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.