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, chicks that are begging more are fed more. When food is scarce, parents instead feed larger offspring. This change could be due to offspring adjusting their behavior, or to confounding factors not directly related to current food availability, such as brood size. Alternatively, it could be due to parents responding to signals differently based on their experience of food availability in the recent past. We tested these competing explanations experimentally by manipulating food availability in wild great tits, Parus major. We then standardized food availability, and manipulated offspring size and behavior by creating mixed cross-fostered broods just before filming. This isolated the effect of parental strategies while holding food availability and offspring begging and size constant across treatments. We found that when parents received supplemented food prior to filming, they were: (1) more likely to preferentially feed chicks that were begging more; and (2) less likely to preferentially feed larger chicks. Chicks, conversely, did not differ in their begging in relation to prior environmental conditions, 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 parents can flexibly adjust how they respond to offspring signals and cues in response to food availability. Consequently, different signaling systems and parental plasticity are favored depending on environmental conditions.