the rainyseason when insect food is abundant.Grasses are often arranged about the en-trance of weaverbird nests in such a wayas to direct away the rain. Skutch (1954)observed that during the early part of thebreeding season, before the rains have be-gun, the nests of the Yellow-rumped Ca-cique (Cacicus cela) in Central America areall open at the top. But as the rains begin,after the eggs have been laid and evenafter the young have hatched, the top ofthe entrance is gradually roofed over andthe nest entrance becomes a bent tube withthe opening downward.

The roof of domed nests has an impor-tant shading effect from solar radiation.Measurements we made of the temperatureinside and outside nests of the VillageWeaverbird (Textor cucullatus) in a cap-tive outdoor colony in Southern Californiashowed the interior of the nests to be some5 to 15°F cooler on a hot day. Brood nestsare somewhat cooler than unlined nests,probably because of the generally thickerroof and walls of the former. The Gala-pagos Finches have an equatorial habitat,and unlike most other Fringillidae, mem-

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bers of this subfamily (Geospizinae) buildroofed nests (Lack, 1947). Since predatorsare rare on the Galapagos, the chief func-tion of the roof of the nest in the Gala-pagos Finches is probably to furnish pro-tection from the elements, particularlyfrom the strong tropical sun. Small nakedaltricial nestlings are no doubt very sensi-tive to direct exposure to strong sun. Thedanger from ultraviolet radiation is pre-sumably greater in the tropics when com-pared with the temperate or colder re-gions than is the heating effect of the sun.However, the direct effects of ultravioleton nestling birds seems not to have beeninvestigated in any controlled fashion.

The roof of domed nests may be com-posed of very different materials in differ-ent birds—woven or thatched grasses in theweaverbirds, a mass of short heterogeneousplant materials bound together by spidersilk in sunbirds and certain titmice, a leafin the tailorbirds, and mud in the cliffswallows. The convergent evolution inthese diverse instances is further evidenceof the great importance of a roof in thelife of nesting birds. Of interest in thisconnection are the few tropical families ofsmall birds that build open cup nests.These cases are not yet fully understood,but various devices seem to substitute fora roof here. The hummingbirds frequent-ly fasten their nests to the underside of aleaf (Ruschi, 1949), while in the bulbulsand cuckoo-shrikes both male and femaleincubate (Van Tyne and Berger, 1959).

The roofed nest among birds reaches itsevolutionary climax of specialization in thependulous or hanging nest and in the com-pound nest.

Pendulous nests are attached by theirupper part while the lower part hangs free.Such nests have evolved independently inbirds of different families, and these birdsmay use very different materials and bind-ing techniques. For example, many sun-birds use spider silk (Chapin, 1953), whilecertain other sunbirds and the AfricanBroadbills (Smithornis) (Chapin, 1953)may use black fungus fibers (Mnrnsmius) asa binding material. The Bush Tit (PMII-

triparus minimus) of the western UnitedStates uses spider silk to hold together anest made of a heterogeneous mass of moss,lichens, oak leaves, and catkins (Addicott,1938). The Cape Penduline Tit (Antho-scopus minutus) of South Africa uses acombination of cobweb and felting to bindtogether a nest made of cottony fibers ofseeds of the kapok tree, wool or hairs(Skead, 1959). Cobweb is an excellent bind-ing material, being very strong, more orless adhesive, and easily pulled out intolong strands. The tiny Black-fronted Tody-Flycatcher (Todirostrum cinereum) oftropical America uses for the main frame-work of its nest a tangled mass of strong,flexible fibers in which are entangled agreat variety of short scraps of vegetablematerial bound together and into the nestby a liberal quantity of cobweb (Skutch,1960).

A woven construction facilitates evolu-tion of roofed and pendulous nests by en-hancing the coherence of the nest. Ac-cording to Skutch (1960, p. 544) "the pen-sile nests of the American Flycatchers arematted rather than woven." This statementpoints up the crudeness of these nests.However, these terms are both difficult toapply in any precise or objective way; thedictionary defines a mat as a piece ofcoarse fabric made by weaving or plaitingmaterials. We have designated as weavingany pattern of interlocking loops of flex-ible materials in the fabric of a nest (Col-lias and Collias, 1957). The orioles andoropendolas of the New World, like thetrue weaverbirds (Ploceinae) of the OldWorld, weave their nests of strips of flex-ible materials. The details of weaving tech-niques have been studied in few species.Herrick (1911) described the shuttle-likemovements of the bill in the BaltimoreOriole (Icterus galbula) which repeatedlypokes in the end of one strip into its nestmass and then pulls back the end of an-other strip. In contrast (except in theearly stages of a nest), the Village Weaver-bird (Textor cucullatus) tends to stay withthe same strip until all the strip is wovenin (Collia!, and Collias, 1902).

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Only one of the half dozen or so sub-families of the weaverbirds can be said totruly weave. The true weaverbirds nor-mally use fresh green materials for theirnests, while the rest of the weaverbirdsthatch rather than weave their nests, oftenusing dry stiff grasses. A whole series ofsteps can be traced from loose, crude, ir-regular weaving to the close, neat, regularpattern that is especially to be foundamong those species of weaverbirds thatbuild pendulous nests with long entrancetubes. For example, Cassin's Malimbe(Malimbus cassini) of central Africa con-structs the most skilfully made nest I knowof in any bird (illustrated by Collias andCollias, 1963).

THE COMPOUND NEST

The compound nest refers to a commonnest mass in which more than one pair ofbirds or more than one female of the samespecies occupy separate compartments.Strictly speaking there should also be somecommon feature of the nest which benefitsall the residents. The compound nest ob-viously grades into instances where differ-ent pairs of birds may build their nests inphysical contact with other nests; suchcases may illustrate early steps in evolutionof a compound nest.

The combination of abundant and con-centrated food supply and relatively safebreeding sites makes possible the gregari-ous breeding and increased social stimula-tion seen, for example, in most sea birds.On the other hand, gregarious breeding israre in small land birds (Friedmann, 1935).An example from passerine birds of highlygregarious nesting in an obviously safenest site is seen in the cliff swallows wherescores of mud nests may be in physicalcontact on the vertical face of a cliff. Infact, Emlen (1954) ascribes the evolutionof the enclosed nest in this species to theneed for neighbors to maintain a certainindividual distance from one another, aneed enhanced by the erection of the physi-cal barriers provided by the contiguouswalls of the nest, which in turn make pos-sible successful nesting under the crowded

conditions and continual social strife with-in the colony. Emlen observed that almostwithout exception the entrance tunnel ofcompleted nests was directed away fromthe nearest neighboring nest entrance.

An example of highly gregarious nest-ing in passerine birds in relation to con-centrated food supply is given by theWattled Starling (Creatophora cinerea) or"locust bird" of Africa, a nomadic speciesfor which some unusually abundant insectlife, as of migrating locusts, is essential forsuccessful nesting by the flock (cf Chapin,1954). A swarm of locusts may be followedby a flock of over 1,000 birds of this spe-cies. The young locusts usually hatch afterrains during the summer, and the WattledStarlings show a remarkable ability to turnup at the right place at the right moment.Locusts are the main food of the nestlings.Recently, Liversidge (1961) has describedsome observations on a colony of this spe-cies in South Africa where over 400 nestswere built, with young being raised inmore than half the nests all within a pe-riod of five weeks. All the nests were situ-ated in thorny acacia trees inside the outerfoliage. The domed nest is mainly of twigswith the entrance usually from one side orfrom above, sloping slightly downwards insingle nests, but in multiple nests the en-trances opened more directly downwards,some even becoming vertical. A few nestswere solitary, but the majority were of twoor three nests combined in one mass oftwigs, and as many as eight in one masswere noted. During incubation both par-ents bring new nesting material to the nestand after the young hatch the males con-tinue bringing fresh nesting twigs fre-quently.

A few species of birds from diverse tax-onomic groups are known to build com-pound nests of twigs in which separatepairs or separate females occupy separatecompartments in the common mass: thePalm Chat (Diilus dominions) of Haiti(Wetmore and Swales, 1931); the Red-fronted Thornbill (Phacellodomns rufi-frons) which is a South American ovenbird(cf Austin, 1961); the Monk Parakeet (My-

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opsitta monachus) of Argentina (Hudson,1920); and the Black Buffalo Weaver (Bu-balornis) of Africa (Chapin, 1954; Crook,1958; Collias and Collias, 1964).

The most spectacular and largest com-pound bird-nest known is built not oftwigs but of grasses, and by a bird smallerthan any of those mentioned thus far. Thissparrow-like bird is the famous SociableWeaver (Philetairus soa'us) of South Afri-ca, whose nest masses have often been com-pared to haystacks in a tree. These nestmasses are not woven but rather arethatched of dry grass stems. Each nestmass, and there may be half a dozen ormore in one tree, is often several feet thick,of irregular extent and up to 5 meters inthe longest dimension. The top of eachnest mass is dome-shaped, the undersiderelatively flat and riddled with up to 100or more separate nest-chambers. The com-mon roof, on which a number of birdsmay build together, may be one key to theevolution of the nest of this remarkablespecies, since it is a communal feature thatenhances protection from predation for all—like the outer thorny shell does in nestsof the Black Buffalo Weaver, or the pro-jecting eaves in nests of the Monk Parakeet(Collias and Collias, 1959, 1963, 1964b).Being domed it also helps to shed the rain.

Possible intermediate stages in the evo-lution of the nest of Philetairus are repre-sented by the nests of its relatives, theGray-capped Social Weaver (Pseudonigritaarnaudi) of East Africa, in which a num-ber of separate nests may be built in thesame tree; but when placed in ant-gallacacias many of the nests are grouped intomasses. We saw up to nine nests in onemass. Colonies of Philetairus are frequent-ly placed in camelthorn acacia trees, andthe base of each of the many thorns maycontain large swellings within which arecolonies of ants. The combination of nest-ing birds with noxious insects and thornytrees is frequent in Africa, and may wellhave been important as a predisposingforce to the evolution of the compoundnest of the Sociable Wea\er.

SPECIATION AND THE EVOLUTION OF NESTDIFFERENCES

It is evident that in analyzing variationsin nest building within the species a num-ber of subsidiary problems are involved:(1) the taxonomic status of the species mayneed to be reexamined; (2) the ecologicalfactors associated with important nest vari-ations must be investigated; (3) the ge-netics and physiology of nest-building, nowalmost untouched fields, must become bet-ter known; and (4) the role of experiencein determining differences in nests betweendifferent populations of the same speciesneeds to be investigated. These four pointsare discussed in sequence below.

1) Stuhlmann's Weaver (Othyphantesbaglafecht stuhlmanni) and Reichenow'sWeaver (O. b. reichenowi) of Africa, for-merly considered separate species are nowbelieved to be one species (Chapin, 1954).Their nests differ in some respects, Stuhl-mann's Weaver builds the outer shell ofpieces and strips of grass leaf, whereasReichenow's Weaver of more arid countrymay use whole grass tops with leaves stillattached in construction of the crudelywoven outer shell of its nest. On the otherhand, after considerable study Stein (1963)now believes that the two song-type racesof Traill's Flycatcher (Empidonax trailli)represent distinct species, E. trailli and E.brewsteri. The former builds a loose bulkynest mainly of grasses as a Song Sparrowdoes, the latter a compact cottony nest,quite like that of a Goldfinch. Both spe-cies occur along streams and lake edges,but trailli prefers more wooded areas andnests closer to the ground and in smallerbushes. Actually, in avian taxonomy dif-ferences in nest form and structure mayoften prove to be of value in delimitinggenera (Mayr and Bond, 1943; Lack, 1956;Collias and Collias, 1963).

2) As already mentioned, in the prairiecountry of Oklahoma where trees are few,the Mourning Doves nest by preference intrees. Those which nest in trees have amuch higher nesting success than do thosewhich nest on the ground, and those which

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nest within forks of trees have a greaternesting success than those which nest far-ther out in the branches. The Magpieoften builds a roofed nest, but where it hasthe protection of dense thorny bushes, itmay build an open nest (Linsdale, 1937).Similarly, the New World Flycatcher, Pi-tangus lictor, makes a roofed nest in ex-posed situations but may build an opennest in concealed places (Smith, 1962). Itwould be profitable to investigate to agreater extent such instances of species inwhich an important characteristic, such asthe presence or absence of a roof to thenest, has not yet become fixed by evolu-tion.

3) Almost nothing seems to be knownabout the genetics of nest-building in birds.I suggest that a small difference in bodysize at certain threshold levels, or in be-havior, such as nest-site selection, may leadto quite large differences in nest-types.Compare, for example, nest-sites and nestsof Barn, Cliff, Bank, and Tree Swallows(cf Bent, 1942). In turn, small differencesin physical traits or behavioral tendenciesthemselves depend on a balanced interac-tion of genetic, physiological, and environ-mental factors for their normal develop-ment.

There is as yet only a little informationavailable on the physiology of nest-build-ing (cf summary by Lehrman, 1959). Lehr-man (1958) finds that an estrogenic hor-mone stimulates nest-building by femaleRing Doves, and we (Collias et al. 1961)have stimulated nest building in male Vil-lage Weaverbirds (Textor cucullatus) out-side the normal breeding season with malehormone treatment. In the latter species,the male weaves the outer shell of the nest.In Ring Doves (Whitman, 1919; Lehrmanet al. 1961) there is some experimental evi-dence that the presence of a nest bowl andnesting material helps stimulate gonado-tropic secretion and breeding behavior. Ithas long been known from naturalists' ob-servations that unsuitable nesting condi-tions may inhibit breeding in birds.

4) The importance of experience to nest-building probably varies with the complex-

ity of the nest built by a species. Canariesmake a simple cup-nest, and Hinde (1958)found that females raised in absence ofnest materials readily built normal nests.In a quite similar experiment, we foundthat young male Village Weaverbirds (Tex-tor cucullatus) raised without nest mate-rials needed considerable practice beforethe complex nest was built (Collias andCollias, 1964a). In the case of different spe-cies of parrots of the genus Agapornis,Dilger (1962) has recently developed someevidence showing the operation of bothgenetic and experiential factors on devel-opment of the ability to gather nest mate-rial.

EVOLUTION OF SPECIAL USES OF

MANIPULATING ABILITY OTHER

THAN NEST BUILDING

The general ability of birds to manipu-late materials probably first evolved for themaking of a nest. Subsequently, there hasalso been an evolution of additional spe-cialized uses for this ability. For example,some birds manipulate materials or makespecial constructions that aid feeding,courtship and copulation, or roosting.

The use of a cactus spine or twig by theGalapagos Woodpecker-finch (Camarhyn-chus pallidus) to probe insect larvae outof crevices or holes is now well known(Lack, 1947).

"Symbolic" nest-building movements andthe holding of nest-material in the bill,especially by the male, during displayspreliminary to pair-formation or coitionare phenomena that occur in many diverseorders of birds (Armstrong, 1942). It is asif the nest-building were projected forwardin the life cycle because of its value as asignal of a given physiological state and inphysiological stimulation. Immelmann(1962) has derived the courtship move-ments of 18 species of Australian estrildinefinches mainly from nest-building activ-ities. In some species the courting malejumps up and down on a branch whileholding a piece of grass in his bill, in otherspecies the grass is dispensed with.

The Australian Magpie Goose (Ansera-

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nus semipalmata) moves about swamps inpairs or trios, builds platforms of rusheson which the birds stand a while to preenand court, and then moves on to buildanother platform elsewhere (Davies, 1962).The Red-throated Loon (Gava stellata)builds a special platform used for copula-tion, the platform being at some distancefrom the nest (Palmer, 1962). Special con-structions used to facilitate courtship andmating activities in birds have culminatedin the remarkable structures built of twigsby the male bowerbirds. These bowers insome species are decorated with flowers orvarious other objects. In fact, the SatinBowerbird (Ptilonorhynchus violaceus)even paints its bower, using fruit-juices andcharcoal. The evolution of bowers hasbeen reviewed by Marshall (1954) and byGilliard (1963). The latter has summarizedthe evolution of arena displays throughoutthe avian orders, and points out that thereseems to be an inverse relationship be-tween the elaborateness of the bower andthe brightness and degree of ornamenta-tion of the plumage of the male. It wouldappear that a large elaborate bower cansubstitute to some degree for conspicuousdisplays of bright plumage features.

Many birds sleep in their nests, and inthe tropics some birds, particularly thosespecies breeding in roofed nests, even buildspecial sleeping nests outside the breedingseason. A number of individuals of thesame species may sleep in one such nest or"dormitory" together (Skutch, 1961). Manywoodpeckers are solitary sleepers and insome species are known to carve specialholes for sleeping in the non-breeding sea-son (ibid).

SUMMARY OF MORE GENERAL CONCLUSIONS

The origin of nests built on land prob-ably traces back to the origin of land lifeand of the land egg in reptiles. The eggsof reptiles are often buried or concealed inpits in the ground, and generally takemuch longer to hatch than do those ofmost birds. Development of eggs in ances-tral birds was speeded up with the originof direct parental incubation which in turn

probably arose in avian evolution in closeassociation with homeothermy and abilityto fly.

The primary functions of a nest are tohelp the parent furnish heat and safety tothe developing eggs. By substituting inpart for these functions, the habit of nest-ing in cavities tends to block evolution ofelaborate increment nests, except as merefilling for the cavity. Size of bird influ-ences the nature of the materials used innests placed on the branches of trees andbushes. Large birds generally make twigplatforms, while small birds as a rule makemore compact and better enclosed nests offiner materials, often binding these firmlytogether and to the substrate by use ofspider or insect silk.

Roofed nests are especially characteristicof small tropical passerine birds. Pensilenests with side or bottom entrances are as-sociated with trees, and presumably en-hance protection from predators. In turn,evolution of penduline nests is facilitatedby a tough, woven construction of the nestas seen in New World icterids and OldWorld weavers, or by firm binding withspider or insect silk as seen in sunbirds,titmice, and a number of other small pas-serines.

Compound nests, in which a number ofpairs or females occupy separate compart-ments in a common mass, is rare amongbirds, and seems to have evolved wherethere is an abundant food supply and un-der conditions where security from preda-tion is increased by special protective de-vices, such as nesting close to biting orstinging social insects. The evolution ofcompound nests is associated with originof some feature of the nest mass that en-hances protection for all the birds, such asa communal roof.

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THE EVOLUTION OF NESTS AND NEST-BUILDING IN BIRDS

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