The secondary adjustment of clutch size in red-winged blackbirds (Agelaiusphoeniceus)

More than a half century ago, the British ornithologist David Lack suggeste d that parent birds may use brood reduction to track uncertain food, a proc ess facilitated by the asynchronous hatching of their young. Lack sketched the logic of asymmetric sibling rivalry: the phenotypic handicap imposed up on last-hatched marginal offspring renders their growth and survival condit ional upon uncertain ecological conditions while buffering first-hatched co re offspring from the inimical effects of overcrowding during periods of st ringency. Though subjected to numerous indirect tests in short-term studies , the central prediction of Lack's hypothesis - that parents use marginal o ffspring to track unpredictable brood-rearing conditions and thus achieve a secondary adjustment of clutch size - has never been tested directly. Here we present the results of a 7-year study of marsh-nesting red-winged black birds (Agelaius phoeniceus) showing that (1) brood size tracks interannual variability in growth and survival of nestlings, (2) the growth and mortali ty of marginal but not core offspring is contingent upon stochastic environ mental conditions (mean air temperature) during brood rearing, (3) the mort ality of marginal but not core offspring is strongly affected by developmen tal uncertainty in the form of both experimental and natural alterations of brood size, (4) the phenotypic handicap of hatching asynchrony buffers cor e offspring from poor growth conditions, but (5) its effects upon marginal nestlings are reversible when growth conditions are favourable and especial ly when brood size is reduced either experimentally or via hatching failure . The presence of marginal offspring ensures that blackbird parents are not left with a too small brood when brood-rearing conditions are favourable. Parents create two castes of progeny: marginal offspring that are strongly affected by both ecological and developmental stochasticity, and core offsp ring that are not.