Daylenght and time allocation in relation to reproductive effort in the Common Treecreeper Certhia familiaris

Abstract

The influence of daylength was studied by seeking answers to the following questions: 1) How does the feeding rate of the Treecreeper relate to the seasonally changing daylength? 2) What are the effects of brood size and nestling demand on the rates of feeding by parents? 3) How do adult Treecreepers allocate their time? Parents fed the nestlings for 89% of the time between sunrise and sunset and the feeding activity varied significantly with the daylength. The feeding rate showed a clear correlation with nestling age. Brood size did not correlate with the number of visits per hour, but a correlation wasfound with the number of visits perday. This suggests that adult birdscannot increase the hourly feedingrate, but, as daylength increases, they can increase the number of visits per day and, in consequence, feed a larger brood. The third question was examined using a time budget study. The foraging time increased with nestling age, reaching 89% of the active time for older nestlings. The results support the conclusion that daylength has some importance as a determinant of the feeding capacity of Treecreepers, together with such factors as the food supply and ambient temperature.