Asymmetric sibling rivalry and nestling growth in red-winged blackbirds (Agelaius phoeniceus)

Many birds hatch their offspring asynchronously, and the adaptive significa nce of this trait, if any, is controversial. David Lack suggested long ago that by facilitating brood reduction when resources are scarce, hatching as ynchrony provides relief from the effects of overcrowding. Some field worke rs interpret this to mean that the growth and survival of survivors should rise following partial brood loss. Here we show in a 6-year study of red-wi nged blackbirds (Agelaius phoeniceus) that the presence or absence of margi nal offspring in experimentally manipulated broods had virtually no effect upon the growth of core offspring, whereas alterations of the size of core brood had strong and significant effects. Nestling growth was, not surprisi ngly, slower in broods with partial brood loss. Intriguingly, marginal offs pring showed significantly greater variation in mass. Core offspring are le ss sensitive to, but not exempt from, the inimical effects of resource shor tfall than are marginal offspring. The phenotypic handicap appears to margi nal offspring a caste of high-valiance progeny whose fitness prospects rest upon levels of parental input (stochastic resources) and the size of the c ore brood (stochastic development).