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Conservation Biology, Pages 447–456Volume 15, No. 2, April 2001

Patterns of Predation Risk and Survival of Bird Nests in a Chilean Agricultural Landscape

MARY F. WILLSON,* JOAN L. MORRISON,† KATHRYN E. SIEVING,‡ TONI L. DE SANTO,§ LEONARD SANTISTEBAN,‡ AND IVÁN DÍAZ**

*5230 Terrace Place, Juneau, AK 99801, U.S.A., email mwillson@ptialaska.net†Department of Biology, Trinity College, 300 Summit Street, Hartford, CT 06106–3100, U.S.A.‡Department of Wildlife Ecology and Conservation, 303 Newins-Ziegler Hall, University of Florida, Gainesville, FL 32611, U.S.A.§Pacific Northwest Research Station, Forestry Sciences Laboratory, 2770 Sherwood Lane, Juneau, AK99801, U.S.A.**Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile

Abstract:

We used experimental nests baited with California Quail (

Callipepla californica

) eggs or clay eggsto examine relative risks of nest predation in an agricultural landscape and in two large forest preserves in asouth-temperate rainforest in Chile. The most common predators, as identified by marks on clay eggs, were acaracara (

Milvago chimango

), a blackbird (

Curaeus curaeus

), and rodents. Nest losses from predation weresimilar in large and small forest patches and lower in patches than in extensive forest. In general, predationrisk was higher (and nest survival therefore lower) on forest edges than in forest interior, in short-grass pas-ture than in tall-grass pasture, in narrow corridors than in wide corridors, and on visible nests than on con-cealed nests. High predation risks in pasture habitat tended to increase the risk of nest predation in adjacentforest edges. For open-cup nesters, the risk of nest predation was relatively high in the present agriculturallandscape, indicating that much of the available wooded habitat ( forest edges, narrow corridors) offers poornesting habitat, although it may be suitable for foraging and traveling. The numerous bird-plant mutualismsin this landscape may be at risk if nesting success of the principal mutualists is consistently low.

Patrones de Riesgo de Depredación y Supervivencia de Nidos de Aves en un Paisaje Agrícola Chileno

Resumen:

Utilizamos nidos experimentales cebados con huevos de codorniz californiana (

Callipepla califor-nica

) para examinar los riesgos relativos de depredación de nidos en un paisaje agrícola y dos reservasforestales grandes de un bosque templado lluvioso del sur, en Chile. Los depredadores más comunes, identifi-cados por marcas en huevos de barro, fueron el caracara (

Milvago chimango

), el mirlo (

Curaeus curaeus

) yroedores. La pérdida de nidos por depredación fue similar en parches de bosque grandes y pequeños y másbaja en parches que en bosques extensos. En general, los riesgos de depredación fueron más altos (y por lotanto la supervivencia de nidos más baja) en los bordes del bosque, que en el interior del bosque, en pastiza-les con pasto corto que en pastizales con pasto alto, en corredores estrechos que en corredores amplios, y ennidos visibles que en nidos ocultos. Los altos riesgos de depredación del hábitat de pastizal tendieron a incre-mentar el riesgo de depredación de nidos en bordes adyacentes a bosques. El riesgo de depredación de nidospara las aves que anidan en copa abierta, fue relativamente alto en la presencia de paisajes agrícolas, indi-cando que una buena parte del hábitat arbolado ( bordes de bosque, corredores estrechos) ofrece un hábitatpobre para anidar, a pesar de que puede ser adecuado para forrajeo y para viajar. Los numerosos mutualis-mos ave-planta en estos paisajes pueden estar en riesgo si el éxito de nidación de los mutualistas principales

es consistentemente bajo.

Paper submitted October 18, 1999; revised manuscript accepted May 10, 2000.

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Introduction

When Darwin visited southern Chile in 1834–1835, heremarked on the extensive rainforests and the lack of ag-ricultural development ( Willson & Armesto 1996). SinceDarwin, however, deforestation and agricultural devel-opment have eliminated large contiguous stands of south-temperate rainforest in many parts of southern Chile. Thepresent-day landscape in much of this area is a patch-work of pasture and woodlots, with wooded or partiallywooded strips extending from or sometimes connectingremnant woodlots. This mosaic landscape provides an op-portunity to describe patterns of predation risk to aviannests across various landscape elements, including forestedges, forest patches of different sizes, forested corridors,and pastures of differing characteristics.

Because so many Chilean forest birds are narrow en-demics (Vuilleumier 1985; Fjeldså & Krabbe 1990), theyare quite vulnerable to habitat modifications (Willson et al.1994). Indeed, areas of high endemism commonly havesuffered high rates of avian extinction and loss of biodi-versity worldwide (Balmford & Long 1994) and thereforeare of high conservation priority (Stattersfield et al. 1998).Temperate rainforests were among the ecosystems foundby Beissinger et al. (1996) to support a higher propor-tion of threatened bird species than expected. Nest pre-dation is an important determinant of reproductive suc-cess in many geographic regions (Reitsma et al. 1990) andmay have contributed to the evolutionary diversificationof avian nest sites (Martin 1993; Schmidt & Whelan 1998;but see also Willson & Gende 2000). Nest predation hasprobably contributed to the decline of forest passerinepopulations in some areas, perhaps particularly in land-scapes heavily modified by agriculture and other humandevelopments (Robinson et al. 1995; Bayne & Hobson1997; Donovan et al. 1997; Tewksbury et al. 1998; Trine1998). Habitat loss and modification directly cause lossof nest sites and can lead to the loss of safe nesting sitesthrough associated increases in the risk of nest predation.Therefore, an understanding of patterns of nest predationin a region with high avian endemism and vulnerability tohabitat modification is important for avian conservation.

We conducted a series of artificial-nest experiments inthe agricultural landscape of northeastern Isla Grande deChiloé, in the extensive forest on the west side of the is-land and in the extensive forest of the nearby mainland.Our primary goal was to describe the relative risk of pre-dation to nests in various landscape elements; our sec-ondary goal was to compare nest placement, nest type,and nest concealment among landscapes elements.

Study Areas and Methods

We conducted our experiments mostly on private landsin northeastern Isla Grande de Chiloé ( lat 41

8

55

9

S, long

73

8

35

9

W ). This landscape is a mosaic of remnant south-temperate rainforest stands and agricultural fields, chieflypasture. Additional experiments were conducted in ex-tensive forests of the Parque Nacional de Chiloé on thewest side of the island and at Proyecto Pumalin Preserveon the mainland, near Caleta Gonzalo, about 100 km eastof Isla Chiloé and about 30 km north of Chaitén. The south-temperate rainforest is floristically diverse and supportsa number of narrowly endemic avian species.

Our experiments were conducted from Novemberthrough January in several years. The various experimentsin forest patches in the agricultural landscape sometimeswere placed in the same patch in different years, but in dif-ferent portions of the patch. Details of methodology andour hypotheses evolved with our increasing understand-ing of the natural history of the ecological system. De-spite this evolution, however, the series of experimentsis sufficient to describe patterns of potential predationrisk to bird nests in this setting and to generate furtherhypotheses.

For all experiments we used locally purchased Califor-nia Quail (

Callipepla californica

) eggs. For most experi-ments we used artificial open-cup nests, constructed ofmoss or grass, in which we placed eggs. In general, theartificial open-cup nests in shrubs were roughly the sizeof the nests of locally breeding

Elaenia

or

Phrygilus

spe-cies. The other nest types we used were meant to repre-sent cavity nesters or open-cup ground nesters. Many lo-cal species nest in cavities in logs, stumps, earthen banks,or treeholes, including several tapaculos (Rhinocryptidae)and furnariids ( Willson et al. 1996). The most commonopen-cup ground nester is the Queltehue or Southern Lap-wing (

Vanellus chilensis

), which commonly nests in pas-tures. In one experiment we used ball-shaped nests tomimick those of the Colilarga or DesMurs’ Wiretail (

Sylvior-thorhynchus desmursii

) (Díaz 1999).Beginning in 1996 we used plasticine clay eggs so we

could better identify predators. Predators were classifiedby the marks left in the clay eggs by bills or teeth; thesemarks were confirmed, in most cases, by matching themwith those made by captive animals. Marks of teeth orbills on eggs were counted as predation events.

Artificial-nest experiments have been criticized ( Wille-brand & Marcström 1988; Roper 1992; Haskell 1995; Ma-jor & Kendal 1996) and do not necessarily indicate ac-tual levels of predation (Major & Kendal 1996; Sieving &Willson 1998). One potential problem with most experi-ments, including ours, is the use of relatively large andthick-shelled quail eggs as bait in the artificial nests. Butpredators whose jaws are too small or weak to grasp quaileggs (DeGraaf & Maier 1996) can nevertheless gain ac-cess to egg contents by other means, including bracingthe eggs against the nest wall (Craig 1998) or pushing theegg out of the nest (K.E.S. and M.F.W., unpublished ob-servations of shrews,

Sorex

sp.). Furthermore, we havefound that clay eggs in experimental nests with quail (or

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Willson et al. Nest-Predation Risks in an Agricultural Landscape

449

other) eggs commonly are attacked by would-be preda-tors as often as real eggs. Therefore, by counting real orclay eggs with tooth or bill marks as evidence of preda-tion, as we did for this study, the possible bias againstsmall predators is countered. Despite the potential prob-lems, this type of experiment remains a useful tool forevaluating the potential risk of nest predation (Sieving &Willson 1998; Wilson et al. 1998), particularly if preda-tors have been identified. Comparisons of experimentaland natural rates of nest loss also provide valuable in-sights into observed distributions of natural nests (Siev-ing & Willson 1998).

Nest appearance can affect predation risk (Martin 1987).Our experimental open-cup nests were constructed ofnatural materials (moss, fine vines, or grass) found onthe site and therefore mimicked the appearance of natu-ral nests far better than wicker nests that are sometimesused for nest-predation experiments. Nevertheless, ournests were less artfully constructed and somewhat moreconspicuous than most natural nests of similar size. Experi-mental open-cup nests were placed in sites similar to thosein which natural nests had been found. Nest locations weremarked with flagging placed at least 5 m from the nest.

In general, experimental nests were placed up to 5 maway from either side of a transect, with about 12 m be-tween nests in 1994 and 25–75 m between nests thereaf-ter. The density of artificial nests exceeded that of natu-ral nests by an unknown amount, and sometimes we musthave placed more than one experimental nest within thehome range of some individual predator, jeopardizingthe independence of the results. But we conducted mostexperiments in so many patches and in so many yearsthat this potential lack of independence was minimal.

There was relatively little annual variation in daily sur-vival rate among years, so we pooled all years for analy-ses. The only comparisons showing annual variation werefor open-cup shrub nests in edge habitats. Inspection ofthese data for each year showed that the survival rate wasconsistently higher in interior than edge habitats for shrubnests (10 of 12 paired comparisons, sign test

p

,

0.02), in-dicating that annual variation in survival did not obscurethe basic pattern and that pooling of years was admissible.

In experiment 1 (1994–1998), we compared nest pre-dation on two nest types in forest patches of differentsize. Small woodlots were defined as

,

6 ha and largewoodlots were

.

100 ha. These woodlots were comparedwith forest sites in the national park. Experiments lasted7–16 days. There were 10–15 nests of each type per rep-licate and 4–6 replicates of each patch size. Both cavityand open-cup nests in shrubs were used. All nests for ex-periments on the effects of forest patch size were placedin forest interior (

$

50 m from the bordering fields).In experiment 2 (1994–1998), we compared nest pre-

dation on two nest types in forest edge and interior. Wedefined forest edge as within 15 m of an adjacent field andforest interior as

$

50 m from the bordering fields. There

were 10–15 nests of each type per replicate and 8–12replicates of each habitat type. Experiments lasted 7–16days and included both cavity and open-cup shrub nests.

In forest edge versus interior experiments (experiment2a, 1997–1999) we used open-cup ground nests. Therewere 10–15 nests per replicate in 8 replicates. Experi-ments lasted 7 days.

In other forest edge versus interior experiments (exper-iment 2b, 1997–1999), we used ball-shaped nests. Eighty-one experimental nests mimicking those of the Colilargawere placed in Colilarga territories in edge and interiorhabitats. These nests were baited with quail and clayeggs, separated by at least 5 m, and monitored for 5 days.

In experiment 3 (1996), we examined the effect ofpasture-grass height and the presence of cows on therisk of nest predation. There were 15 open-cup groundnests per replicate and 5 replicates. Experiments lasted2–3 days each. We decided to conduct this experimentbecause of observations we made in the previous year of10 open-cup nests we placed on the ground along a pas-ture fence. Nine of these nests were depredated within3 days, suggesting that potential predation in pasturemight be high. Several factors could produce this pat-tern, including greater nest visibility in short vegetation.In addition, the Tiuque or Chimango Caracara (

Milvagochimango

), a small raptor that forages extensively forearthworms in cow droppings, might be attracted by thepresence of grazing cattle or their feces. If so, they mightencounter ground nests in the pastures and cause higherrates of predation on eggs where cattle were present (orhad been recently). We therefore conducted experiment3 to examine predation rates on ground nests in pastureswith tall grass versus short grass, with or without cows.

In experiment 4 (1998) we examined “contagion” ef-fects between pasture and edge. Knowing that preda-tion rates on ground nests in pasture tended to be high,we tested the hypothesis that predators (particularly thecaracara) drawn to pasture nests also cause high mortal-ity on nests in adjacent forest edge. The test consisted oftwo treatments, “adjacent” and “separate,” at each of foursites. In the adjacent treatment, the two transects, oneeach in pasture and edge, were placed parallel to eachother at least 100 m apart for their entire length. In theseparate treatment, the two transects were separated byat least 200 m between their ends. There were 15 nestsper transect. The experiment lasted 7 days. Differences indaily survival rates were assigned positive values if survivalwas greater in edge than pasture and negative values if sur-vival was greater in pasture than edge. These differenceswere then compared by means of a one-tailed paired

t

test.In experiment 5 (1998), we compared nest predation

on two nest types in wooded corridors of different width.We examined nest predation patterns on ground andshrub nests in corridors of different widths (

,

10 m vs.

.

50 m); these widths are the same as those found bySieving et al. (2000) to serve solely as travel corridors or

450

Nest-Predation Risks in an Agricultural Landscape Willson et al.

Conservation BiologyVolume 15, No. 2, April 2001

to be suitable as living space, respectively, for terrestrialunderstory birds. There were 10 nests per transect and 6replications, and the experiment lasted 7 days.

In experiment 6 (1998), we compared forest interiorversus forest edge versus wide corridors versus narrowcorridors; all four habitat types were tested at the samesite. There were 15 nests per replicate and 4 replications,and each experiment lasted 7 days.

In experiment 7 (1998), we compared concealed ver-sus visible nests at the mainland site of Pumalín and innortheastern Chiloé. We tested the hypothesis that pre-dation was lower on shrub nests that were relativelyconcealed (to a human observer and presumably to visu-ally searching nest predators) than on those that werereadily visible. Artificial nests were placed in the shrublayer at a height of 0.7–1.7 m at about 12-m intervalsalong transects on edges between forest and field or for-est and roadsides. Nest concealment was estimated byL. Santisteban based on a modification of the techniquedeveloped by Holway (1991). “Concealed” nests were

,

25% “visible,” and “visible” nests were

.

75% visible.There were 10 nests per replicate, 4 replicates per site,and the experiments lasted 5 or 13 days.

We made one-way comparisons of untransformed May-field survival rates using the program CONTRAST (Sauer &Williams 1989). We based analyses of variance (ANOVAs)and

t

tests on arcsin-transformed Mayfield daily survivalrates (Mayfield 1961; D. H. Johnson 1979). In addition,we reanalyzed the data using an arcsin-transformed sim-ple percentage of nests surviving, separating experimentsof different durations, to see if statistical outcomes weresimilar to those of the primary analyses. Outcomes wereidentical in both analyses, except that survival in largeforest patches was lower than in small patches when weused percent survival and equal when we used Mayfielddaily survival. In addition, for the Colilarga experiments,we used simple percentages. No interactions in ANOVAswere significant.

Results

Identification of Predators

In pasture, most egg depredation was attributed to birds,principally the caracara, but in forest most depredation

was attributed to rodents (Table 1). In edge habitat be-tween pasture and forest, depredation was attributed toa mix of birds and rodents. Corridors had a mix of avianand rodent predators on eggs, although the mix differedbetween wide and narrow corridors (Table 1). The “otherbirds” category usually reflected marks left by a probing,stabbing bill, which are unlike the marks of the caracarabill. In the Colilarga nest experiment, the major preda-tors were rodents and small, unknown birds.

Patterns of Nest Predation

Nest survival was lowest in the national park but did notdiffer significantly between large and small woodlots (ex-periment 1; Table 2). Nests in forest edge were less safethan nests in forest interior (experiment 2 & 2a; Fig. 1),marginally so for the Colilarga-type nests (experiment 2b).Hole nests were no safer than open-cup shrub nests (ex-periments 1 & 2).

The presence of cows had no effect on the risk of nestpredation, although cows sometimes trampled the unde-fended nests; trampled nests were excluded from theanalysis. Nests were significantly safer in tall grass thathad not been grazed recently (

.

30 days) than in shortgrass (experiment 3; Fig. 2).

The contagion experiment demonstrated that high risksof predation in pasture may lead to increased risks ofpredation in adjacent forest edges (experiment 4; Fig.3). If the initially presumed high risk of predation ( lowsurvival of nests) in pasture were contagious to adjacenthabitats, one would expect to find that pasture and edgenests would have more similar survival in the adjacenttreatment than in the separate treatment. This was ob-served: the average difference in daily survival rates be-tween pasture and edge nests in the adjacent treatmentwas negligible but positive, whereas the average differ-ence in the separate treatment was greater but negative(indicating greater average survival in pasture nests). Theseresults are more easily understood if one inspects thesimple percentages of nest success. The difference in suc-cess between habitats in the separate treatment ( pasture88%, edge 62%) was much greater than in the adjacenttreatment ( pasture 76% success, edge 73%).

Nests in narrow corridors were significantly less safethan those in wide corridors (experiment 5; Fig. 4); thispattern was similar for both ground and shrub nests. In

Table 1. Predators of eggs in experimental nests in different habitats, as identified by marks on clay eggs, 1997–1998.

Habitat No. eggs depredated

Proportion of depredated eggs attributed to each type of predator

caracara other birds all birds rodents unknown

Pasture 96 53 32 85 3 11Edge 122 36 25 61 20 19Forest 48 6 6 13 42 46Wide corridor 26 8 23 31 42 27Narrow corridor 44 20 43 64 27 9

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451

experiment 6, survival rates differed significantly amongthe four habitats; post-hoc pairwise comparisons identi-fied the source of significance in the lower nest survivalin forest edge than in interior (Fig. 5). In addition, how-ever, there was a trend for nests in narrow corridors tohave low but variable survival. Concealed nests were dep-redated less often than visible nests in forest-edge habitatat both study sites (experiment 7).

Discussion

Most of the egg predators were rodents, in forest andwide corridors, and the small caracara, in pasture andforest edge, although in forest edges and narrow corri-dors another bird often attacked eggs with a probingbill. The most likely candidate for the probing bird is theTordo or Austral Blackbird (

Curaeus curaeus

), which isknown to be an egg predator and often moves along for-est edges in pairs or flocks (A. W. Johnson & Goodall1967; Jaramillo & Burke 1999; our observations). We

have so far identified three kinds of predator on 319depredated eggs, although the rodents may be repre-sented by two or three species. In contrast, Sieving andWillson (1998) identified seven species of predator in186 photographs or other evidences of predation eventsin north-temperate rainforests in southeast Alaska. Sevennest predators were identified in 14 predation events atexperimental shrub nests in New Hampshire (Reitsma etal. 1990), and eight species of mammals were identifiedin 28 predation events at experimental ground nests inMinnesota (Fenske-Crawford & Niemi 1997). Chileanforests may have fewer kinds of egg predators thannorthern forests, although this has not been confirmed.

Small mammals, including shrews and mice, are im-portant nest predators in the northern hemisphere (e.g.,Reitsma et al. 1990; Nour et al. 1993; Sieving & Willson1998, Craig 1998). Of the rodents found in Chilean rain-forest,

Irenomys tarsalis

is arboreal but chiefly herbivo-rous (Meserve et al. 1988). Mice of the genus

Akodon

are more omnivorous and terrestrial, although they canclimb well and may be common in rainforest (Meserve

Table 2. Comparisons of Mayfield daily survival rates of experimental nests in different landscape elements

a

in southern Chile.

Experiment Comparison Outcome Category Mean SE Statistics

b

1 patch size

3

nest type large

5

small

.

park; small 0.995 0.002 two-way analysis of variance; patch size,

F

5

3.30,

p

5

0.05; MS

5

2

0.016, df

5

2; nest type, n.s.

shrub

5

cavity nest large 0.996 0.002park 0.982 0.007cavity 0.991 0.003shrub 0.991 0.005

2 edge/interior

3

nest type

interior

.

edge; edge 0.978 0.004 two-way analysis of variance; edge vs. interior,

F

5

4.28,

p

5

0.05, MS

5

0.024, df

5

1; nest type,

F

5

3.01,

p

5

0.09, MS

5 2

0.017, df

5

1

cavity

$

shrub nests interior 0.988 0.003cavity 0.988 0.004shrub 0.980 0.004

2a edge vs. interior (ground nests)

interior

.

edge edge 0.890 0.026 CONTRAST chi-square

5

10.8,

p

5

0.001interior 0.977 0.0052b edge vs. interior

(Colilarga nests)interior

$

edge edge 92%

t

test,

p

5

0.08interior 98%

3 pasture: grass height

3

presence of cows

c

tall

.

short grass tall 0.853 0.078 two-way analysis variance; grass,

F

5

13.06,

p

5

0.005, MS

5

0.318, df

5

1; cows, n.s.

cows

5

no cows short 0.571 0.049cows 0.638 0.052no cows 0.488 0.077

4 contagion difference between pasture and edge: adjacent

,

separate

adjacent 0.003

d

0.031 one-tailed paired

t

test on differences,

t

5 2

4.05,

p

5

0.01

separate

2

0.076

d

0.037

5 corridor width

3

nest type

wide

.

narrow wide 0.984 0.006 two-way analysis of variance; width,

F

5

13.19,

p

5

0.002, MS

5

0.0168, df

5

1; nest type, n.s.

ground

5

shrub next narrow 0.922 0.022ground 0.940 0.023shrub 0.966 0.012

6 forest vs. edge vs. narrow corridor vs. wide corridor

forest

.

edge forest 0.973 0.007 CONTRAST chi-square

5

6.61,

p

5 0.03; forest vs. edge, chi-square 5 4.99, p 5 0.03 all other pairwise contrasts, n.s.

edge 0.940 0.013wide 0.957 0.012narrow 0.915 0.044

7 nest concealment concealed . visible concealed 0.986 0.004 CONTRAST chi-square 5 51.20, p , 0.01visible 0.954 0.002

aNo interactions were significant.bn.s., not significant.cSome pasture nests were lost to trampling by cows, so the number of nests available for analysis was sometimes fewer than designed.dIn this directional comparison, a positive difference indicates that the daily survival rate of nests was higher, on average, in edge than pas-ture, and a negative difference indicates that survival was greater in pasture than edge.

452 Nest-Predation Risks in an Agricultural Landscape Willson et al.

Conservation BiologyVolume 15, No. 2, April 2001

et al. 1988; Redford & Eisenberg 1992). A tiny forest opos-sum, the monito del monte (Dromiciops gliroides), maybe an egg predator also, although we found no toothmarks to indicate its activity. Mustelids are nest predatorsalso, but they appear to be uncommon in our study areaand are probably incidental predators (Vickery et al. 1992).

Many kinds of birds in addition to those reported hereand the well-known corvids may be important nest pred-ators in some regions of the world. For example, severalspecies of blackbirds (Icterinae) and wrens (Troglodytidae)appear to be regular egg eaters (Purcell & Verner 1999).Wrens occur in our study sites in Chiloé, but we did not ob-serve egg predation by wrens.

We detected little effect of patch size on daily survivalrates of experimental nests, which parallels the resultsfrom natural nests (T. L. D. & M. F. W., unpublished data).Studies in the northern hemisphere have reported mixedresults with respect to relationships between patch sizeand nest survival (e.g., Huhta et al. 1998; Farnsworth &Simons 1999; Porneluzi & Faaborg 1999).

Natural open-cup nests were significantly more vulnera-ble to predation in forest edges than nests in forest inte-rior or in open habitats such as shrubby pastures androadsides ( T. L. D. & M. F. W., unpublished data). Preda-tion risk for Colilarga nests tended to show an edge effectas well. The greater vulnerability of experimental nests innarrow than wide corridors also suggests the existence ofan edge effect and reinforces the opinion of Major et al.(1999) that linear forest remnants may be of mixed valueto avian conservation—possibly useful as travel corridorsbut unsuitable for nesting. Nevertheless, the negative ef-fects of narrow corridors or forest edges may not apply to

some species in some situations (Haas 1997). In general,forest edges in these south-temperate rainforests appearto offer relatively unsafe nest sites. Edge effects are oftenfound in agricultural landscapes in the northern hemi-sphere (Paton 1994; Bayne & Hobson 1997; Donovan et al.1997; Hannon & Cotterill 1998; Hartley & Hunter 1998;but also see Filliater et al. 1994; Friesen et al. 1999). Inaddition, however, landscape configuration and predator

Figure 1. Mayfield daily survival rates in forest edge and interior habitats (experiment 2). Survival was consistently lower in interior than in edges.

Figure 2. Mayfield daily survival rates in pastures of different grass heights (experiment 3).

Figure 3. Difference in Mayfield daily survival rates of ground nests between adjacent and separate treat-ments in pasture and forest edge (experiment 4).

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Willson et al. Nest-Predation Risks in an Agricultural Landscape 453

ecology must also be important in determining spatialpatterns of predation (Andrén 1992, 1995; With 1994;Hanski et al. 1996; Danielson et al. 1997; Hannon & Cot-terill 1998; Söderström et al. 1998; Villard et al. 1999).

Some of the birds in the south-temperate rainforest thatcommonly nest in or near forest edges are critical mutu-alists for the many plant species that are pollinated ordispersed by birds (Armesto et al. 1996). Of special im-portance as mutualists are a hummingbird (Sephanoidesgaleritus), flycatcher (Elaenia albiceps), and thrush (Tur-dus falcklandii ). Although all three of these species ap-pear to be habitat generalists, occurring commonly inthe fragmented-forest landscape, their risk of nest preda-tion is high there (T. L. D-S. and M. F. W., unpublisheddata). Populations of these mutualists in much of the ag-ricultural landscape may be sinks rather than self-main-taining. If so, the mutualisms essential to reproductionof many plant species may be at risk.

Natural cavity nests were significantly more successfulthan natural cup nests (T. L. D-S. & M. F. W., unpublisheddata), as has been reported by others (Martin & Li 1992).Nevertheless, the relative safety of natural cavity nestsmay decrease, especially along streams, as the introducedmink (Mustela vison) increases its range in Chile (Me-dina 1997 ). Predation patterns for natural nests contrastwith those we found in artificial nests. We suspect that inour first experiment (with patch size) some cavities usedfor experimental eggs were rodent tunnels in logs, stumps,and rootwads. Because rodents were subsequently shownto be the most important egg predators in forest habitat,placement of eggs in their runways probably exagger-ated the apparent level of predation on experimentalcavity nests. Although subsequent experiments avoidedprobable rodent runways, our selection of natural cavi-ties may have been biased.

The results of several of our experiments support theidea that visible nests were more vulnerable to predation,implying that major predators probably use vision to de-tect this kind of prey (see also Yahner & Cypher 1987;Willebrand & Marcström 1998; L. S., K. E. S., & M. Avery,unpublished data). Visibility was examined directly in for-est edge, but the greater safety of nests in tall grass andwide corridors and (in some experiments) of cavity nestssuggest that nest concealment is likely to improve sur-vival of the nest contents. Some studies have failed to findany association between concealment and nest safety(Filliater et al. 1994; Howlett & Stutchbury 1996), sug-gesting that the idiosyncrasies of predator identity andecology are important in determining the presence ofsuch associations (Reitsma et al. 1990; Sieving & Willson1998; Söderström et al. 1998).

Predation risk on artificial nests in pasture was high.The most common pasture predator, the caracara, is subsi-dized by agricultural practices and by nearby beaches.Not only do farms provide suitable habitat for these birds(Morrison & Phillips 2000), they augment the predator’sfood resources, providing poultry, the placentae of cowsand sheep, and earthworms that utilize aging cow dung.As a result, caracaras are abundant in the agricultural land-scape and are capable of exerting intense predation pres-sure. Similarly, the Austral Blackbird commonly forages inpastures and fields and is generally more common in frag-mented forests than in continuous forest tracts (A. W.Johnson & Goodall 1967; our unpublished data). Nestpredators in north-temperate forests are also subsidizedby other resources. For example, Steller’s Jay (Cyano-citta stelleri ) is more common in suburban forest edges

Figure 4. Mayfield daily survival rates in wooded cor-ridors of different widths (experiment 5).

Figure 5. Mayfield daily survival rates in four habi-tats at the same site (experiment 6).

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than in natural edges, and nest predation by jays is higherin suburban edges (De Santo & Willson 2001; K. E. S. &M. F. W., unpublished data), a pattern similar to thatsometimes found for Blue Jays (C. cristata; Fretwell 1973;but see also Danielson et al. 1997). Similar levels of nestpredation in Ohio woodlots in rural and suburban set-tings was attributed to subsidization of predator popula-tions by human activities (Melampy et al. 1999). Like-wise, the abundance of the red squirrel (Tamiasciurushudsonicus) in southeastern Alaska reflects the pres-ence of large spruce trees and the availability of sprucecones, and predation by squirrels increases with the rep-resentation of large spruce trees in Alaskan rainforestT. L. D-S. & M. F. W., unpublished data).

In our study, the patterns of predation on experimen-tal nests sometimes mirrored those on natural nests, butsometimes they did not, perhaps as a result of our choiceof nest placement. In general, artificial nests do not nec-essarily indicate the vulnerabilities of real nests (Major &Kendal 1996; Sieving & Willson 1998; Wilson et al. 1998).Artificial nests provide an index of potential predationpressure, which may exceed actual pressures if birds haveadapted to the threat by changing nest sites, habitats, orbehavior. In this study, however, artificial shrub nestswere safer than natural shrub nests. The lack of parentalactivity near the artificial nest gives fewer cues to a po-tential predator. We observed the small forest cat orguiña (Oncifelis guiña) depredating nests with nestlings,and Tordos appear to be attracted by the activity ofsmall birds near the nests (M. F. W. & K. E. S., personalobservations).

Predation risk, resulting in part from the foraging pat-terns of the principal predators, should select for partic-ular patterns of nest protection, including shifts of habitator nest site and nest-defense behavior. Where predationis intense, selection may favor increased crypticity of nests,perhaps especially in small birds, or highly developedaggressive nest-defense behavior, perhaps especially inlarger birds. Thus, the small nests of finches and spar-rows tend to be cryptic, but the large Southern Lapwingexhibits loud and active nest defense. Similar patternsare observed in southeastern Alaska, where small wrens,warblers, and sparrows tend to make cryptic nests, butthe larger robins and Varied Thrushes (Ixoreus varius)appear to rely more on active defense.

Major changes in land-use patterns in the Chilote land-scape are unlikely in the foreseeable future. We can there-fore expect increasing forest fragmentation and increas-ing agricultural development, to the detriment of forestbirds. If forest birds require some management action topreserve their populations, there may be two possibleoptions. First, the edge-to-interior ratio of forest frag-ments could be minimized by leaving them in compactrather than linear shapes and leaving wide wooded cor-ridors to maintain the connectivity of fragments. In addi-tion, reforestation of fencerows and small ravines would

increase the connectivity of habitat in the landscape. Ifthese new corridors were sufficiently wide (Sieving et al.2000), they would also provide breeding habitat for manyspecies. Second, populations of nest predators whosepopulations have expanded as a direct result of agricul-tural development could be controlled. A recent reviewof the outcomes of predator-control efforts and the ef-fects of predators on numbers of breeding birds showedthat predator control commonly increased the nestingsuccess of the prey species (mostly ducks and grouse inthe reviewed studies) but did not necessarily increaseprey population size (Newton 1998). In our Chilean sys-tem, populations of some species, including the cavity-nesting tapaculos (Rhinocryptidae), are more likely to belimited by habitat availability and accessibility than by nest-ing success. In contrast, for many species with greatervagility and less specific nesting requirements, a reduc-tion of nest predation might permit increased popula-tions. But a program of predator control would increasesuch prey populations only if nest success were the chieffactor limiting prey population size. The option of pred-ator control, therefore, should not be pursued withoutfurther research on the ecology of the local predatorsand their specific patterns of nest predation, in additionto experimental assessment of the effects of control pro-grams on the nesting success of prey species and theprey population. Furthermore, it is likely that a predator-control program would need to be continual, becauseof recruitment from other predator populations and be-cause continuing habitat destruction creates ever morehabitat for some of the important nest predators. Forthese reasons, the option of habitat maintenance or res-toration appears both more feasible and more effective.

Acknowledgments

We thank our field assistants for their participation in thiswork: C. Anderson, J. Arnett, K. Bardon, M. Bruscia, T. Fort-ner, L. McConnell, S. McGehee, H. McPherson, L. Phil-lips, J. Pinkston, H. Puckey, M. Reetz, L. Solchenberger,and N. Wright. Statistical consultation was provided byG. Packard and G. White. J. F. Eisenberg and P. Meservegave us advice on rainforest mammals. Numerous Chilotelandowners graciously gave permission to use their landfor our studies. This research was funded principally bythe National Science Foundation (International Programs)and the National Geographic Society. This is publicationNo. R–07996 of the University of Florida Agricultural Ex-periment Station.

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