A.fi·. J. Eco/. l9S7, Volume 25, pages 35-45

Foraging site selection by three raptors in relation to grassland burning in a montane habitat

PH 0 E BE BARN A R 0* Department (~/Zoology, University of the Witwatc:rsrand, Johannesburg 2001, RSA

Summary

Factors affecting the use of flight-hunting areas by three common raptors (rock kestrel Fa/eo tinnunculus L., jackal buzzard Buteo rufofuscus (Forster), and black­shouldered kite £/anus caeruleus (Desfontaines)) were analysed in a montane grassland area. Seasonal variation in foraging site selection was examined, based on five habitat parameters: regrowth time since burn, cover density, grass height, rockiness and slope. Kestrels and buzzards moved into recently burnt areas to hover, while kites hovered over undisturbed grassland. Increased small mammal and large insect availability characterized newly burnt savanna and are suggested as reasons behind the habitat change by two of the three raptors. An increase in the use of rocky grassland by kestrels and buzzards coincided with the availability ofjuvenile skinks in this patchily-distributed habitat.

Resume

Les facteurs inft uen<;ant !'utilisation de terrain de chasse par trois rap aces communs (Crecerelle des rochers. Fa/eo timwnculus L., Buse-chacal, Butl!o ndi?{ilscus (Forster) et Milan a epaules noires, £/anus caeruleus (Desfontaines)) ont ete ana­lyses dans unc zone de prairie montagneuse. On a examine la variation saisonniere du fourragc sur le site choisi scion cinq parametres:temps de rcpousse apres brulis, densite de couverture, hauteur de l'herbe, caructere rocailleux et pente. Des crecerelles et des buses se sont rendues auwdessus des endroits recemment brl'iles pour plan~r tandis que les milans survolaient des pres n'ayant pas connu de perturbation. La savannc fraichemcnt bn1lee se caracterise par une plus grandc disponibilite en petits mammifcrcs et grands insectes, ce qui expliqucrait le changc­ment d'habitat de deux des trois rapaces. Une utilisation accrue des prairies rocheuses par les crecerelles et les buses correspondait a la presence de jeunes scinques dans cct habitat clairseme.

Introduction

In discussing the responses of montune small mammal populations to fire, Rowe­Rowe & Lowry ( 1982) speculated about the differential availability of grassland mammals to avian predators. Grassland protected from fire, they suggested, should iL1hibit predation by raptors and carnivorous mammals. By contrast. mammal populations in periodically burnt veld arc theoretically more susceptible to aerial and terrestrial predators.

35

36 Phoebe Bamard

The numerical and functional interactions of predator and prey populations have been examined in open-country boreal and temperate ecosystems (Pitelka, Tomich & Treichel. 1955; Hagen, 1969; Hornfeldt, 1978; Phelan & Robertson, 1978; Erlinge et al., 1983), but the interactions of such communifies in the southern hemisphere are less well understood in this context (Ogen-Odoi & Dilworth, 1984). A growing body of information is being gathered on the large and small mammal communities of fire-climax grassland in the Natal Drakensberg of South Africa (e.g. Mentis & Rowe-Rowe, 1979; Rowe-Rowe, 1982, 1983; Rowe-Rowe & Lowry, 1982; Rowe-Rowe & Meester, 1982; Scotcher, 1982), although the role of predators in this system is still widely misunderstood (but see Rowe-Rowe, 1983; Manry, 1982). Fire, a tool in the management of grassland controlled by the Natal Parks Board, profoundly afrects the seasonal distribution of herbivores and, therefore, their predators. It was in this light that the fo!Jowing study, part of a broader investigation of the foraging energetics of montane raptors, was initiated.

~latcrials and methods

Observations were made from October 1983 to April 1984 in Giant's Castle Reserve (29°08'-22' S; 29°24'-35' E) in the Drakensberg mountains bordering Lesotho and Natal. This 36,000 ha reserve encompasses a 30 km stretch of the Drakensberg escarpment and its foothills to the cast, 1380--3350 m above sea level. Precipitation is concentrated in the austral summer from October to April and, in this study, ended a drought during 1982-1983. Mean daily minimum and maximum temperatures for January, the warmest month, are l3oC and 23°C (RowcwRowe & Meester, 1982).

Giant's Castle Reserve consists of fire-climax grassland characterized by Themeda triandra Forsk., with isolated riverine forest and scrub in protected areas. Sandstone outcrops and low boulder-beds act as shelters for small mam­mals, reptiles and invertebrates during flre (Rowe-Rowe & Lowry, 1982). The rotational biennial spring-burning pattern at Giant's Castle Reserve during the study was similar to that described by Rowe~Rowe & Lowry ( 1982); vegetation at different altitudes is detailed by Killick(1963).

Spot observations. of unmarked rock· kestrels Fa/eo tinnunculus L., jackal buzzards Buteo ruj~fusc:us (Forster), and black-shouldered kites £/anus caeruleus (Desfontaines) were made both incidentally and during systematic 1-11 h observation periods. Hover-hunting birds were followed using 8 x 32 mm Zeiss binoculars until a hunting bout ended or the bird was lost to view. The vast and deeply-dissected terrain made continuous observations of individuals impractical, so this paper concerns aerial foraging only. Here, a 'hover' is defined as a sustained period of stationary aerial foraging, with or without wing-flapping. The hover is the main unit used in most analyses in this paper. Sequential hovers and gliding or flapping 'interhovers' comprised a hovering 'bout', which began when a bird entered the observation area to forage or resumed hunting after a kill, and ended when .the bird perched, caught prey or began soaring.

Vegetative cover hovered over by raptors was coded at the start of each hover­ing bout, as well as after all patch changes, according to relative grass height. density, time since last burn and rockiness. Vertebrate and invertebrate prey

Foraging site selection hy three raptors 37

captures were identified for rock kestrels and black~shouldered kites. but normally jackal buzzards consumed unseen prey on the ground in < 20 s.

Prey populations were indexed by insect sweep-netting and small mammal snap-trapping during February and February to April. respectively. Netting transects were run linearly or in plots at each of six favoured raptor foraging sites and two unforaged sites, for a total 400-600 sweeps per transect. Six transects were in 'new' grass ( < 1 year since burnt) and two were in 'old' grass (;:::: 1 year since burnt). Transects were sampled during each of four daylight periods (0600---0900 h, 0900---1200 h, 1200-1500 h, 1500-1800 h) in warm, sunny weather, and four of the eight were additionally sampled in foggy, cool conditions at mid­day. Grasshoppers, katydids (Orthoptera) and beetles (Coleoptera) of ;?:-0·5 cm body length were collected and measured to the nearest 1·0 mm. Small mammals (all rodents) were sampled by removal, using break-back traps set in linear transects at 15 m intervals and baited with oats and peanut butter. Two major cover types used by hovering raptors were sampled using four to ten traps set for twenty-four to thirty-six consecutive 24 h periods, for a total of 200 trap-nights per cover type. Reptiles, mainly skinks A1ahuya spp., were not systematically cen­sused, but were abundant on boulder-beds and in rocky veld. Female skinks were pregnant from December to March, with juveniles conspicuous from January to April.

Data were analysed with the SPSS statistical package (Nie et al .. 1975) and hand-calculated chi-square tests with Yates' correction factor. Unless noted, all means are expressed as x + 1 so.

Results

Seasonal sh(fi in use a_( newly humt vegetation

Two of the three species, rock kestrels and jackal buzzards, exhibited marked shifts in their use of flight-hunting habitat, moving into first-year or 'new' grass­land as spring growth increased the cover density of second~yea r veld. This shift occurred in December (Fig. 1 ), when new herbage became conspicuous after the rains of October to November. Rock kestrels and jackal buzzards hovered over new grass after November significantly more than expected by chance alone. Not only were more hovering bouts initiated over new than old grass (X 2 RK = 73· L df=3, P<0·005; x2

18 =88·l, df=3, P<0·005), but more individual hovers also occurred over new grass (X 2RK=346·1, df=5, P~0·005; X2

J8 =1028·2, df=4, P~0·005). The latter analysis is more meaningful, as birds frequently began a hovering bout over one cover type and made subsequent patch changes to another.

Statistical evaluation of the seasonal trend for black-shouldered kites could not be done by the chi-square test, as black-shouldered kite hovers occurred over­whelmingly over old grass· (Fig. 1 ), rendering expected cell values too small. The anomalous data for December (Fig. I) were mainly gathered on an immigrant female kite which was at first excluded from, then admitted to, a predominantly old grass area controlled by a territorial male (Barnard & Simmons, 1985). As the sample size of individual hovers was smaller for black-shouldered kites (n = 548) than for rock kestrels (n=2044) or jackal buzzards (n= 1718), such an anomaly appears more prominent. However, clear interspecific difTerences in the use of

38 Phoehe Bamard

~ 50 "' 11)

)it

V! .... <V ro > I() r<l N 0 a; r<l r<l 0 ..c:; 1-- <:1" r<l -0 -.c 0

,1::.

"' "' 0 "'0 ..... .... 0\ 0

50 N -a N

0 ::l ro .,_

0

OJ

"' ::>

"0 OJ > ~ tf)

_Q

0

:::-!: 0

<!) -50 ::X::

Month

Fig. I. Sl)asonalusc of old grass habitat (%of month's hovers) by hovering kestrels, buzzards and kites. Numbers in histogram indicate total monthly hovers.

ne\vly burnt grass were apparent. Jackal buzzards used new grass for 80·W~~ of all observed hovers, while rock kestrels hovered over new grass 66·9% of the time and black~shouldered kites only 32·8% of the time (Fig. 3 ). These difl'crcnces were highly significant (X 2 = 442· 3, df= 2, P ~ 0·005), particularly in late summer (cf. Fig. l ),

Seasonal shtfi in use (~f'hou/der-bed habitat

Rocky 'boulder-beds' were actively foraged by all three raptors. Both rock kestrels and jackal buzzards noticeably increased their use of rocky habitat during the 6-month study (Fig. 2); this trend coincided both with increasing herbage pro­duction in non-rocky second-year grass. and with the reproduction of skinks. Although prey selection in this study was incompletely quantified, I observed at least twenty-three skink captures by rock kestrels and five by jackal buzzards, showing that these ra ptors did exploit this resource. Both rock kestrels and jackal buzzards showed significantly linear increases in rocky habitat use during the summer (rock kestrel r = 0·77, one-tailed t = 2·38 .. P < 0·005; jackal buzzard: r = 0·83, t = 3·0 L P < 0·025), but black-shouldered kites were not increasingly

I I

10 11

Foraging site selection by three raptors 39

,A... I ',

I ' I ' I '-,

I "

I 12

Month

Availabilily of ~.......... . ........... > ' juvenile sklnks

--·'--.... . .--- ~

I 2

'---..

Buzzard

Keslrel

-. Kile

I 3

Fig. 2. Seasonal use or boultler-bcd habitat (% of month's hovers) by hovering kestrels. buzzards and kites. Timing of juvenile skink availability is indicated at top. Sample sizes are the same as in Fig. I.

';;;'

1ii > 0

.<::.

4> "-;::-

.::; u 0

"' 0 50

<))

"' ::>

u Q)

"' 0> <f) .0 0

~ Q

Burn Density Rock Gross Crest

Foraging site porumeler

Fig. 3. lnt~rspccitk difl'crences in foraging site selection by hovering kestrels, buzzards and kites (top). For cuch parumetcr. black and shaded bars indicate the two options open to foraging raptors: description of each option is in box (below). Yerticaliblack bars,_~ rock kestreL horizontal hluck bars= jackal buzzard: diagonal/ black bars = black -shouldcrl·d kite.

Para me! er

Burn Density Rock Grass Crest

Black bars

First-year gruss Pah:hy grass 13oulder· bed ha bit at Short ( ~ 30 cm) grass Hill slope or crest

Shaded bars

Second-year grass Homogeneous grass Grassy habitat Long ( > 30 cm) grass Flat land

attracted to boulder-beds (r=0·60, t= 1-49, P>0·05). Use of rocky habitat was lowest among kites (23% ofull hovers), followed by kestrels (27·2%) and buzzards ( 38·1 °/u) (Fig. 3). The difference was signincant (X 2 = 70·6. df = 2, P < 0·005), but not as marked as the difference in use of new grass (Fig. 3).

40 Phoehe Bamard

Other parameters c~fforaging habitat

In Fig. 3, relative use of habitat types as judged by five parameters is compared for the three raptors. lnterspecific differences in the parameters 'burn' and 'rock' (Fig. 3) have already been presented. In addition, note that the parameters 'den­sity' and 'grass' (grass height) show similar histograms to that of the parameter 'burn'. This similarity arises from extensive covariance between the three l~'lctors. 'Density' describes the homogeneity of grass cover, and consists of two relative terms: patchy and homogeneous. Patchy grass tended to be first-year, short growth; homogeneous grass was frequently second-year growth which had become more luxuriant and even. These subjective assessments were made during weekly walks through specific foraging areas, and not on the basis of quantitative criteria such as quadrat analysis. Reflecting the associated use of first-year grass, rock kestrels and jackal buzzards made greater use of patchy grass (68·6% and 80·8% of all hovers, respectively) than did black-shouldered kites (34·3% ). a highly significant difference (X 2 =420·9, df=2, P ~0·005).

The parameter 'grass' describes relative grass length. An arbitrary value was set to distinguish short ( ~ 30 cm) from long ( > 30 cm) grass. In spring and early summer this distinction, based on new (green) and old grass (brown) categories, was clearcut. However, 8 months following a burn the two types appeared similar from a distance and were visited periodically for measurement. Jackal buzzards used proportionally more short grass for hovering (81·9%) than did either rock kestrels (73·1 %) or black-shoulde1"ed kites (39·4%), again a highly significant diflerence (X2 = 375·?, df = 2, P ~ 0·005).

Finally, a fifth parameter, 'crest', was established to distinguish, for energetic purposes, the relative use of hills and updrafts by raptors (Barnard. 19g6). \Vhen hovering, a bird either used a crest (steep slope or hilltop) or flat land. Crests were most frequently used by jackal buzzards (95·5<Yo), less often by rock kestrels (81·6%), and least often by black-shouldered kites (74·6% ), a signifkant diflerence (X 2 =222·8, df=2, P~O·OOS). The energetic implications of this pattern are discussed by Barnard ( 1986).

Indices of prey popular ions

Small mammal snap-trapping. To estimate small mammal abundance in old and new grass, trapping equivalent to 200 trap"nights in each of two grassland sites (6 and 18 months since burning) was carried out at 1850-2000 m. Trap success for sites 1 (new.grass) and 2 (old grass) was identical (Table 1), despite a lengthy trapping pcdod. Trapping lasted from 26 February to 9 April. with a 17-day overlap period in which both habitats were trapped simultaneously. To estimate possible effects of differential rainfall during the trapping of each site, a rank score (0-5) based on rainfall duration per 24 h period (min) was used. Slightly more rain fell during the trapping of site 2 (:X score= 2· 2 + 2·0, n = 30 days) than of site I (x=2·6+2·0, n=29 days), where a score ofO indicated pro­longed rain and 5 indicated dry weather. It is unlikely that this small dificrence affected trap success over a !-month period.

Insect s!l'eep-netting. The relative abundance, estimated availability and mean body length of orthopterans and coleopterans at each of eight sites arc shown in

Foraging site selection by three raptors 41

Table l. Small mammal trapping success in two Themeda grassland habitats at I 850-2000 m at Giant's Castle Reserve

Months since

Site Date buming Species

3/3 6 Rhahdomys pumilio 4/3 6 Rhahdomys pumilio

16/3 6 Rhahdomys pumilio 2 14/3 18 DeiJ(/romus melunotist 2 28/3 l!i Rhabdomys pumillo 2 31/3 IH Rhahdumys pumi/io

•Expressed as captures/l 00 trap-nights, or% trap success. tPartially eaten in snap-trap.

Sex Age

M Adult? M I mm.? M I mm. M I mm.? M Adult M Adult

Weight

45,} 37 g :?.2g

lOg} 53 g 50 g

Trap index*

1,5

1.5

Table 1. Body length. abundance, and availability index or orthopterans and coleoptcrans• in eight transects sweep-netted at I HSO 2000 m in Themeda grassland at Giant's Castle Reserve, arranged in decreasing order of estimated availability

Mean Months Mean abundance

since body length SD (insects l 00 Transect burning (cm) (cm) sweeps- 1)

H F Et G B A os s c

5 1·03 0·44 5 1·00 0-47 5 2·07 0·86 5 1·10 0·46

17 1·26 0·31 5 2·11 0·98 5 1·95 1·08

17 1·35 0·67

•93·9% of individuals measured were or1hopterans. tCalculated by the equation Ax = (L~ .../n~)/S,B~ (terms defined in text). tUnforaged area: fairly dense grass cover. *U nforagcd area: grazed, with minimal grass cover.

22·8 14·2 11·0 IO·K 13·5

5·5 3·4 6·0

Availability indext

11·2 8·0 K·O 7·9 7·5 5·0 3·3 2·5

Table 2. An availability index, reflecting the expected 'reward' of an insect capture~ was calculated for each transect by the formula

A)(= (Lx A)/SxBx

where for transect x, A= estimated availability, L =mean body length (cm), n=number of insects collected per 100 sweeps, S=standard deviation of L, and B= time since transect was burnt (years). In general, first-year grassland supported a larger range of insect size classes, prominently including large grass· hopper instars (x = 1 · 35 + 0· 78 cm; n = 347), than did old grass (x = I· 30 + 0· 50 cm: n = 96), although the difference in means was not significant. The related avail­ability indices tended to be higher for first-year than second-year plots (Table 2).

42 Plwebe Burnurd

Large, robust grasshoppers were fur more frequently collected in new grass than old, although the presence of very small individuals in the sample obscured the difference statistically. There was no difference, however, in availability index or mean body length between rocky and non-rocky areas. Mean availability of insects in two rocky sites was 6·5 and mean body length was 1·44 +0·83 cm (n = 65), as compared to corresponding means for non-rocky grassland of 6·8 and 1·4 + l·Ocm (n=384).

Discussion

Cover density The availability of two major grassland types at Giant's Castle Reserve, one recently burnt, patchy and short, and the other thickly regrown and long, created two foraging options for raptors. In the former, large insects and rodents represented a more available prey source due to the paucity of cover, although they appeared to occur in numbers comparable to those in the denser, second" year cover. Monthly measurements of herbage production were made in Giant's Castle Reserve by Scotcher, Clarke & Lowry (1980) at the same altitudes (2000 and 2200 m) and under the same burning regime (biennial spring burn) as my study area. Their data also suggest that following an August burn, herbage pro· duction rises sharply from November to December; new growth peaks after 7 to 8 months and is followed by a fairly rapid decline through the winter. This trend is a general one, although the peak standing crop in late summer varies with altitude (Rowe-Rowe & Meester, 1982), treatment plot (!hid.; D.T. Rowe-Rowe, inlitt.), season of burn and year (Scotcher et al., l9~W).

Prey selection

Rock kestrels and jackal buzzards, which prey on a wide variety of animals includ­ing lizards and large insects as well as small mammals (Steyn. 1982; Barnard, unpubl.), could exploit the greater availability of prey after considerable ground cover was removed by tlre. Black-shouldered kites, by contrast, are rodent special­ists; Mendelsohn ( 1982) found that 90· 3% of prey items represented in the pellets of a Transvaal population were rodents, and 6·6% shrews. A fully quantified analysis of prey selection was not possible in this study, as birds often captured prey out of my sight. and few pellets were located. By early December. however, kestrels and buzzards had begun catching lizards and large grasshoppers: kestrels usually ate these items in mid-air during gliding flight.

Qualitatively, only broad generalizations about prey selection can be made. All three species preyed upon small mammals and kites preyed nearly exclusively on Otornys and Rlwhdomys; one skink was recorded, accounting for 2·3% of forty-two items eaten by adults or brought to a nest (P. Barnard unpubl.). By contrast, kestrels preyed heavily on large insects, skinks and mammals in that order of frequency, while buzzards most probably preyed on large lizards, small mammals and large insects (cf. Steyn. 1982). Insect predation by kestrels and buzzards became apparent only after November, coinciding with their shift to newly burnt grassland. I cannot explain why kites apparently preferred old grass foraging habitat. Although only 35% of the study area consisted of this grass type, kites concentrated on it for 67% of hovers (this study). It is possible that new grass

Foraging site selection by three raptors 43

areas acted as 'dispersal sinks' for immature rodents, as at least two of the three mice trapped in new grass (Table I) were immature. Kites may have remained in old grass habitat to exploit larger adults, including nesting females. Yet although the trapping periods were lengthy, the results could be misleading and should be interpreted with caution.

Prey avai/ahili!J'

Other trapping results from Giant's Castle Reserve are significantly higher than those reported here (Rowe-Rowe & Lowry, 1982; Rowe~Rowe & Mcester, 1982). The former paper presented seasonal data detailing trap success relative to time since burning. Six months after spring burning, Themeda grassland at 1900 m supported small mammals at a trap index of 12·5%, while the same habitat 18 months after a fire was trapped at 8·3% (Rowe-Rowe & Lowry, 1982). Spring­burnt grassland at 2200 m, trapped by the same authors, showed similar values at 6 and 18 months but exhibited a different overall pattern of peaks in trap success. Rowe-Rowe & Lowry ( 1982) further fc>iir}d that the highest small mammal diver­sity and moderately high trap success were obtained in structurally heterogeneous habitats, including boulder-beds. Thus it is possible that in this study, kestrels and buzzards foraged over rocky habitats for mammals in addition to skinks, although the former must have been much less available in these areas than in homogeneous grassland. Although trapping results from different studies are seldom directly comparable due to vagaries in trap sensitivity, sampling design and prevailing weather conditions, the extremely low trap success of this study is inexplicable.

Similarly, patterns of insect abundance as estimated by sweep-netting (Table 2) may indicate a possible dispersal of large size classes into regenerating veld from unburnt reservoirs; equally they may simply reflect differential trappability in long and short grass.

Two generalizations may be cautiously drawn from these two sampling treat­ments. First, small mammals were equally trappable in first- and second-year grassland plots at 1950 m, so that due to reduced cover, they may have been more available to raptors in the former areas. Second, the orthopterans available in first-year grass included much larger individuals (up to 4·3 cm); these insects were presumably more available there than in sccond~year grass, although the mean calculated expected reward wns similar in both habitats. A third point is that an area unused by hovering raptors, a 6 km long jeep track through the study area (transect E in Table 2), had the highest insect availability index and second highest mean insect body length of the eight sites sampled. Grasshoppers, par­ticularly large instars, appeared to bask in large numbers in the grassy centre of the track on warm, sunny days. The importance of nearby cover to these insects is suggested by the contrasting transect D, an adjacent section of jeep track where grass cover had been heavily grazed by ungulates. In transect D, the availability index was the second lowest of the eight calculated, and abundance was the lowest. As kestrels and buzzards were ,observed taking only grasshoppers larger than an estimated 2 cm as prey, they were expected to forage over areas where the largest size classes were mo'st available. The non-use of the jeep track by raptors may indicate a preference for areas where the expected rate of capture is highest, rather than for where prey individuals are largest.

44 P/wehe Barnard

Foraging strategies and grassland burning

Small mammal predators, therefore, face several foraging options based on cover density and height, irrespective of other aspects of prey availability. Hovering raptors, which should seck a balance between minimum cover density (to detect prey movement) and maximum density (to encourage a threshold level of prey density), move to ditferent sites if local cover density exceeds an optimum and if alternative palatable prey arc more available elsewhere. In this study, where these options were made available through the prevailing burning policy, this scenario was the case for kestrels und buzzards~ but not for the rodent specialist black­shouldered kite. Although in other arc,as this und related kites actively forage over pastured farmland and Jield verges with low to medium cover (Warner & Rudd, 1975; Mendelsohn, 1982), undisturbed road verges of high cover are preferred due to the high rodent densities they support (Mcndelsohn, 1982). The lack of response to boulder~ beds and burnt areas by kites in this study corroborates the concept of this species as a more etncient hunter in dense, long-grass sites than either kestrels or buzzards, both of which could avoid these areas by exploiting alternative prey types.

In Giant's Castle Reserve, areas undisturbed by fire support stu ble small mammal communities, although total numbers and species richness are below those of biennially-burnt plots (Rowe-Rowe & Lowry, 1982). Raptors were not observed hovering over plots protected from fire at Giant's Castle Reserve, even when wind conditions favoured it. It seems, then, that in choosing foraging sites, raptors balance adequate visibility with maximal prey densities: protected areas or very thick cover do not attract them.

Patterns of foraging site selectiori by another avian predator, the bald ibis Geronticus calvus (Boddaert), have been described in the Drakensberg by M anry ( 1982). The bald ibis is primarily insectivorous and restricted to montane cultivated lands, where it prefers recently burnt, ploughed or harvested sites for foraging. In this species. also, predation is presumably more eflicient in areas of minimal cover when prey density is held constant, as prey ur~ rendered more available by the structural changes in vegetation caused by fire or agriculture. In Giant's Castle Reserve, foraging avian predators such as kestrels, buzzards and black-hcudcd hl!rons Ardea melanoccplwla (Vigors & Children) were present at plots being burnt, even us smoke billowed from the ground. Manry ( 1982) argued that in addition to the immediate increase in insect availability due to mortality, recently burnt veld conditions such as increased surface temperature may enhance insect recoloniz­ation and reproduction. Both Rice (1932) and R. Miller (unpubl.) have docu­mented the reappeu ranee of insect taxa in post-burn grassland. For phytophagous insects u nd small mammals, the earlier spring re growth and greater nutritional quality of burnt veld (Rice,. 1932; Scotch er et al., 19gQ) creates favourable con­ditions for Coraging and reproduction. This enhanced recolonization by con­sumers, coupled with decreased cover density, fosters attractive foraging prospects for avian predators.

Acknowledgments

This study wus carried out and financed independently as an extension of related \York done at Acadia University in Canada. I am deeply grateful to the Natul Parks

Foraging site selection by three raptors 45

Board for the privilege of working in the Drakensberg. Logistical assistance was otfered by D. T. Rowc~Rowe. D. N. Johnson and P. M. Brooks (NPB); A. E. Bowland. P. Govender, S. P. Henzi and R. Miller (University of Natal), J. M. Mendelsohn (Durban Museum}. and Jl. E. H. Paterson and R. E. Simmons (University of the Witwatersrand). For discussion on prey sampling I thank P. Govender, R. Miller and partlcularly D. T. Rowe-Rowe. I am grateful to the Masson, Miles and Yunnie families for help in many ways, and to S. K. Eltringham, R. E. Simmons and an anonymous reviewer for clarifying the manu­script. For sometimes checking small mammal traps at 2000 m during lightning squalls. I am eternally indebted to Robert Simmons and his rubber boots.

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(Mmtuscript received 7 December /984)