Ornis Hungarica. vol.7. (1997) p.1-17.
Weight change patterns in breeding Great Tits (Parus major)
Between 1993 and 1997 we weighed 12 Great Tit nests automatically using electronic balances for periods of 6-47 days during the breeding seasons. We used "The Wisitor" software package for collecting and processing the data. In this paper we publish the results of the analyses of the day to day changes of morning and evening body weights of females and males. 1. None of the 12 males lost or gained weight during the observation periods. 2. Those two females whose weight was measured front the beginning of egg-laying until fledging had three different weight levels: average roosting weight during egg laying, during incubation and during late feeding differed. None of these females lost weight during egg laying, but one female's egg weight decreased. The females' morning weight levels were similar at egg laying and incubation. 3. Each female maintained her weight through incubation (N = 4). Females lost weight (10% on average, N = 4) after hatching, in the brooding period. Ten females maintained their body weight, while two others lost weight in the late feeding period. 4. We did not find any significant difference either in the average weight gained per hour dur- ing a day or in the average weight lost per hour during a night between the four periods of breeding (N = 5 females). Based on other published data and on our own results we think that a substantial gain in weight before egg laying and some %%,eight loss of females after hatching is a general phenomenon among Passerines. The substantial increase in body weight just before laying and its mainte- nance until the brooding period can partly be explained by the high energetic demands of lay- ing and incubation. Reserves can serve as buffers against unfavourable conditions during these periods. Flight cost of high body weight makes weight loss before the peak feeding pe- riod adaptive but the exact timing and amount of this weight loss may depend on the actual en- ergetic demands of brooding. Thus, optimal clutch size may depend on the amoimt of reserves a female can mobilise right after incubation.