Differential food allocation by male and female great tit, Parus major, parents: are parents or offspring in control?

The distribution of food among altricial bird nestlings is the result of an interaction between parental feeding decisions and scramble competition between nestmates. Both young and parents can potentially be in control of the outcome of this interaction. In great tits, each parent feeds from a fixed location on the nest rim, thereby forcing nestlings to choose between the father's and the mother's location. It was previously found that hungry nestlings approached the female preferentially and were more likely to be fed, appearing as if females showed a stronger preference to feed hungry young than males. However, nestlings were free to move in that study, and the effects of nestling positioning could not be disentangled from those of parental food allocation decisions. Here, we experimentally divided broods into two halves and randomly assigned each half of the brood to one side of the nest cup where only one parent could feed them. One nestling in each half-brood was food deprived to manipulate short-term hunger state. Both parents showed a similar preference to feed the more hungry nestlings, suggesting that the previously observed difference was due to offspring positioning rather than active parental choice. Our study shows that food allocation is partially under nestling control and suggests that nestlings adjust positioning and begging behaviour to the profitability of a given position in the nest cup.