Efficiency of enhanced capture methods and age-class structure of dispersing boreal woodpeckers

Abstract

American Three-toed ( Picoides dorsalis ) and Black-backed Woodpeckers ( Picoides arcticus ) are irruptive species for which yearly movements and abundance are linked to higher productivity years due to forest fire or large-scale insect outbreaks in boreal forests. Studies have found that, in Black-backed Woodpeckers, younger birds are the main colonizers of recent burns, and thus related to natal dispersion. However, age structure of dispersing boreal woodpeckers in the fall have yet to be studied. The Observatoire d’oiseaux de Tadoussac is a migration monitoring station located at the southern limit of Québec’s boreal forest and where a special effort has been made to count and capture transient boreal woodpeckers in the fall between 2000 and 2006. In this study, we investigated (1) the age structure and sex ratio of dispersing boreal woodpeckers and assessed (2) if the use of enhanced capture methods (ECM) improves their rate of capture and (3) the correlation between capture rates and visual counts. The age structure of Black-backed Woodpecker (1:13.9 [1:4.5–1:64] adult:juveniles ratio) and American Three-toed Woodpecker (1:16.7 [1:4.22–0]) was strongly skewed toward juveniles, and suggests that dispersing individuals were mainly related with post-fledging movements. The use of ECM to capture boreal woodpeckers was an efficient way of capturing these species. This method showed capture rate four times greater than the passive method. Moreover, capture rates were correlated with visual counts. Therefore, our study supports the importance of post-fledging movements in boreal woodpecker population dynamics, and shows that the use of ECM is an efficient method to capture and count these species outside their breeding season.

Keywords

• age ratio
• american three-toed woodpecker
• banding
• black-backed woodpecker
• dispersal
• mist-netting
• population monitoring