Variable parental responses to changes in offspring demand have implications for life history theory

摘要

Parental care, a component of reproductive effort, should evolve in response to its impact on both offspring and parent fitness. If so, manipulations in brood value should shift levels of care in predictable ways, provided that appropriate cues about the change in offspring value are altered. Prior brood size manipulations in birds have produced considerable variation in responses that have not been fully investigated. We conducted paired, short-term (2 h) reductions and enlargements in brood size (+/- 2 nestlings) of house sparrows in each of 4 years. Parents at reduced broods shifted parental care downward in all four seasons. Parents experiencing increased broods responded significantly variably across years; in some, they increased care, but in others, they decreased care compared with control periods. Nestlings in both treatments gained less mass than during control sessions, with year producing variable effects. We found evidence that parents experiencing reduced broods behave as if recurring predation is a risk, but we found no evidence that parents with enlarged broods were responding to inappropriate cues. Instead, parent sparrows may be behaving prudently and avoid costs of reproduction when faced with either broods that are too small or too large. We modified a published model of optimal care, mimicked our empirical manipulation, and found that the model replicated our results provided cost and benefit curves were of a particular shape. Variation in ecology among years might affect the exact nature of the relationship between care and either current or residual reproductive value. Other data from the study population support this conclusion.