Crop use by introduced bird species in winter in relation to crop structure and seed resources

Abstract

Capsule: Most species preferred to forage in grassland where seed resources were more abundant and the vegetation was uniform in height but heterogeneous in density. Aim: To determine the influence of crop composition and structure, and seed food resources on the distribution of introduced bird species on New Zealand farmland in winter. Methods: Birds were counted on 90 ha of mixed cropping farmland during mid- and late-winter periods, 2004. The presence or absence of six introduced species (Blackbird Turdus merula, Skylark Alauda arvensis, House Sparrow Passer domesticus, Goldfinch Carduelis carduelis, Yellowhammer Emberiza citrinella and Starling Sturnus vulgaris) in fields was investigated in relation to crop composition and structure and seed abundance. Results: Species-habitat associations varied over time and between species. Blackbird, House Sparrow, Yellowhammer and Starling were associated more frequently with grassland than arable crops during at least one survey. Seed abundance was a significant predictor of the occurrence of all species except House Sparrow, while crop structure was a predictor of only Goldfinch, Yellowhammer and Starling occurrence. Conclusion: During the winter, the distribution of six introduced bird species in the lowland cropping area was related to habitat composition, vegetation structure, and seed food resources. The preference for grassland habitats was probably determined by the vegetation structure and seed resources in these areas. The trend for increased use of supplementary animal feed in livestock farming in New Zealand may help sustain introduced bird populations. A larger-scale study is needed to investigate the generality of the species-habitat associations detected in this study at the landscape scale. © 2007 British Trust for Ornithology.