Abstract
Optimal foraging theory predicts that prey selection by breeding birds is governed by tradeoffs among diverse prey types with different energy gains per unit energy expended. This is particularly so in central-place foragers such as dippers (Cinclidae), which must provision nest-bound young using prey gathered along a linear habitat. In this study, we examined changes in the dietary composition of nestling Brown Dippers Cinclus pallasii over the course of the nestling period in the mountainous Dajia River of central Taiwan. Prey preference was associated with prey morphological traits: fish and trichopterans, with relatively large body sizes, were the preferred prey items, while smaller prey such as ephemeropterans, plecopterans, and dipterans were least preferred. However, the nestling dietary composition significantly shifted over the 1st 1/2 of the nestling period, with the proportion of large prey and daily maximum prey size increasing as the nestlings grew. Our data suggest that the increasing energy demands of the nestlings are responsible for the increase in prey size, while nestling gape-size possibly limited the maximum prey size early in the nestling period. These results bear a striking similarity to data reported for other dipper species, implying identical constraints and strategies in nest provisioning.