Nest Insulating Capacity during Incubation and after Fledging are Related

Abstract

Most birds build nests to hold eggs and nestlings. An important property of nests is their ability to keep eggs and nestlings at an optimum temperature. This is usually measured as the insulating capacity (IC); nests with a higher IC will keep their content warm for longer. The usual protocol to estimate IC involves collecting nests after fledging of the young. However, nest properties change throughout the nesting period, potentially affecting IC. Therefore, a relevant question is whether the nest IC, measured after fledging, actually reflects its IC during incubation and early nestling development, when it is most crucial. In April 2015, we collected 18 Great Tit (Parus major) nests 3-4 days after incubation had started, measured their fresh mass and IC, and returned them to the nestboxes. We collected these same nests again after fledging, and measured their IC and fresh and dry masses. IC tended to be lower after fledging than during incubation, but both measures were correlated. Fresh nest mass was higher after fledging than during incubation, and dry nest mass was correlated to both fresh masses. Nest mass gained between incubation and fledging was positively related to clutch size. As expected, nest mass increased throughout the nesting period, but the deterioration of nest structure as the nestlings grew up tended to negatively affect its IC. However, the correlation between IC in the two nesting stages allows the use of post-fledging nests for comparative studies of nest insulating capacity.