Do old great tits forego breeding

There is considerable evidence that young or inexperienced birds are less effective parents than older individuals, but only limited information is available on the reproductive performance of older birds, especially in short-lived species. In Great Tits (Parus major) territory size increases with age, reaching a maximum at 4 yr and then decreasing in birds of 5 yr or older (Dhondt 1971). In some cases old Great Tit males (5 yr or more) do not defend a territory or raise young in a first brood, but pair with a female to raise young in her second brood of the season. Perrins and Moss (1974) found that in Great Tit females "The birds of 5 or more years of age seem to be becoming less efficient at breeding, rather than curtailing their breeding effort. " They showed, in particular, that older females lay later, have a lower hatching rate, and produce fledglings with a lower chance of survival. The birds of 5-7 yr did somewhat poorer than 1-yr-old females. Webber (1975) showed that at Oxford the annual survival rates of Great Tits increased gradually up to the age of 4 yr and then decreased steadily for birds of 5 or more years. This suggests that all old females will breed, but less efficiently than when younger, and that older males occasionally will miss a reproductive season. It is generally extremely difficult to document nonbreeding in small passerines. In the populations studied at Ghent (Belgium), however, a large proportion of Great Tits use nest boxes for roosting in winter, so that evidence for the presence of birds can be obtained from routine nightly visits to the nest boxes. I present evidence here that Great Tits of both sexes sometimes forego breeding once they reach 5 years of age. I also verify for the Ghent populations the extent of nonbreeding among first-year birds, as has been shown by Bulmer and Perrins (1973). In the course of a long-term study of Great Tits at Ghent begun in 1959, all nestlings were ringed and a variable proportion of breeding adults trapped on the nest. Adults were trapped when entering the nest box to feed 1-2-week-old nestlings. Details on study areas and basic information on the study can be found in Dhondt and Eyckerman (1980). Two evening visits to all nest boxes were made each winter, and roosting birds were trapped. Of the 962 nestlings born between 1959 and 1976 that were recaptured after fledging, 417 were caught at least once on the nest. The analysis presented here used two subsets of these birds. The first data set consisted of 118 birds (74 males, 44 females) from the cohorts 1961-1971 that met two criteria: they were regularly found roosting during winter, and they were trapped on the nest in at least half of the possible years. The first condition gives information on a bird's presence in the study area even if it was not caught on the nest. The second criterion excludes birds that were difficult to trap or were irregular breeders in our nest boxes. The 74 males in this set were trapped 178 times out of a possible 239 (i.e. in 74.5% of the possible instances), and the 44 females were trapped 123 times out of a possible 144 (85.4%); although this difference was statistically significant (G-test, Sokal and Rohlf 1969; G = 6.39, 1 df, P 0.05). Because age does not seem to affect the probability of nest failure, the proportion of individuals actually captured should reflect differences in the proportion of the birds in each age class that breed in the nest