PROPERTIES OF FOOD LOADS DELIVERED TO NESTLINGS IN THE PIED FLYCATCHER - EFFECTS OF CLUTCH SIZE MANIPULATION, YEAR, AND SEX

Experimental manipulation of the number of altricial offspring is supp osed to modify parental expenditure in birds. In addition to the obser ved increase in parental feeding rate, it is also possible that the ch oice of prey or the size of load may change with the changing demand f or food. Sexual differences in the provisioning response are also expe cted, on the basis of earlier studies. We examined the effect of brood size manipulation on choice of prey brought to nestlings and load siz e in the pied flycatcher. The composition and size of loads differed b etween years, possibly depending on varying availability of different prey types. Males responded to brood size enlargement by gathering hea vier loads, whereas females showed no response. The alteration of load size in males was not explained by a larger number of prey items or m ean prey size, but was a combination of these components. It is likely that males also increased their work rate in response to increased fo od demand at the nest. The absence of response in females might be bec ause they are unable to increase work rate any further, or because foo d delivery rate in females can not be optimized by changing load prope rties.