The Auk 113(1):189-199, 1996 BROOD REDUCTION AND SIBLICIDE IN BLACK-BILLED MAGPIES (PICA PICA) P. S. REYNOLDS Divisionof BiologicalSciences, University of Montana,Missoula, Montana 59812, USA ABSTR•CT.--Inmany avian species,brood reduction is consideredto be adaptive and may be attributed either to sibling competition (passivestarvation, active sibling aggression)or parentaleffects (manipulation of hatching spread,active infanticide). However, nonadaptive factorssuch as environmentaleffects, may contributesubstantially to nestling mortality. I determinedmechanisms of brood reductionand survivalprobabilities of nestlingBlack-billed Magpies(Pica pica) in relation to intraclutchegg-size variation, brood size, nestlingage and size, and weather. Median brood survival time was weakly, but positively, correlatedwith intraclutchegg-mass variation. Starvationaccounted for most nestlingmortality within the first 9 daysposthatching, although 12 deadand moribund nestlingswere found badly bruised around the head. Siblicide and sibling cannibalismwere observedin two broodsand impli- cated in the deaths of nestlings in three other broods. Siblicidal events occurred when nestlingswere between 15 and 20 daysold. Expectedmedian survival times and probability of survival to fledging of nestlingswas not linearly related to brood size at hatching,being highest for broods of five, lowest for broods of three, and intermediate for broods of four and six.Dead nestlingswere smallerand lighter than survivingsiblings at any age;however, asymptoticbody massand linear measurementswere negativelycorrelated with brood size at fledging.The probabilityof mortality wasaffected by prevailing weatherconditions; rain and low temperaturesdoubled the estimatedrisk of deathfor young nestlings,independent of brood size. Thesedata suggestthat factorsinfluencing the occurrenceand maintenance of brood-reductionstrategies in a populationmay be morecomplex than previouslythought. Brood-reductionmechanisms are affectedby the interactionof biotic and abioticfactors, and may vary in responseto factorsoutside of either parentalor offspringcontrol. Received9 September1994, accepted 25 April 1995. IN MANY SPECIESof birds, brood reduction can 1976,Clark and Wilson 1981,Husby 1986,Mock be attributed to starvationof the youngestor 1994). In particular, hatching asynchronywas weakest nestlings in a brood; this may be re- interpretedby Lack (1954, 1968)as an adaptive garded as a type of passivesibling competition mechanismfor producing marginal offspring. (Lack 1954, 1968, Ricklefs 1965, Clark and Wil- Because hatch order determines size differences son 1981,Mock 1994).Brood reduction also may between siblings, and the relative size differ- result from direct sibling aggression,in which encesbetween young in a brood contribute to "marginal" young are killed by siblings(Mey- competitive outcome, this leads to the second burg 1974, Edwards and Collopy 1983, Mock assumption,namely, that mechanismsof brood 1985,1994, Stanback and Koenig 1992).Parental reduction should be directed towards the small- manipulation of hatching spread (hatching est and weakest brood members.Finally, food asynchrony)may influence both the occurrence supply is assumedto be the critical factor lim- and outcomeof sibling competition by gener- iting nestling survival. Sibling competition is ating competitively--disadvantagedoffspring expectedto increasein importancewhen there (Lack1954, 1968, Mock 1994);additionally, par- are major discrepanciesbetween food supply entsmay directly manipulatebrood size by kill- and offspringdemand. Total offspringdemand ing young (Stanbackand Koenig 1992). will increaseboth with brood size and nestling There are at least three basicassumptions in- age; thus, becauseof the relative increaseof herentto manymodels of broodreduction. First, potential competitors,nestlings in large broods both brood reductionand hatching asynchrony may be proportionately more restricted in ac- are assumed to be adaptive, inasmuch as the cess to food than those in smaller broods. decrease of the brood to a size which can be fed Becauseadaptive explanationsfor brood-re- results in the maximization of the number of duction mechanisms have received most atten- viable young fledged (Lack 1954, 1968, Howe tion, the effectsof nonadaptivefactors on brood 189 190 ?. s. REYNOLDS [Auk,Vol. 113 loss have rarely been considered. However, METHODS breedingsuccess and nestlingsurvival for many I studiedbreeding Black-billedMagpies in Missou- birds are strongly affected by environmental la County, Montana (46ø55'N 114ø6'W,elevation 973 factors(Murphy 1985).Extremes in temperature m), from late March to late June 1994. I measured and rainfall will significantly impact nesting length and width of 118eggs in 25 completeclutches successof many speciesof birds by depressing to the nearest0.01 mm with digital calipers.I obtained insectavailability, therefore affecting the major morphometricdata for a total of 110 nestlingsfrom food sourcefor nestlings and curtailing avail- 22 broodsevery two days from hatch to fledging or able foraging time (Lack 1954). Ambient con- death/disappearance.Nestlings were individually ditionsalso may affectnestlings directly. When marked with nontoxic permanent marker on the un- ambienttemperatures are low, young nestlings dersideof the manusand tarsusuntil old enoughto color band. I measuredlength of head, culmen,ma- may show thermoregulatorycompromise and rius, and tarsus,to the nearest0.01 mm with digital chilling independent of brood size (Hill and calipers;body masswas measuredwith Pesolascales. Beaver1982); very high-temperatureand solar- Whenever possible,dead young were collected,mea- radiationloads also may be implicatedin deaths suredand examinedfor injuries.When nestlingsdis- of exposednestlings (Murphy 1985). Thus, the appearedbetween censusdays, I used data obtained incidenceand timing of brood-reductionmech- during the last censusday young were seenalive in anismsmay be the result of nonadaptivecon- comparisonswith surviving young on that day. I ob- straintson individuals,rather than purely adap- taineddaily weatherdata from the National Weather tive responsesto current conditions. Service, Missoula; the weather station was within a radiusof 2 km of the study sites. In this paper, ! present data on patternsand Mortality.--I examined the effectsof biotic and abi- probability of nestling mortality for the Black- otic factorson nestlinglifetime using survival anal- billed Magpie (Picapica). Magpie clutch size is ysis.The key featureof survivalanalysis which dis- unusually large in comparisonto most corv- tinguishesit from other typesof statisticalanalysis is ids--clutchesgenerally average six to eight eggs, that it handles "censored"data. Censoring occurs in contrastto the three to five egg clutch char- when certain individuals cannot be observed for their acteristicof many corvidspecies (Goodwin 1986, entire lifetime, and the event of interest(in this case, Birkhead1991). However, magpiesrarely fledge death) has not occurred within the duration of the more than two to four young (HSgstedt1981, study;thus, exactsurvival times are known only for Goodwin 1986, Buitron 1988, Birkhead 1991). a certainsubset of the studypopulation. Survival dis- I examined the combined effects of three bio- tributionsand instantaneousprobability of deathwere modelled by survival functions S(t) and the cumu- logical factors (nestling age, brood size, and lative hazard function h(t); thesefunctions are related within-brood nestling-size differences) and to each other by h(t) = -log S(t) (Lawless1982). severalweather variableson the probabilities I usedthe Kaplan-Meier, or product-limit, survival and distribution of nestling survival over the estimator (Kaplan and Meier 1958) to estimate the nesting period. I compared the relative influ- cumulativesurvival probabilitiesof nestlingsfrom ence of these factors on the incidence of two different brood sizesat hatch. The Kaplan-Meier es- major types of brood-reductionmechanisms-- timator is given as: starvation and siblicide. Starvation has been demonstratedexperimentally to be a majorfac- õ(t)=h n,-d,, (1) tor in magpienestling mortality (HSgstedt 1981, Hochachkaand Boag1987); however, siblicide where n• is the number of nestlingsalive at the be- ginning of a given time interval (specifiedas t•_• to and sibling cannibalismhave not been previ- t•),and di is the numberof nestlingsthat died during ouslyreported for magpies(or, in fact,for any that interval. The standard error of S(t) is: other passerinespecies). The occurrenceof both brood-reductionmechanisms in the samepop- SE[g(t)]= S(t) ((d,)/[n,(n,- d,)])0.s (2) ulation, and even within a single nest,suggests that factorsinfluencing the both the incidence and the timing of specific brood-reduction (Kaplan and Meier 1958, Lawless 1982, White and Garrott 1990). The median survival time is the first mechanismsmay be more complexthan pre- observed time when the cumulative survival is 50% viously thought. Brood-reductionmechanisms or less. are affectedby the interactionof biotic and abi- I usedpairwise log-rank tests(Lawless 1982) to test otic factors,and may vary in responseto factors the null hypothesisthat survival did not differ be- outsideof either parentalor offspringcontrol. tween sizesof brood at hatching.The test statisticis January1996] MagpieBrood Reduction 191 selectedfor in magpiesand other speciescharacter- U= • [do•,(i)- d,xp(i)], (3) ized by large clutchsizes, hatching asynchrony, and broodreduction