Patterns of activity and habitat use by a population of bullfinches (Pyrrhula pyrrhula) in relation to bud-feeding in orchards.
Abstract
A farmland population of bullfinches on the Weald of Kent was studied for 2 years to determine how changes in its size, composition, or activity corresponded to bud-feeding in pear orchards. The birds' habits required indirect methods of study, including a transect count technique, and analysis of captures of ringed and unringed birds by mist-netting. An index of bullfinch presence derived from transect counts revealed 2 peaks, in mid-winter and early spring, the first associated with a shift of habitat use from woods to thickets and hedges. The spring peak did not coincide with habitat changes, nor could it be explained as the result of changes in detectability or weather. Relative values of the index implied a greater population density in 1981-82 than in 1980-81. Bud-feeding in orchards occurred between the autumn moulting period and the spring-summer breeding season, but coincided with a period when birds' bodyweights were high, due to daily deposition of fat stores. During the winter the sex-ratio of the population did not change, but a progressively smaller proportion of first-winter birds was captured, following their initial dispersal in September. In all months, most bullfinches occurred singly or in pairs, although large groups (>6 birds) were encountered only in winter. The rates of capture of ringed and unringed bullfinches diminished little during the winter, in 2 netting sites. That this reflected a resident, but wide ranging population, rather than a continued turnover of immigrants, was confirmed by examining the recapture histories of individual birds. Largely sedentary behaviour was also implied by the distances between successive captures, by associations between individuals, and by the lack of ringed birds among those shot on neighbouring farms. Bud-feeding within a pear orchard started to increase sharply in January in both years, but was stopped by control measures in 1981-82. Most damage was located at the orchard margins, near to woods and thickets. Counts of the numbers of blossoms and fruits in 1981 suggested that only very heavy damage would affect the yield. The results imply that most bud-feeding was attributable to a small number of birds, in a resident population whose composition changed little through the winter, though the habitats and ranges of individual birds may have altered.