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

Abstract: 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. Significance statement: Parent animals often adjust their levels of care in response to manipulations of offspring value, but considerable variation in these responses exists. This suggests either a mismatch between manipulation and natural cues or undetected subtleties in the fitness consequences of care. Over 4 years, we conducted manipulations of offspring number in the biparental house sparrow (Passer domesticus). We found little evidence that parents misinterpreted cues regarding the change in number, but they behaved differently depending on the year of the manipulation. A model recovered the observed patterns if a parameter influencing the curve relating offspring fitness to levels of care was altered. This parameter should vary with food supply, and our data suggested that this varied in the years of our study. Our results emphasize that predictions about patterns of parental care are risky without attending to the shapes of fitness curves and that some organisms may be particularly sensitive to food supply.