Bullfinch Pyrrhula pyrrhula breeding ecology in lowland farmland and woodland: comparisons across time and habitat.

摘要

Bullfinch Pyrrhula pyrrhula populations decreased by 33% in Britain over the years 1977-82, the period of steepest decline. The timing of this decline and its greater severity on farmland (-65%) than in woodland (-28%) point to agricultural intensification as a likely environmental cause, but previous analyses of survival rate and breeding success data have not been able to identify a clear demographic mechanism. As part of an investigation into the causes of the decline of Bullfinch populations, we conducted an intensive study of breeding phenology and success in farmland and woodland habitats of Oxfordshire and neighbouring counties in 1999-2001, using a combination of nest monitoring and autumn mist-netting. Breeding success data were supplemented with nest record and mist-net capture data collected in the same geographical area during the 1960s and 1990s to compare pre- and post-decline productivity. We found a change in the distribution of first-egg dates, with median first-egg dates 17-18 days later in the 1990s. This reflected a change in the centre of gravity of the breeding season rather than the timing of its start and end, with proportionally fewer birds laying in May and early June, and proportionally more between then and the end of July. We found no significant difference in any aspect of productivity per breeding attempt between the two periods, although there was a trend towards higher egg period failure rates in the 1990s. October age ratios indicated higher annual productivity in the 1990s. We found no significant differences between farmland and woodland in Bullfinch breeding success or timing of breeding. The cause of the decline of Bullfinch populations in both woodland and farmland, and the greater severity of the decline in farmland, seems more likely to be found in: (a) the impacts of habitat deterioration on breeding densities, (b) constraints on survival probability outside the breeding season and/or (c) the impacts of increasing populations of Sparrowhawks Accipiter nisus on the ability of Bullfinches to exploit resources in some habitats. Further investigation of the breeding ecology and phenology of Bullfinches would best be focused on understanding the causes and possible demographic consequences of the shift in first-egg date distribution in favour of later-summer nesting attempts..