A 51-year study of Eurasian Bullfinches Pyrrhula pyrrhula in Treswell Wood (UK)

Capsule Eurasian Bullfinch numbers in Treswell Wood (Nottinghamshire, UK) declined over the 51-year study period, but the 2,065 individuals caught and ringed in these years provided useful information on age and sex ratios, pairing behaviour, various aspects of reproduction, and seasonal changes in body weights.Aim To examine the numbers, ages and sexes, body weights, mate selection, and breeding output of Eurasian Bullfinches in Treswell Wood over a 51-year period.Methods The study involved annual counts of breeding pairs, and regular mist-netting. Each bird caught was ringed and details taken of age, sex, wing length and body weight.Results Eurasian Bullfinch numbers greatly declined over the 51-year period, although fluctuating from year to year. The broad changes recorded within the 48 ha wood paralleled trends from national monitoring programmes. The sex ratio among mist-netted birds showed an excess of males over females, even in winter, presumably reflecting a surplus of males in the population. Only a slight difference in wing length was detected between sexes, and no significant difference in body weights, except in the breeding season when females were significantly heavier and more variable than males. The population contained more adult–adult and yearling–yearling pairs, and fewer mixed-age pairs, than expected from random pairing between age groups. Mating was therefore selective by age, but not apparently by body size (wing length). Although breeding occurred over four to five months each year, the majority of young were produced from nests initiated in the second half of the season, probably due to heavy predation on the contents of earlier nests. Annual productivity reached a peak averaging 4.6 young per adult in September or October, but, probably because of higher mortality of young than adults, this ratio declined to ∼1.6 young per adult by the following breeding season.Conclusion This study provided data on several aspects of Eurasian Bullfinch biology, including population trend, mortality rates, breeding success, sex ratio and assortative mating.