Abundance,feeding, and morphology of passerine birds at Kowhai Bush,Kaikoura, New Zealand

Abstract

Five-minute stationary counts of birds at Kowhai Bush over 17 months suggest that the scores for grey warblers and chaffinches may reflect vocal conspicuousness, rather than abundance, and that the scores for shining cuckoos, goldfinches, and redpolls may reflect vocal and visual conspicuousness in combination. Counts in three types of kanuka forest show that cuckoos, robins, and bellbirds favoured the more dense and diverse suocessional stages; that riflemen, brown creepers, fantails, chaffinches, goldfinches, and redpolls were most abundant in less mature habitats; and that warblers and silvereyes were almost uniformly common. Cuckoos and robins overlapped the most in use of habitat, robins and redpolls the least. For both warblers and robins, indices of abundance varied between two of the habitats in proportion to the densities of resident adults, permitting calibration of the indices. Excepting creepers and robins, native species were apparently less abundant at Kowhai Bush than in climax forest near Reefton. At Kowhai Bush in winter, creepers, warblers, and silvereyes (three of the four small native gleaners of foliage) collected prey almost entirely from kanuka, the dominant tree. Warblers foraged on 80% of occasions from living foliage, whereas creepers fed almost equally from trunks, branches, twigs, and leaves, and silvereyes concentrated on leaves and trunks. Creepers and silvereyes overlapped the most in use of feeding stations. Also, they were exclusively gleaners, whereas warblers caught prey on 40% of occasions by hovering. Warblers gleaned only in the upright position, but creepers and silvereyes often gleaned from vertical surfaces or by hanging upside down. The greatest overlap in feeding behaviour was between creepers and silvereyes. Data for four of the five small native insectivores show that warblers were half as heavy as creepers and silvereyes, and lighter on average than fantails. The tail was longer than the wing in fantails, shorter than the wing in silvereyes, and equal to the wing in warblers and creepers. The ratio of wing length to tarsometatarsus length was greatest for fantails (3.4), as befits an aerial feeder. Warblers, silvereyes, and fantails had bills that were wider than deep; the creeper’s was slightly deeper than wide. Silvereyes had the longest bill, and creepers the longest tarsometatarsus. Indices of morphological difference show that silvereyes and creepers differed least.