Document: KIM-3-53-11

A comparison of age ratios, site fidelity, and spatial arrangement of a shrub-nesting songbird in heavily deer-browsed and less-browsed habitats.

HALL, K.R.*

University of Michigan, Ann Arbor MI 48105, USA ¹

Abstract:
Heavy browsing of low vegetation by white-tailed deer is likely to change the suitability of forest habitat for songbird species, such as the Black-throated Blue Warbler (Dendroica caerulescens), that depend on understory habitat. My work examines the male age ratios (first year breeders vs. older birds), return rates, and spatial arrangement of breeding blue warblers in heavily-browsed and less-browsed hardwood forests in northern Michigan. All study sites had patches of dense understory; in the two heavily-browsed sites the understory was a mosaic of browsed hardwood stems and patches of unbrowsed balsam fir, while in the two less-browsed sites it was primarily hardwoods with little browse evidence. In both heavily-browsed sites and one less-browsed site, most males were older, and the ratio of older birds to yearlings was similar between years. Age ratios in the other less-browsed site varied yearly. In all sites, most returning males established territories that overlapped areas used in previous years, although some movements, especially of yearlings returning as older males, were documented. Females showed less of a tendency to reoccupy the same territory. Territories in the heavily-browsed sites were concentrated in areas with dense patches of fir saplings, but were more evenly spaced in less-browsed sites. In all sites, 38-60% of banded adults returned to the site per year, with no consistent difference between returns to heavily- and less-browsed sites. The density of breeding pairs in the heavily-browsed sites ranged from 1.6 - 2.0 pairs per 10/ha, while in less-browsed sites densities ranged from 1.8 - 2.4 pairs/10 ha. In these sites, blue warbler populations tended to have similar characteristics, although the distribution patterns were less even in heavily-browsed sites. Manipulation of balsam fir in the understory may be a useful management tool for providing short-term habitat for shrub-nesters in Michigan forests with high deer densities.

Keywords: deer browse, songbirds, population density, age ratios, habitat use

This abstract is being presented at: 9:15 AM in session:
Oral Session #3: Avian Ecology.