Nestling Weight and Survival in Individual Great Tits (Parus major)

(1) The aim of this paper is to estimate the shape of the curve relating first year survival to nestling weight in individual great tits (Parus major) and to study the causality of this relationship. (2) Data were collected in a mainland and an island population. Nestlings were weighed and sexed in the nest when 2 weeks old. A recapture programme provided data for recapture-rate estimates in the winter. Local survival until next breeding season was estimated by capturing the breeding population. Brood-size manipulation experiments were performed in the mainland population in order to manipulate nestling weights. (3) The relation between local recapture rate and nestling weight was described using logistic regression techniques. The descriptive model included positive weight and negative squared weight regression coefficients, if controlled for year, sex and date. Recapture rate approached zero at weights of c. 70% of the adult body weight. The curves for both populations showed an approximately linear part over a rather long range of weights. At high weights, the curve levelled off in the mainland population and curved down in the island population. (4) Survival from weighing till fledging and recapture rate from fledging till winter were related to nestling weight, but recapture rate from winter till breeding was not. (5) The effect of brood-size manipulation on nestling weight and subsequent recapture rate suggests causality of the recapture rate-nestling weight curve. Additional information from a comparison of the association between recapture rate and nestling weight within and between broods leads to the conclusion that weight does play a causal role in this relationship. Recapture rate-nestling weight curves can thus be estimated from non-experimental data.