Feeding strategies in great tit (Parus major): Do parents adjust prey size to nestlings' size?

Abstract

The feeding behavior is favored by natural selection in the past and is based on getting a high fitness by maximizing net energy intake. Reproductive success among altricial birds is closely related to feeding strategies because nestlings are totally dependent on being fed by their parents from the day they hatch to the day they fledge. We have studied the patterns of feeding behavior in great tit parents and their ability to adapt to the current feeding situation by adjusting prey size to nestlings size. This has been an experimental study of parental care by great tits of 10-14 days old great tit and blue tit nestlings in their natural environment. We recorded the feeding behavior inside 30 nest boxes that were divided into three groups. Group great tit was an unmanipulated control consisting of the original great tit nestlings, Group blue tit consisted of only blue tit nestlings and Group mixed consisted of one original great tit nestling and the rest of the nestlings replaced with blue tit nestlings. We found that the ability to adjust prey size would be an important skill in tits reproduction ecology because large prey increases the swallow time for all the nestlings which in time increases the feeding time for the parent. We assumed that the smaller body size of the blue tit nestlings would imply a more constrained capacity to swallow larger prey items so that prey size should have more critical consequences for blue tit than for great tit nestlings. However we did not find any indication of this since the blue tit nestlings were fed with prey of approximately same size as the great tit nestlings and that the proportion of long swallow times did not increase. We discuss possible reasons for the surprising result.