BREEDING BIOLOGY OF THE DARTFORD WARBLER SYLVIA-UNDATA IN ENGLAND

The breeding biology of Dartford Warblers was studied during 1975–1976 in 1010 ha of heathland on the Purbeck peninsula, Dorset; comparisons are drawn from other areas of the species' range. Clutches were of 3,4 or 5 eggs, with the mean clutch size increasing progressively from early May to mid‐June, declining thereafter. Incubation and brooding of the young were shared unequally between the sexes, with the female taking the greater part. In the number of feeding visits to nestlings there was no difference between the sexes until the last four days of nest‐life, when the male greatly increased his share. Nesting was comparatively successful (about 80% of nests producing some young), though the incidence of single infertile eggs was high. There were large variations between years and sites in the timing of the starting of laying. Late‐laying pairs reared only one brood, while early starters could rear two. Timing of starting, influencing the number of broods reared, appeared to be the most important factor causing variation in breeding productivity. No vegetational characteristics were entirely adequate to explain variation in starting dates. Past data, however, showed earlier starting in Sussex and Jersey where gorse was a more abundant component of the vegetation of Dartford Warbler habitat than in Dorset. It is suggested that the abundance of gorse is directly related to the quality of the habitat for Dartford Warblers, and hence to breeding productivity. Copyright © 1979, Wiley Blackwell. All rights reserved