170 S. Afr. 1. Zool. 1996,31(4)

Territory and nest defence in polyandrous pale chanting goshawks: do co-breeders help?

G. Malan' and A.R. Jenkins Percy FitzPatrick Institute, University of Cape Town, Rondebosch 7700, South Africa

Received 19 February 1996; accepted 30 July 1996

The behaviour of polyandrous and monogamous pale chanting goshawks Melierax canarus was investigated to determine if co-breeders, by defending the territory and nest contents, helped to increase the fitness of polyan­ drous trios. Polyandrous trios consisted of a female and male breeder, as well as a subordinate co-breeding male. Pale chanting goshawk males performed most of the interspecific territorial maintenance duties, as well as participating in aggressive intraspecific interactions against other males on territory borders. Intraspecific territo­ rial interactions were almost exclusively recorded in high-quality habitat, Karroid Broken Veld, and probably functioned in the defence of foraging habitat and potential mates. In this habitat, males of polyandrous families largely occupied exclusive sections of a territory, It is suggested that the cost of defending territories in Karroid Broken Veld was offset by the co-breeder's contribution to high-risk, intraspecific territorial defence, During the nestling period females of polyandrous trios stayed at nesting sites for longer periods than did monogamous females and all males, enabling these females to guard the nest and act as sentinels. Predation of nestlings was recorded at the nests of monogamous pairs, but not at those of polyandrous trios. Co-breeders did not guard the nesting site but contributed directly to nest defence by either coming to the female's aid when solicited, or attack­ ing potential predators when present at the nesting site. Co-breeders may also have helped indirectly, through activities such as provisioning prey to females that relieved them of their hunting duties. We suggest that the female's nest guarding and the resulting lower nest predation may hold reproductive and fitness benefits for pol­ yandrous breeders.

* To whom correspondence should be addressed

Family groups evolve when limitations are placed on breed­ were allowed to delay dispersal in the natal territory, but were ing opportunities of high quality for sexually mature individu­ not tolerated near the nesting site in the breeding season. als (Koenig, Pitelka, Carmen, Mumme & Stanback 1992; Since these offspring continued to interact with their parents Emlen 1994). Group living holds no automatic advantages into adulthood, PCG groups were referred to as families .

) and may even be disadvantageous to an individual (Alexander (Emlen 1994). 9

0 1974). However, once a group has formed, individuals may be The aim of this research was to investigate territory and 0

2 nest-site defence by breeding peGs under conditions of expected to alter their behaviour to take advantage of the pres­ d e ence of group members and derive benefits from group-living monogamy and polyandry, and in habitats of different quality. t a (Em len 1994). In birds, such benefits may include the early In this article it is discussed whether co-breeder males, by d ( detection and deterrence of predators at nests and of intruders participating in territory and nest defence, increase the inclu­ r e at territory boundaries (Gayou 1986; Rabenold 1990; sive fitness (Hamilton 1964) of PCGs in polyandrous trios. h s i

l Mumme 1992). Apart from the fundamental question as to b Methods

u why helpers help and do not disperse, it is necessary to deter­ P

mine if they really contribute significantly to the group, since Study area e h helping may be no more than an unselected consequence of t The study area was located ncar Calitzdorp (33'32'S, y non-dispersal (Craig & Jamieson 1990; Emlen 1991). If such 2]" 48'E) in the Little Karoo, South Africa. Mean annual pre­ b

d help does bendit individuals in groups, they can expect to cipitation is approximately 200 mm. Three karroid vegetation e t increase their Jitness through enhanced survival and reproduc­ types occur in the study area, lying in broad parallel bands n a tion (Em len & Wrege 1994). from Spekboomveld in the north, through Karroid Broken r g Tn the pale chanting goshawk (PCG) Melierax canorus, pre­ Veld, to Succulent Karoo in the south (Aeocks 1988). Open e c Spekboomveld is an open shrub community comprising 2-5

n dation of eggs or nestlings was the major cause of nest fail­ e c ures in four peG study areas in South Africa and Namibia m high trees and shrubs, Karroid Broken Veld conSIsts of low i l succulents « 75 em) with dwarf trees and shrubs scattered r (Malan, Crowe, Biggs & Herholdt in press). However, only in e

d one study area (see below) and only in one vegetation type, throughout, and Succulent Karoo is open with a sparsely dis­ n Karroid Broken Veld, were PCGs found to breed in polyan­ tributed layer of dwarf succulents. u

y drous trios. The Karroid Broken Veld supported a very high a Sampling w bioma'ls of otomyinid rodents, Otomys unisulcatus and Parot­ e t omys brantsii, the preferred prey of PCGs, and a suitable A monogamous pair comprises one male and one female a

G hunting habitat incorporating prey visibility and perch availa­

breeder (Malan et al. in press). A polyandrous trio comprises t e bility (Malan & Crowe 1996). Within PCG polyandrous trios, one female and two male breeders. A breeder is an adult par­ n i a co-breeding male participated fully in all reproductive activ­ ticipating in all reproductive activities (Malan et al. in press). b a ities such as nest construction and prey provisioning to the In the PCG, as with most other raptors (Newton 1979; S

y female and offspring at the nesting site. In addition to co­ Faaborg & Bednarz 1990), the female is larger (on average b breeders, non-breeders, either adult or juvenile offspring, 22% by body mass) than the male (Malan 1995). In the breed- d e c u d o r p e R S. Afr. 1. Zool. 1996,31(4) 171

ing season the dominant female was able to displace the male demonstrated an occupational preferencc for a scr.;tion of a from prey he brought to the nesting site, as well as from eggs territory, we divided each territory into two equally sized sec­ during the incubation period. In a polyandrous trio, a co­ tors along a central axis. During instantaneous sampling. the breeder is an additional adult male identified hy his subordi­ position of the focal male in cach sector was recorded every nate status to both the dominant female and male. Dominant 60 seconds. For each observation period recorded. males were males were in all cases larger than co-breeding males, and assigned to a sector according to the majority of minutes were able to displace co-breeders from prey and eggs but not spent there (i.e. more than 50% of time). vice versa (Malan 1995). During [he non-breeding and pre-laying (from first copula­ Predation of nest contents tion until cgg laying) pert ods of 1988, we studied the territo­ For each of the 51 visits made to nests during the breeding rial behaviour of peGs in the Karroid Broken Veld and Succulent Kama vegetation lypes. Employing instantaneous seasons of 1988-1992, the following data were recorded: the saJllpling (Lehner 1979), 86 observation periods totalling presence/absence of eggs/nestlings, disturbance to nest lining 12368 minutes (mean = 144 ± 73 min) were completed. Dur­ and structure, and the presence of chick remains on the ing the nestling period, the hehaviour of all breeding peGs ground below the nest. When previously occupied nests were within a 100 m radius from the nesting site was recorded from found to be empty, predation was only considered as a r.;ause a hide overlooking the nesting sites (scan sampling), during of nesting failure if the nest cup or rim had been disturbed, or 34 eight-hour observation periods (16320 min). These data if offspring remains were found near the nest. were collected for two different polyandrous trios and two monogamous pairs in Karroid Broken Veld (\ 988 and 1989), Results and for a monogamous pair each in Succulent Karoo (19gg) Attendance at the nesting site and Open Spekboomveld (1989). Since breeding raptors spend more time away from the nest during the latter part of During 1988 and 1989. time spent away from the nesting site the nestling period (Newton 1979), we analysed nest attend­ did not differ significantly between either monogamous males ance data by employing a one-way analysis of covariance or females from two families in Karroid Broken Veld (GM with days after hatching as the independent variable, and min­ unpublished data), so data were combined for males and ute~ per ohservation period outside the 100 m radius as the females respectively. During 1988, time spent away from the depenJent variable. nesting site did not differ significantly hetween polyandrous males (ANCOVA; f ~ 4.60; dl~ I; p > 0.05), or he tween pol­ Territorial behaviour yandrous males and monogamous male~ (ANCOVA; ,.. ;:: .

) 1.07; df ~ I; p < I).U5), so data were comhined for all males. 9 A territorial interaction was defined as an encounter between 0 On average, the polyandrous female was away rrom the nest­ 0 a territory holder and an intruder (interspecific or intraspe­ 2

ing site 28o/c) of the time (n = 480 minutes). monogamous

d cific), which elicited an action or reaction from the focal

e females 73%, and all males 95% (ANCOVA; F ~ 49.1; dl ~ 2; t PCG. We recognized six interaction modes. sequenced in a p < 0.00 I). During 19R9, on average. the polyandrous female d order of increasing aggression according to physical and (

spent 6% of the time (n = 480) away from the nesting site, the r vocal expression: e polyandrous breeder male 45%, the co-breeder male 530'0, h Fly diving at - PCG dived at intruder, dives were shallow; s i monogamous females 61 % and monogamous males 89%· l Fly mob - PCG dived repeatedly, deep and directly at b (ANCOVA; F ~ 528; dl~ 4; p < 0.00 I). u intruder; P

e Fly mob and call - PCG new mobbed and alarm-called, a h Predation of nestlings t

quavering ee-e-e-e-e-e-e (Maclean 1985); y

b Fly stnking - PCG flew mobbed and physically struck No predation of eggs was recorded. The frequency of nestling

d intruder; predation did not differ signilicantly between Open e t II n Spiral night and aggressive-call - PCG thermalled in small Spekboomveld (19%; n ~ 16). Succulent Karoo (8%; ~ 13) a r circles with very fast and shallow wingbeats, with aggressive­ and Karroid Broken Veld (7%; n ~ 73; G2 ~ 1.9; I' > 0.05). g

e calling in a loud fast burst of staccato kikiki-kikiki-kikiki­ Nestling predation was highest during 1988 and 1992, and c

n kiktki; and lowest during 1989 and 1990 (Table I). During years of inter­ e c i Cartwheel night - PCGs locked feet in mid-air and whirled mediate PCG reproduction (1988 and 1992), when 39 fami­ l

r down vertically. lies produced 27 broods, seven incidents of predation at the e d All interactions were recorded during instantaneous and scan nest were recorded. During years with high reproductive suc­ n u sampling during 1988-1989 and ad hoc observations during cess (1989 and 1990), when 49 families produced 53 broods. y 1988-1992. Interactions were classed as interspecific or only two cases of predation were recorded. During 1991, a a w intraspecific (between neighbouring PCG families), and year when 29 families produced only three broods, no preda­ e t occurred either on the border of two territories or within a ter­ tion was recorded. or the 83 hreeding attempts recorded over a G

ritory (more than 500 m from territory houndary). five years (1988-1992), nine (14%) of the 66 nests of monog­ t e During all observations in the study area, we searched for amous pairs were subject to predation (Table I). No nests of n i breeding birds marked with colour rings and patagial tags (n polyandrous trios were raided by predators. Most cases (891ft) b a = 28) and mapped their positions. We studied the territorial of nestling predation were of first broods (or single broods in S

y behaviour of the males of two polyandrous trios and observed years when only one brood was raised) and the majority of b each male for more than seven hours. Since each mare nestlings (56%; n ::::: 9) were preyed upon in the firs! half of the d e c u d o r p e R 172 S. Afr. J. Zool. 1996.31(4)

Table 1 Instances of predation of pale chanting gos­ 84% (/I = 31) and were the aggressors in 96% (/I = 26) of hawk nestlings, all from monogamous pairs, in five these, whereas females were the aggressors in three out of breeding seasons (1988-1992). OSBV = Open Spek­ only five interactions (Table 2). boomveld, KBV = Karroid Broken Veld and SK = Succu­ Apart from two incidents, intraspecific interactions in the lent Karoo non-breeding and pre-laying periods were all between mem­ Number Second Nc,,' Remains bers of the same sex (Table 3). Males were involved in more of successful Iiningl on ground aggressive interactions, for example spiral and cartwheel Vegetarion nestlings Age brood per structure (5-7 tlights, and females in less aggressive interactions. Males Year tyre taken (weeks) sca"on disturbed weeks) were also involved in more interactions in total (77%; n = 17) 19H8 aSBV 2 ~o than females. Since 20% of the 80 families studied in Karroid

(l~BV ) No Yes Broken Veld were polyandrous trios, coupled to the 22 KHV 6 No No' Yes monogamous pairs studied in Open Spekboomveld (Malan et al. in press), the sex ratio was biased in favour of males 1989 KHV 6 ~o No Yes (I1R:102). However, even if the observed male:female 1990 KBV 7 Yes Yes Yes aggressive interactions frequency is corrected (from 13:4 to No pretlation 1991 II :4), males were still involved in 73% (n = 15) of interac­ 1992 aSHV 2 No Yes tions, Males were involved exclusively in interactiuns along KBV 2 No Yes the borders of territories (n = 13), whereas females where also KHV 2 No Yes involved in some disputes within territory borders (75%; n = 4). Thirteen intraspecific interactions were observed in Kar­ SK 2 ~o Yes roid Broken Veld, four in Open SpekboomveJd and none in I second chick still in nest Succulent Karoo (Table 3). seven week nestling period. Remains of offspring found Overall, males and females shared equally in defending the helow nests were all of large nestlings. nest contents when both sexes were present, although only females were involved in physically striking potential preda­ Interspecific and intraspecific interactions tors (Table 4). Males called in 50% (" = 12) of interactions Of the interspecific territorial interactions observed during the and females in 33% (" = IS) (G, = 0.23; P > 0.05). peGs nun-breeding and pre-laying periods, males were involved in appeared most agitated (e,g. flying with continuous wing- . )

9 Table 2 Interspecific interactions recorded for pale chanting goshawks in 0

0 the non-breeding and pre-laying periods of 1988 and 1989. All intruders 2

d were mobbed. KBV = Karroid Broken veld, SK = Succulent Karoo e t VcgclLllioll Noof a d Aggre,.~or Aggr~:-.:,e~ interactIOns

( type Status r

e Male h

s 1 i PCG KBV Co-breedcr Blackshouldered kite Elanus caerufeu.\ 4 l b PCG KHV Polyandrous Two black crows Corvus ca{Jer!Ji.~ u

P br~eder 2 e h PeG KBV Co-hreeder White necked ra\'enl C/lrvu.{ u/b,Clllli.\· t

y peG Kev Co-breeder Jackal buzzard2 Butell rufllfuscU_f 1 b

d PCG SK' Monogamous Two black nows 4 e t

n peG SK' ~onogamou:-. Black harrier Circus ftI(1urus a r PeG SKI Munogamous Lanner fakon Falco bianni('us g

e peG :-"lonogamous Hlackshouldered kite1 6 c SK n e PCG SK Monogamous Rock kestrel F(1IL'l! tinnu"t'ulu.f c i l

PCG SK Monogamous Two Egyptian geese Afll{Jrlc/wrI a('';YfJti(1( U.\' r e PCG Monogamous Yellow mongoose Cl'Ilicli,( penicif/uw d SK n ~onogamous peG

u Two black crows SK I y a Fl'm(1fe w e t peG KBV Monogamous Steppe buzzard Buteo buteo 2 a 2

G peG KBV Polyandrou'i Blackshouldered kite t e Two bla(k- n i shouldered kites KeV Polyandrous PCG . I b a Black crow SKI Monogamou~ PCG S

y I interaUions recorded in SK territory on Ihe border of KHV b

! birds pur~ued to the burder of the territory d e c u d o r p e R S. Mc. 1. Zool. 1996,31(4) 173

Table 3 Intraspecific interactions recorded for pale chanting goshawks in the non-breeding and pre-lay­ ing periods from 1988 to 1992. See text for explanations of interaction modes. Birds in parenthesis present but did not participate in interaction. KBV = Karroid Broken Veld and OSBV = Open Spekboomveld Internal or bordel No of Aggressor (vegetation type) Interact mode Aggressee Interaction interactions Males

Co-breeder male (KBV) Fly mob Monogamou.~ male Rorder 2 Polyandrous breeder male (KRY) Fly mob Monogamous male & female Bordel Co-breeder male (KBV) Fly striking Monogamous male (& female) Border Polyandrous breeder male (KBV) Spiral flight and call, Monogamous male cartwheel night Border Monogamous male l (KBY) Spiral flight and call Monogamous male Border 4

Polyandrous males I (K8V) Spiral tlight and call Monogarnou,~ male Border Monogamous male] (OS8Y) Spiral flight and call Monogamous male Harder 2

Monogamous male j (USH V) Spiral flight and call. Monogamous male (& female) cartwt\eel night Border

Fellll.1fe.\'

Polyandrous female (KBV) Fly mob Non-breeder female (from polyandrous trio) Internal

Monogamous female I (KBV J Fly mob Non-breeder female (from monogamous pair) Internal Monogamous female (KBV) Fly mob each other Polyandrous breeder male & female Border

Monogamous female I (OSBY) Fly mob Monogamou.~ female Internal

I interactions recorded in ca~ual observations

heats) during interactions with gymnogenes (Polyboroides was ever observed in the territory of another family. Om: typus) and martial eagles (f'o{emaetus bellicosus). marked breeder was observed for five breeder-years in Open . ) Spekboomvcld, six marked hreeders for 29 hreeder-years in

9 PCGs were ahle to actively defend their nest contents as 0 Succulent Karoo, and 21 marked hrceders for 57 breeder­ 0 demonstrated hy the following two incidents. In a particularly 2 aggressive interaction, a polyandrous trio attacked an uniden­ years in Karroid Broken Veld. Of those in Karroid Broken d e Veld, four polyandrous females were studied for 10 breeder­ t titicd intruder on thc ground. Although we were unahle to a years, three polyandrous breeder males for 10 hreeder-years

d ohserve any tracks, this intruder was terrestrial. less than 75 ( and five co-breeder males for 11 breeder-years. r em in height as its movements were obstructed by the lower e

h vegetation layer, and probably mammalian as it proceeded at Clear-cut differences were found in the spatial arrangement s i l the pace of a fast-walking man. The perched female spotted of polyandrous males within territories. In one family. the b

u the intruder approaching down a drainage line towards the polyandrous hreeder male occupied the eastern sector of the P nest and gave an alarm call, which brought the two males territory (n = 3 observation periods), whereas the co-breeder e h quickly to the nest. All three hirds started calling and diving male occupicd the westcrn sector (n = 3) (G, = 4.9; P < 0.05). t

y at the animal continuously, forcing it to follow a circular route The polyandrous breeder male occupied a higher proportion b around the nest. The same animal, it is presumed, returned to of quadrats in the east (90%; n = 474 min) and the co-breeder d e t the area of the nest during the same ohservation period, hut male in the west (91 %; n = 579 min). Another family's poly­ n a was again successfully driven away. androus breeder male occupied the northern sector of the ter­ r g (n = In Succulent Karoo, territorial interspecifIC interactions ritory R ohservation periods), whereas the co-breeder e c ~ere observed at a rate of one every 311 min (n = 5 595 min). male occupied the south (n = 3) and north (n = 2) (Gc = 5.4; I' n e < 0.05). The polyandrous brecder male occupied quadrats at a

c In Karroid Broken Veld these interactions were observed once i l II every 599 min by polyandrous breeders (n = 7 188 min). higher proportion in the north (84%; = 607 min) and the co­ r e Although monogamous breeders from this vegetation type breeder male in the south (64%; n = 758 min). d n were only ohserved for 349 minutes, two interspecific interac­ u Discussion y tions were ohserved. At nesting sites in Karroid Broken Veld, a interspecific interactions were recorded once every 675 min Territory defence w e

t (n = 12145 min), once in 5248 min at sites in Succulent In general, helpers within a group of social hirds usually huve a Karoo and no interspecific interactions were recorded in G

a positive impact on each individual's inclusive fitness t I 440 min at sites in Open Spckboomveld. e (Emlen 1990). Hclping may, however, have no impact at all. n i or may even disrupt reproductive and survival activities (Sta­ b a Territory occupancy cey & Koenig 1990; Du Plessis 1991). Defence against poten­ S

y Twenty-eight marked breeders were ohserved for 91 breeder­ tial predators of young may be achieved more successfully by b years within their breeding territories, and no marked bird groups containing helpers (Bckoff & Wells 1982; Stacey & d e c u d o r p e R 174 S. Mr. J. Zoo!. 1996.31(4)

Table 4 Interspecific and intraspecific interactions ror territorial PCGs. males predominantly defended the ter­ recorded at nesting sites of pale chanting goshawk in the ritory against interspecific intruders. Against intraspecifics, breeding seasons of 1988-1989. See text for explana­ territorial males were also involved in the highest frequency tions of interaction modes. KBV ~ Karroid Broken Veld, of interactions. Interactions between territorial males were SK = Succulent Karoo particularly aggressive, involving among other things, spiral and cartwheel flights (Simmons & Mendelsohn 1993). Spiral Aggrt!~sor (vegetation type) Interaction mode Aggressce and cartwheel flights by raptors involve a high risk factor as All hrccders altcu-k birds sometimes injure themselves as they grapple in mid-air Co-breeder male uod female Fly lI10b (and rob) Booted cagle (3 xl and crash into the vegetation below, and fatalities arc known (KRY) Hif'IWlf'tU.~ pennaru.~ in some cases (Simmons & Mendelsohn 1993). Males were Monog

. Fell1u/c attack, mule !llIr preselll is not known which predator species were responsible. Yellow )

9 mongooses (Cyniclis penicillala) were frequently and vio­

0 Monogamous female (KHY) Fly striking Yellnw mongmlse (2 x)

0 lently chased from near nest trees, but are not known to climb 2

Monog

d trees or to prey heavily on birds (Smithers 1983). Caracal e MOIlOPlnlOllS female (KHY) Fly mob Jackal buzzard I t (Felis caracal), large-spotted genet (Gef/ella tigrina) and a d I birds pur~ued 10 Ihe border of the territory African wild cat (Felis lybica), all proficient climbers and (

r hird eaters (Smithers 1983), were observed in the study area. e h Ligon 1987). Groups with helpers that participate in territorial s Since signs of struggle were found in the nests, often with i l dcfem:e may also be more successful in defending such terri­ traces of blood on the nest lining, some nestlings and/or par­ b u tories (Gayou 1986). ents probably resisted predation. Remains of larger nestlings P

e In the peG. there was tirst of all a need to defend territories discovered below nests could have been the result of either­ h t against interspecific intruders. The chasing of predominantly nestlings accidentally falling out of the nest and then being y

b eaten. or their being flushed out or the nest during the preda­

rodent-eating raptors such as blackshouldered kites (Elcmus d tinn act itself. At some nests (n = 5; GM unpuhlished data) e caeru/eus) and jackal buaards (Buteu rufofuscus) to the bor­ t

n nestlings or eggs disappeared without any obvious damage to ders of territories in these vegetation types may have occurred a

r the nest itself. While these losses could have been the result

g because these species share the peG's diet (GM, personal of factors other than predation, avian predators (e.g. martial e

c observation). Blackshouldered kites also bred in the study

n cagle PolemaelUs bellicosus or whitenecked ravens Corvus e area. whereas Egyptian geese (Alopochen aegypliacus) fre­ c albicotlis) or snakes (boomslang Dispholidus typus) could i l

quently utilized nests after occupation by PCGs (GM, per­

r have been responsible. The majority of incidents of predation e sonal observation). Interspecitic interactions were recorded in

d occurred in this study during the first three weeks of the nes­ n more or less equal proportions in Succulent Karoo and Kar­ tling period, when females were most attentive (see below). u mid Broken Veld. Intras'pecitic interactions, however. were y Evidently, some nest predators were sufficiently large and/or a recorded almost exclusively in Karroid Broken Veld. It there­

w skilful to prey on nestlings. despite the defensive efforts of the e t fore appears that not only resources such as prey and nesting female. a habitat. hut also the possibility of establishing a territory and G Because predation was only recorded if both offspring dis­ t e hreeding, was of particular importance to peGs. There was appeared from the nest (ruling out hunger-induced sihling n i thus not only potential benefits to be gained from residing in aggression) and disturbance to nest lining and structure was b a high-quality Karroid Broken Veld, but also a cost as this habi­ noted, we may have underestimated predation. Since hoth S

y tat had to be defended, not only against intcrspecifics, hut in high predation years, 1988 and 1992, were preceded by years b partIcular ngainsl intraspecifics. of helow average rainfall (1987 = 103 rnm; 1991 = 121 mm; d e c u d o r p e R S. Afc J. Zool. 1996,31(4) 175

annual mean = 199 mm), we suggest that potential predators prey-provisioning rate of either one or two males, as well as may have switched to 'alternative' prey, such as PCG eggs the selection of prey by potential nest predators, the abun­ and nestlings, in years when availability of more traditional dance of otomyinid rodents may playa bigger role in ensur­ prey (assumed to be rodents) was depressed. A similar mech­ ing successful reproduction of peGs than the help provided anism has been put forward to explain predation at the nests by the co-breeder per !ie. Nevertheless, we suggest that the of arctic waders, which vary annually in response to fluctua­ participation of subordinate co-breeders in the defence of nest tions in lemming abundance. Predators such as Arctic foxes sites may hold fitness benefits, not only for the co-breeder. (A/apex lagupus) switch to wader eggs and nestlings in poor but also for the dominant breeder. The lower nest predation lemming years (Underhill, Prys-Jones, Syroechkovski, and the resulting increase in reproductive success of polyan­ Groen, Karpov, Lappa, Van Roomen, Rybkin, Schekkerman, drous trios may hold fitness benefits for all three polyandrous Spiekman & Summers 1993). Nest guarding by PCGs may breeders. therefore be in greater demand in years of intermediate (e.g. 1988) and even low prey abundance (Ligon & Ligon 1990), Acknowledgements when potential predators switch to alternative prey and peGs, This research was supported in part by grants from the Foun­ especially nest-guarding females, are more likely to leave the dation for Research Development (M.Sc. bursary to GM), the nesting site to hunt. Frank M. Chapman Memorial Fund (American Museum of Natural History), the Bob Blundell Memorial Scholarship, the Nest-site defence Leslie Brown Memorial Grant, the University of Cape Town During the nestling period raptors generally practice biparen­ Equipment and Research Committee and the Percy Fitz­ tal care; males provide the prey 10 the female and nestlings, Patrick Institute. We thank Marne du Plessis and an anony­ while the females, that are larger, remain at the nesting site mous referee for valuable comments on an earlier draft. We and protect the young (Newton 1979; Ward & Kennedy also thank the landowners at Calitzdorp for access to their 1996). In peGs, polyandrous females were more attentive at properties. nesting sites than monogamous females, and since the nests of polyandrous trios were not subjected to predation, the nest­ References site presence of polyandrous females may have resulted in ACOCKS, J.p.H. 1988. Veld types of South Africa. Mem. Bot. Swv. superior nest defence. If helpers are present at the nesting site, 5. Air. No. 57. they may contribute to nest defence by either guarding, solic­ ALEXANDER, RD. 1974. The evolution of social behavior. Ann. iting help or physically attacking potential predators Rev. Eco/. 5)'51. 5: 325-383. BEKOFF, M. & WELLS. M.C. 1982. Behavioural ecology of . (McGowan & Woolfenden 1989; Zahavi 1990). In peGs, it ) coyotes: social organization, rearing palterns. space u.~e. and 9 was the female that acted as a sentinel, solicited help and 0 resource defence. 1. Tierpsychol. 60: 281~3()5. 0 physically attacked potential nest predators. Co-breeders may 2

BROWN, l.L, DOW, D.D., BROWN, E.R .. BROWN, S.D. 1978.

d have helped by assisting in nest defence when solicited by the

e Effects of helpers on feeding of nestlings in the grey-crowned t female. This strategy was clearly demonstrated by one poly­ a babbler (Pamatostomus temporali.~). Rehav. t.·wl. Sociohio/. 4: d androus trio. Co-breeders also participated in nest defence ( 43-59.

r when present at the nesting site. e CRAIG, I.L. 1984. Are communal pukeko caught in the prisoner's h s To meet the energetic needs of the growing young in the lat­ dilemma? Behav. Ecol. Sociobiol. 14: 147~150. i l & b ter parts of the nestling period, raptor females are forced to CRAIG, J.L. JAMIESON, I.G. 1990. Pukcko: different u leave the nesting site to hunt (Newton 1979). The female approaches and some different answers. In: Cooperative breeding P

e therefore faces a potential conflict between foraging for the in birds, (cds.) PB. Stacey & W.O. Koenig. Cambridge University h

t Press, Cambridge.

young versus guarding and defending the nestlings (Martin y DU PLESSIS, M.A. 1991. The role of helpers in feeding chicks of b 1992). The nest-site presence of the peG female may be cooperatively breeding green (red-billed) woodhoopoes. Rehav. d attrihuted to the prey abundance and therefore the rate at e t Eeo/. 5ociobio/. 28: 291-295.

n which prey were provisioned to her, as well as the number of

a EMLEN. S.T. 1990. White-fronted bee-caters: helping in a r males provisioning that in turn releases females of hunting g colonially nesting species. In: Cooperative breeding in birds,

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