Habitat selection, nest survival, and nest predators of Rusty Blackbirds in northern New England, USA

Abstract

Rusty Blackbird (Euphagus carolinus) populations have plummeted since the mid-20th century. Recent research in New England, USA, suggests that an ecological trap, created through timber harvesting on the breeding grounds, may be responsible. Red squirrels (Tamiasciurus hudsonicus) were hypothesized to be the primary nest predator, but definitive identification was lacking. The potential for mast cone crops to affect Rusty Blackbird nest predation via trophic interactions also remains unexamined. Our objectives were to identify the mechanisms by which an ecological trap may be operating in New England through a multiscale analysis of Rusty Blackbird habitat selection and nest survival, as well as predator identification and quantification. We located 72 Rusty Blackbird nests in Maine and New Hampshire in 2011 and 2012, and modeled habitat selection and nest survival as a function of habitat characteristics at the nest patch (5 m) and home range (500 m) scale. We placed camera traps at 29 nests to identify nest predators, and conducted ground surveys to obtain an index of squirrel abundance each year. We found that Rusty Blackbirds selected nest patches with a high basal area of small conifers and low canopy closure. Nest survival was not reduced in harvested stands, but increased with increasing basal area. Percent cover of wetlands and young softwood stands were the best predictors of Rusty Blackbird selection at the home range scale. At the home range scale, we found that nests that were closer to a road were less successful in 2011, but not in 2012. Red squirrels were the most frequent predator of Rusty Blackbird nests in 2012, when they were abundant following a mast year in 2011. These results suggest that dense cover of small softwoods is important for habitat selection and survival of Rusty Blackbird nests, and that precommercial thinning and possibly road-building could reduce habitat quality for this species.