The effect of fledging mass on the lives of Great Tits Parus major

Studies of ringed Great Tits Parus major have shown that the mass of fledgl ings has a major impact on their futures. Heavier chicks an more likely to survive to breed and birds which are larger at fledging remain larger birds for the whole of their lives. The mass at fledging does not affect the bre eding characteristics of males, but lighter females lay slightly earlier th an heavier ones, while heavier ones lay slightly larger clutches. In years with low breeding numbers (when presumably there is low competition for ter ritories) the new territory holders tend to have been heavy as fledglings, while in years with high breeding numbers (when presumably there is high co mpetition for territories), more chicks of lower mass recruit into the bree ding population. Hence, it does not look as if the number of new breeders i s related, at least in any simple way, to the numbers of fledglings of high mass in the previous year.