NOTES on the BREEDING CYCLE of CAPE VULTURES &Lpar
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RAPTOR RESEARCH A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC. Voi. 20 SVMMER 1986 NO. 2 NOTES ON THE BREEDING CYCLE OF CAPE VULTURES (Gypscoprotheres) ALISTAIR S. ROBERTSON ABSTRACT- Observationswere made of the pre-laying,incubation and nestlingperiods of the CapeVulture (Gyps coprotheres)at a colonyin thesouthwestern Cape Province, South Africa. Some members of thecolony were colour-ringed as nestlings;this allowedthe sex of breedingpartners to be determinedby subsequentobservation of copulation. Informationon occupancyof nestsites, nest-building, a sex-related behavioural difference, dates of egg-laying,incuba- tion and nestlingperiods, parasite infestation (Prosimuliium spp.), and associatedblood parasites(Leucocytozoon) of nestlingsis presented. The Cape Vulture (Gypscoprotheres) isthe heaviest All activenest sitesand roostingledges were visiblefrom the endemic accipitrid in southern Africa and is the southeastside of the ravine,300-400 m away.Observations were madewith binoculars and a 15-60xtelescope. Observations began only vulture in the region t(; breedin colonieson in May 1981,continued for 12 d eachmonth regardless of weather cliff faces (Mundy 1982). Approximately 3500 conditions,and ended in May 1982.One "day"represents the time Cape Vulture nestlingshave been colour-ringed period 0730 H - 1630 H. The concludingstages of the 1980 (banded) in southern Africa since 1974; studies of breedingseason, all stagesof the 1981season and the initiationof the 1982 season were covered. Presence of birds at sites and marked birds, however,are mostlyconfined to as- roostingledges was noted every 30 min during eachobservation pects of dispersal and survival (Houston 1974; day;the proportionof eachclass (i.e., one, both or neitheroccup- Piper et al. 1981; Mundy 1982). ant presentat eachcount during an observationday) wasdeter- Here I present results of observationsmade mined and converted to an arcsinc transformation (Schefler during a casestudy of a particularcolony, to sup- 1969).Although most of the breedingpairs were not individually recognizable,some ringed breeders were present. At a nestwhere plementMundy (1982) and provideintbrmation on both birds were colour-ringed,strangers alone at the nest for the behaviourof colour-and metal-ringedbreed- longer than 5 min were seenonly 3 times.Unless shown to the ing adults.Sexes of Gypsare morphologicallyindis- contrary,vultures observed at a sitewere regardedas the occup- tinguishablein the field (Houston 1976; Mundy ants.Individual vultures were age-estimated using the characters 1982), therefbre marked birds provided an op- givenby Mundy (1982).At nestswhere at leastone partner was ringed,sex was determined by observationofcopulation attempts. portunity to investigate behavioural differences Layingdates for 1981 and 1982 were determinedby checking between sexes, as well as nest-site fidelity the colonyregularly, although precise dates tbr 3 nestswere not (Robertson1984). The pre-laying,incubation and obtained in 1982. Dates for 1977, 1979 and 1980 as well as the nestling periods (Newton 1979) are considered datesof layingin both 1981and 1982 at Aasvogelvleiwere esti- matedusing an averageincubation period of 57 d and a known- separately;observations on the post-fiedgingde- age/winglength curve (Mundy 1982).Wing lengthsof nestlings pendenceperiod and copulatorybehaviour are de- measuredbefore 1981were obtained h-om unpublished records; scribedelsewhere (Robertson 1983, 1985). thus,only laying dates of sitesthat producedlarge nestlings were obtained for yearsother than 1981 and 1982. STUDY AREA AND METHODS Nestlingsat both colonieswere colour-ringed,weighed and Observationswere madeat a colonyin the Potberg(34 ø 22' S; measuredas part of the colourringing schemeon Cape Vultures 20ø 33' E), approximately10 km from the Indian Oceancoast and (Ledger 1974). Nestswere visitedon 5 other occasionsin order to approximately50 km northeastof the southerntip of Africa. collect addled eggs or shell fragments (measurementsin About 50 vultures inhabit a ravine of the mountain, and use cliffs Robertson1984) and to replacean egg that an adult had knocked facing southeastand southwestfor nest and roost sites.In addi- from the nestcup. In 1981,nestling blood samples were obtained uon, a smallercolony of about 20 at Aasvogelvlei,120 km from from both colonies.Smears were preparedfollowing Greiner & Potberg,was monitored for numbersand breedingsuccess. Mundy(1979) and scannedfor bloodparasites by M.B. Markus. The regionhas a temperateMediterranean climate and receives mostof its rainfall (• 530 mm/y) from aboutMay to September 0 e., winter). 5 1 R^VTORRES^RCI-I 20(2):5 1-60 52 ALISTAIRS. ROBERTSON VOL. 20, NO. 2 PRE-LAYING INCUBATION NESTLING POST-FLEDGING (16) 100 (?) 80 ÷ (10) I- (5) z 60 • I.Cl 4O 2O , 0 I 2 o I 2 o I I 2 VULTURES AT SITE Figure1: The averagelevel of occupancyof nestsduring different periods of thebreeding cycle. The numberof nests usedin the calculationsfor eachperiod is indicated. birdsbent overand pickedat the nestcontents. The RESULTS nest was examined on 8 October and consisted of a Pre-laying period. -- Becauseof the length of the few sticksand green sprays-- an "intermediate" breedingcycle, certain pairs displayed an overlapof neststructure. The first intruder repulsionat this post-fledgingand pre-laying periods (Robertson sitewas observed on 20 December.This site pro- 1985). Other pairs that failed in breeding retained duceda fledglingin 1982/3,thus it waseffectively their nestsite (by aggressivelychasing off visitors) activatedin mid- 1981,if the parentsare assumedto from the date of failure. be the same individuals. Figure 1 depictsthe average occupancyof 10 A probablemate replacement (certainly the in'iti- nestsfor the 1981 pre-layingperiod. On average, ation of breeding) occurred at one site in 1983, nest sites were left vacant for 5% of the observation wherea colour-ringed,6 y old femalewas observed time. from late December 1982. This site was active in (a) Activation of nest sites. -- Of 17 breeding 1980and 1981(both partners adult and unringed). sitesactive in 1981, 14 were activeagain in 1982.Of The 6 y old'ssex was determined by observationof a 4 additional nests active in 1982, one was active in copulation attempt with an unknown adult at a 1980. Thus only 3 siteswere initiated during the perch on 18 May 1982. observationperiod. (b) Nest-building. -- No signof nest-buildingat At one site on 29 October 1981, 2 birds were seen any site was observedbefore the end of March to stand very closetogether, facing inwards and 1982, and at only 2 sites,successful in the previous pickingat stickson the surface.Seven min later, the cycle,were there any remainsof the neststructure. ringed female occupantlanded at the site and re- By the end of December 1981, nestmaterial on the pulsedboth, whichthen repeatedthis behaviour at sites of all failed breeding attempts had disap- another(vacant) site. The impressionwas of a pair peared. Collection of nest material in earnest was "prospecting"for a possiblefuture breeding site, first observedon 25 April 1982,about 10 wk before about 7 monthsbefore the next seasoh'saverage the averagelaying date. Branchescomprising the laying date. nest structure were collected in most cases from 2 At another site,an immature male (est.3rd y) and separateareas on the ravineslopes and carried back female (est. 4th y) were observedto copulate 36 to the nest in the bill. timesbetween 30June and 30 December1981. On At 6 nests where male and female were distin- most occasionsafter the copulationattempt, both guished,frequency of collectingforays by eithersex SUMMER1986 BREEDINGCYCLE OF CAPEVULTURES 53 wasdetermined. Collecting forays were mainly de- 1980 1981 198'2 termined for 3 nestsonly -- thosesituated on the cliff directlyopposite the observationpoint where the collector'ssex was determined with greater certainty.Of 98 collections,males collected 75 times 2oo (76%), far more than did females,although at one I- nest the difference was not so marked. The female uJ generallypacked the materialinto the structure, (g 0 althoughboth sexeswere often seenbending over uJ andrearranging material. -2oo Incubation period. -- In 1981, 356 nest-d of observationof 16 nestswith eggswere made,and I Jan Feb Mar Apr May Jun made a further 99 nest-dof observationduring an MONTH 8-d consecutiveperiod in 1982. No double clutches(Mundy & Ledger 1975) or Figure 3: Rainfall in the six months preceding egg-lay- ing, expressedas a percentagedeviation above replacementclutches were recorded,although the and below the monthly mean. egg at nest #55 was laid 40 d later than the esti- mated5June mean for that yearand maythus have These siteswere unoccupiedonly 0.09% of this been a replacementclutch. observation period, where an average of 359 For 16 incubation periods in 1981, which in- thirty-min countswere made per nest.(Fig. 1). On cludesone egg lost after 41 d, one adult wasat the average, 2 birds were at the site 2.0% of the time siteat 97.7% of the counts(range 91.2% - 100%). (range 0% - 8.8%). Birds generallyincubated the potberg 13 1982 1979x 1980x 1981 16 1981 All POTBERGYEARS xx AVV1982x 8 1980 -• I 1982x 1981xx 1979 1982x 1981xx 12 1977 •, I 1981xx aasvogelvlei 1982 AVV1981x 1981x AVV1982x 1977x 1979x may 18 24 31 june 14 21 28 jury 12 '19 F•gure2: Datesof egg-layingat Potbergand Aasvogelvlei.The mean(+ s.d.)and rangeare shown,as well as significant differencesin datesof layingbetween the various years (0.01 level:yearXX; 0.05 level:yearX). 54 ALISTAIR