BICKNELL'S THRUSHES (CATHARUS BICKNELLI) IN NEW BRUNSWICK CLEAR CUTS: THEIR HABITAT ASSOCIATIONS AND CO-OCCURRENCE WITH SWAINSON'S THRUSHES (CATHARUS USTULATUS)

Abstract

We studied the use of regenerating clear cuts in the central highlands of New Brunswick by Bicknell's (Catharus bicknelli) and Swainson's (Catharus ustulatus) thrushes during the 1997 breeding season. Bicknell's Thrushes were found at 57 of the 90 points surveyed on six mountains. Swainson's Thrushes occupied 89 of 90 survey points. Bicknell's Thrushes had a lower probability of detection (P = 0.40) than Swainson's Thrushes (P = 0.76). Fixed-radius point counts yielded breeding density estimates for birds on individual mountains ranging from about 3–22 pairs/100 ha (average 16 pairs/100 ha) for Bicknell's Thrushes, and from about 39–69 pairs/100 ha (average 51 pairs/100 ha) for Swainson's Thrushes. White birch (Betula papyrifera), balsam fir (Abies balsamea), and pin cherry (Prunus pensylvanica) were the dominant tree species on regenerating sites. Points occupied by Bicknell's Thrushes tended to be at higher elevations than unoccupied points and to have a greater proportion of white birch stems, a greater number of stems in the 5–10 cm diameter class and a lower diversity of trees and shrubs. No attempt was made to characterize Swainson's Thrush habitat since this species was present at virtually all survey points. We believe that relatively young (average height about 4 m) mixed stands of regeneration with moderate to high stem densities provide important habitat for Bicknell's Thrushes in New Brunswick. Suggestions are made about the direction of future research needed to formulate habitat management and conservation plans for this species.