WEIGHT CHANGE AND THE BODY RESERVES OF FEMALE BLUE TITS, PARUS-CAERULEUS, DURING THE BREEDING-SEASON

A female blue tit lays a large clutch that is commonly heavier than he r own body weight. Body reserves cannot provide for the entire clutch and little is known of the importance of these reserves during the bre eding season. This study reports on weight changes in adult female blu e tits during the breeding season, observed by the use of electronic b alances. In addition, data from dissecting 67 adult female blue tits, at different stages of the breeding cycle, were used to investigate th e changes in body weight. There was a rapid increase in body weight du ring a period of 4-5 days before the first egg was laid, which was exp lained mostly by the development of the oviduct and the production of an egg on the final day. Body weight remained constant during the layi ng period, before decreasing by approximately one gram after clutch co mpletion owing to the absence of an egg and the absorption of the ovid uct. Body reserves increased during the day after clutch completion, w ere maintained throughout the incubation period and were mobilized dur ing the first Eve days of the nestling period. Breeding female blue ti ts appear to deposit small body reserves to act as a short-term buffer against adverse conditions or to support the brooding phase.