Breeding Seasonality and Reproductive Success of White-Fronted Bee-Eaters in Kenya
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BREEDING SEASONALITY AND REPRODUCTIVE SUCCESS OF WHITE-FRONTED BEE-EATERS IN KENYA PETER H. WREGE1'2 AND STEPHEN T. EMLEN 1'2 •Sectionof Neurobiologyand Behavior, Cornell University, Ithaca, New York15853 USA, and 2NationalMuseums of Kenya,Nairobi, Kenya ABSTR•CT.--White-frontedBee-eaters (Merops bullockoides) at Nakuru, Kenya, reproduce in an environmentwhere foodsupply for provisioningnestlings is unpredictableand generally scarce.With data from 8 yr of study on three populationsof bee-eaters,we examinedhow these environmental conditionshave influenced the timing of breeding and patterns of nesting success. Breedingcoincided with the generalpattern of rainfall in the Rift Valley. Kenyaexperiences two rainy seasonsper year, and individual bee-eatersbred with eitherthe long or the short rains,but not both. Membersof a givenpopulation bred coloniallyand quitesynchronously, but membersof adjacentpopulations were often out of phasewith one another.As a result, the distributionof long- and short-rainsbreeding populations formed a spatialmosaic. Reproductivesuccess was highly variablebut typically low. Mean (+SD) fledging success was 0.57 + 0.83 young, and only one nest in four produced an independent offspring (6 monthsof age). Most prefledgingmortality (88%)was due to egg lossesand the starvation of young. Very little (7%) was attributableto predation.Social factors, primarily intraspecific nest parasitism,were responsiblefor nearly half of all egg losses,and representan important costof group living. Starvation,the singlemost important source of prefledgingmortality, claimedthe lives of 48%of all hatchlings.Starvation was greatestat times of low food availability,in nestswith large broods,and in neststended by pairs without helpers. Hatching was asynchronous.This enhancedthe ability of older nestlingsto monopolize limited food supplies,and resulted in the selectivedeath of the smallestnestling first. Such brood reduction, coupled with the ability of nestlingsto slow their developmentrate in responseto food stress,is consideredan adaptationfor copingwith the unpredictablevari- ation in food supply commonly faced by these birds. Received17 August1990, accepted18 January1991. EVOLUTIONARYprocesses act through vari- of breeding as it relates to the annual pattern. ability in the lifetime reproductive output of We then provide reproductive successstatistics, individuals. An understanding of how selec- quantify sourcesof egg and nestling mortality, tion hasshaped the different componentsof life and discuss the influence of environmental con- histories depends on knowledge of the sources ditions on reproductive output and the evolu- of variability in reproductive success. tion of life history patterns. For 8 yr we have studied the social dynamics and reproductivesuccess of individually marked METHODS White-fronted Bee-eaters(Merops bullockoides) in the central Rift Valley of Kenya. These birds This study was conducted in Lake Nakuru Na- are nonmigratory, cooperativebreeders that live tional Park, Kenya. The bulk of our data come from throughout the year in coloniesof 25-400 in- 8 yr of concentratedstudy of 3 populations:Makalia dividuals. Three populations were studied in- (1977-1979), Badlands (1977-1978), and Baharini tensively, and known individuals were moni- (1980-1984).A fourth population (Enderit) was mon- tored continuously for 3-7 yr (depending on itored in 1977,and thesedata are included in analyses population). Helpers at the nest were a con- of seasonalityand overall reproductive success.Ad- ditional data obtained from the Baharini population spicuousfeature of the social system,and we (1985-1986)as well asother populationsin and around detailed their influence on measuresof repro- the national park are included in someanalyses, as ductive successelsewhere (Emlen and Wrege noted. 1991). White-fronted Bee-eaters are colonial, roosting In this paper we describethe salient features throughout the year, and breeding, in holesexcavated of resourceavailability and examine the timing 1-2 m into vertical earth banks or cliffs. These birds 673 The Auk 108: 673-687. July 1991 674 W•aE ANDEMLEN [Auk, Vol. 108 feed almost exclusively on flying insects,and ex- predatorsat Nakuru to snakesand mongooses,both tended family groups(clans) defend foraging terri- of which are known to consume the entire nest toriesin savannahabitat surrounding the colonysite contentsin one visit. MadagascarHarrier Hawks (for a general descriptionof the socialstructure and (Polyboroidesradiatus) and various small falcons cooperativebreeding behavior, see Emlen 1990).All might take single nestlingsfrom hole entrances, adultswere identified individually by permanentleg but we rarely saw these predatorsat our study bands and plastic wing tags,and were sexedby lap- colonies. arotomywhen in adult plumage. 4. Other eggloss--all lossesof eggs not meeting the criteria above. Mr•SUR•M,wrr oF REPRODUCrIVE SUCCESS 5. Nestlingstarvation--young that disappearedsingly and for which there was prior evidence of mor- Behavioralobservations.--Behavioral aspects of nest- phological retardation of the individual (83% of ing phenology of the breeding colonies were ob- cases) or of nestmates. Often we found the car- served daily (2-5 observationhours per colony).In- cassesof suchnestlings either in the nestchamber formation on nest activity and failure, as well as or discardedbelow the colony. changesin the compositionof groupstending specific 6. Other nestlingloss--losses of nestlingsnot attrib- nests, was obtained from such observations. utable to either starvationor predation. Nestchecks.--We checked all activenests at regular 7. HorusSwifts--occasionally Horus Swifts (Apusho- intervals with a "ripariascope": a tube of adjustable rus)actively took over bee-eaternest chambersby length that contains lenses, lights, and a terminal enteringand pushingout eggsor killing nestlings mirror (Demongand Emlen1975). During egglaying, by puncturing them with their claws. nestswere checkeddaily (Baharini population)or ev- 8. Catastrophiclosses--eggs or young lost to severe ery secondday (Makalia, Badlands,and Enderit pop- environmental conditions (e.g. flooding or cave- ulations).Checks usually beganbefore the first egg, ins causedby heavy rains, rising streamlevels, or with supplementalchecks whenever unusualbehav- both). ior was suspected(e.g. intraspecificnest parasitism). Fledglingsurvival to independence.--Wecalculated Nest contentswere monitored periodically through- survivorshipfrom fledging to independencefor the out incubation but daily throughout the expected Baharini population from monthly censusdata. Al- hatching period. Checkscontinued at intervals of 3- though fledglings reach adult foraging proficiency 5 daysto monitorthe growth and fate of all nestlings. approximately6 weeksafter fledging (2.5 monthsof Beforefledging, eachsurviving nestlingwas removed from its nest hole, banded with a numbered alumi- age), they remain with the family unit and do not molt into the definitive adult plumageuntil 6 months num band, weighed, measured,and returned to the nest. of age. We usedthe plumagecriterion to define full independence.Survivorship was defined as the per- Nestingsuccess.--Actual fledging from the nest was centageof nestlingsthat reachedmorphological age not always observed, primarily becausethe age at 25 that were still alive at 6 monthsof age. fledging varied from a norm of 28-30 daysup to 42 Data from the Makalia and Badlands populations daysfor individuals with slowed development(Em- were not used in survival analyses.From 1978-1979, len et al. in press).Although chronologicalage varied, nestlingswere fitted with wing tagsjust before fledg- all nestlingsthat successfullyfledged first attained ing, and we suspectthat mortality was substantially morphologicalage 25 (i.e. the stageof morphological higher on fledglingswith tagsthan on thosewithout developmenttypically completedin 25 daysby well- (Emlen unpubl. obs.).Consequently, young from the fed nestlings).We thereforeused this morphological criterion as a standardized estimate of successful Baharini population were not given wing tags until approximately6 months of age. fledging. Sourcesof eggand nestling mortality.--All losseswere classifedto presumedcause of mortality accordingto ENVIRONMENTAL MONITORING the following criteria: Insect availability was considereda possiblepre- I. Unhatchedeggs--eggs still present in the nest dictor of reproductive success.Rainfall influenced in- chamberwhen the oldest nestling was 5 days old sect abundance, and local weather conditions (hours (longest span for entire clutchesto hatch was 4 of rain, strength of wind, etc.) affectedboth the flight days). activity of insects and the ability of bee-eatersto 2. Desertion--eggsstill present in the nest chamber capturethem. The availabilty of insectsto foraging when, before full-term incubation, the birds ceased bee-eaterswas therefore representedin analysesby incubation and roosting at that nest. variables of insect abundance, rainfall, and their in- 3. Predation(of eggsor young)--entire nestcontents teraction. that disappearedbetween two successivechecks RainfalL--Monthly total precipitationand number separatedby <6 days. Such losseswere usually of dayswith >0.5 mm of rain were derivedfrom daily detectedwithin 2 days.Hole-nesting limits likely precipitationdata recordedat the national park head- July1991] ReproductiveSuccess inBee-eaters 675 quarters,3 km from the Baharini population (Fig. 1). There was no significantdifference in the amountor