Parental control: ecology drives plasticity in parental response to offspring signals.

Birds differ in their parent-offspring interactions, and these differences may be caused by environmental variation. When food is plentiful, the chicks that are begging the most are fed the most. When food is scarce, parents instead feed the largest offspring. This change could due to offspring adjusting their behaviour, or to confounding factors not directly related to current food availability, such as brood size. Alternatively, it could equally be due to parents responding to signals differently based on their experience of food availability in the recent past, for example, over the past weeks. We tested between these competing explanations experimentally, by manipulating food availability in a population of wild great tits, Parus major. We then standardised food availability, and manipulated offspring size and behaviour by creating mixed cross-fostered broods just before filming. This isolated the effect of parental strategies while holding food availability, offspring begging and size constant across treatments, but with sufficient variation within broods to generate usable information for parents. We found that when parents had experienced plentiful, supplemented food prior to filming, they were: (1) more likely to preferentially feed the chicks that were begging the most; and (2) less likely to preferentially feed larger chicks. Chicks, on the other hand, did not differ in their behavior in relation to the environmental conditions they had experienced previously, but instead begged in relation to their immediate feeding history and their nestmates' begging intensity. Overall, our results suggest that parents have more control over food distribution than suggested by scramble competition models, and that they can flexibly adjust how they respond to both offspring signals and cues of offspring quality in response to food availability. Consequently, different signalling systems are favoured depending on environmental conditions and predictability and parental plasticity.