Brood reduction and siblicide in black-billed magpies (pica pica)

Abstract

In many avian species, brood reduction is considered to be adaptive and may be attributed either to sibling competition (passive starvation, active sibling aggression) or parental effects (manipulation of hatching spread, active infanticide). However, nonadaptive factors such as environmental effects, may contribute substantially to nestling mortality. I determined mechanisms of brood reduction and survival probabilities of nestling Black-billed Magpies (Pica pica) in relation to intraclutch egg-size variation, brood size, nestling age and size, and weather. Median brood survival time was weakly, but positively, correlated with intraclutch egg-mass variation. Starvation accounted for most nestling mortality within the first 9 days posthatching, although 12 dead and moribund nestlings were found badly bruised around the head. Siblicide and sibling cannibalism were observed in two broods and implicated in the deaths of nestlings in three other broods. Siblicidal events occurred when nestlings were between 15 and 20 days old. Expected median survival times and probability of survival to fledging of nestlings was 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 nestlings were smaller and lighter than surviving siblings at any age; however, asymptotic body mass and linear measurements were negatively correlated with brood size at fledging. The probability of mortality was affected by prevailing weather conditions; rain and low temperatures doubled the estimated risk of death for young nestlings, independent of brood size. These data suggest that factors influencing the occurrence and maintenance of brood-reduction strategies in a population may be more complex than previously thought. Brood-reduction mechanisms are affected by the interaction of biotic and abiotic factors, and may vary in response to factors outside of either parental or offspring control.