AGE AND FORAGING SUCCESS IN EUROPEAN BLACKBIRDS - VARIATION BETWEEN AND WITHIN INDIVIDUALS

Variation in foraging success among 284 colour-ringed European blackbirds, Turdus merula, was studied from 1988 to 1990, in Cambridge, U.K. One-year-old birds foraging on lawns were half as successful at getting large prey as 2-year-old birds, which in turn had slightly but significantly less success than older birds. There was no measurable change in foraging success among birds aged 3-5 or more years. Males and females of the same age had similar foraging success. Age-specific differences when foraging on leaf-litter were similar to those on lawns. Repeated measures from the same individuals over 2 years or more showed that the variation in foraging success among age groups at a given time was primarily due to an individual's own improvement. Given yearlings were cosistently 'poor' or 'successful' foragers at different periods of the year, but differences between individuals were smaller later in life. There was no relationship between the mean foraging performance of yearlings and their survival to the following year. There was significant seasonal change in bill length, but bill growth was probably insufficient to explain the variation in foraging success with age. © 1992.