Nest-Patch Characteristics of Bicknell's Thrush in Regenerating Clearcuts, and Implications for Precommercial Thinning

摘要

Catharus bicknelli (Bicknell's Thrush) is a rare and globally vulnerable songbird often found in regenerating clearcuts in the Canadian maritime provinces and Quebec. Previous studies have shown correlations between vegetation characteristics and occurrence and abundance of this species, but no study has described vegetation associated with Bicknell's Thrush nests in managed forests. From 2007-2010, we investigated nest-habitat selection of Bicknell's Thrush in the industrial forestry landscape of north-central New Brunswick. We compared vegetation composition and structure in 5-m-radius patches around nests to vegetation in a random control-patch within the home range of each Bicknell's Thrush. Precommercial thinning (PCT) is a forest-management treatment that may reduce the suitability of habitat for Bicknell's Thrush, thus we also examined the percent of the landscape treated by this practice around Bicknell's Thrush nests. We found that Bicknell's Thrush preferentially selected nest sites witha significantly lower proportion of deciduous trees and higher overall tree density than randomly sampled habitat within their home range. We also found that an average of 44% of the area within 500 m of Bicknell's Thrush nests was treated by PCT, and most had been treated within 3-5 years of our study. We suggest that small patches of dense, Abies balsamea (Balsam Fir)-dominated forest within a thinned matrix may be sufficient to provide nesting sites for Bicknell's Thrush; however, it remains unclearif these areas support production of young or if they are population sinks. PCT could have serious negative consequences on Bicknell's Thrush breeding success and on the long-term survival of the species in Canada; thus, we encourage silviculture treatments that leave unthinned areas for nesting of Bicknell's Thrush in managed forests.