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 (2h) 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 to control periods. Nestlings in both treatments gained less mass than during control sessions, with year producing variable effects in enlarged broods. 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.

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Parents provisioning their offspring can adopt different tactics to meet increases in offspring demand. In this study, we experimentally manipulated brood demand in free living great tits (Parus major) via brood size manipulations and compared the tactics adopted by parents in 2 successive years (2010 and 2011) with very different ecological conditions. In 2011, temperatures were warmer, there were fewer days with precipitation, and caterpillars (the preferred prey of great tits) made up a significantly larger proportion of the diet. In this “good” year, parents responded to experimental increases in brood demand by decreasing mean inter-visit intervals (IVIs) and reducing prey selectivity, which produced equal average long-term delivery of food to nestlings across the brood size treatments. In 2010, there was no evidence for effects of brood size manipulations on mean IVIs or prey selectivity. Consequently, nestlings from enlarged broods experienced significantly lower long-term average delivery rates compared with nestlings from reduced broods. In this “bad” year, parents also exhibited changes in the variance in inter-visit intervals (IVIs) as a function of treatment that were consistent with variance-sensitive foraging theory: variance in IVIs tended to be lowest for reduced broods and highest for enlarged broods. Importantly, this pattern differed significantly from that observed in the “good” year. We therefore found some support for variance-sensitive provisioning in the year with more challenging ecological conditions. Taken together, our results show that variation in brood demand can result in markedly different parental foraging tactics depending on ecological conditions.

Differences between individuals in correlated responses across contexts have both functional and mechanistic implications. Such syndromes may have either beneficial or harmful consequences when novel changes in the environment occur. We used wild-caught house sparrows, Passer domesticus, to test in functionally relevant circumstances, whether neophobia (initial fear of novelty), habituation, and the learning of novel cues (discriminant learning) were linked by a common underlying mechanism or reflected separate processes. We repeatedly measured individual latencies to approach and also to feed from a familiar feeding site in three contexts: a baseline control for mild disturbance, in the presence of a novel object, and to a novel cue indicating hidden food. House sparrows on average exhibited neophobia, habituated to novel objects, and learned to associate new cues to a reward. We also found evidence for consistent individual differences in in both latencies within most contexts, but there was no evidence of individual differences in plasticity with repeated trials within either the novel object (habituation) or novel cue (learning) contexts. There was also little or no correlation between the two latencies within individuals within contexts. Individual differences in latencies to arrive at the food station exhibited strong correlations across contexts, but latencies to feed were weaker. These results suggest a personality trait that exists regardless of novelty, but no syndrome affecting reactions to different forms of novelty. House sparrows appear strongly plastic when responding to novel environments. Such plasticity is likely favored by the varied consequences of novelty across environments.

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