Adaptive parental feeding as a factor influencing the reproductive rate in the grey starling

Abstract

Summary Adaptive parental feeding of chicks is one of the factors influencing the reproductive rate of a local population. The food resources in the rural and urban colonies in Tokyo were entirely different as proved by collar experiments of the chicks. In the rural habitat the mole‐cricket of fair size (1 g) and of a high nutritive value was the ‘key food' to all broods and only a few other items were added for larger broods. In the urban habitat the food consisted of both animal and plant (fruits) items of various kinds, but the animal matter was mostly small and of poor nutritive value, and fruits are much less nutritive than animal matters as experimentally proved with captive chicks by using cherries which are the most abundant and favoured fruit. Thus the food preference and the feeding ability of parents had more important effect upon growth rate and flying success of chicks in the urban colony than in the rural, especially in larger broods. In the rural colony, the adaptive reaction of parents to a large brood size (experimentally increased) was evident in a pair which adequately switched the normal food, the secretive mole‐cricket, to an easily obtainable kind, the small white pupae and caterpillars. Such focussed or concentrated foraging was also shown to some other food items to be found in clustered condition and selection for large size was also suggested, since such large foods as the gecko or lizard were brought though they were disgorged by chicks in some cases. Subject to various factors, there is the maximum possible feeding frequency for the parents and therefore, however hard the parents might work, there were limits of brood size they could successfully raise. Such limits were 6 chicks for some parents or 7 for others, and a single parent could raise not more than 3 chicks. Thus in the grey starling the broods of 5–6 chicks are of the most efficient size in reproductive rate and are most common, though are subject to difference of local food situation.