Influence of habitat structure and fruit availability on use of a northeastern stopover site by fall songbirds

Abstract

Many studies have suggested that early successional habitats are important for fall migrant and resident birds. In light of this, we studied habitat relationships, preferred fruiting shrub species, and fruit consumption by birds in early successional habitats during the fall at Iroquois National Wildlife Refuge, Alabama, New York, USA. We used transects to count birds in 12 shrub and 6 early successional forest sites during 2008–2009, and collected data to construct habitat models. We examined frugivory using a paired open/enclosed fruit branch experiment and by analyzing fecal samples of birds captured in mist nets. More fall songbirds were detected in shrublands than in forests across the 2 years, although forests with well-developed understories also had high detection levels. Total bird abundance was positively related to fruiting species richness and negatively related to small woody stem (0.0–2.5 cm) abundance. Abundance of American Robins (Turdus migratorius) was positively related to total shrub cover and fruit abundance on common buckthorns (Rhamnus cathartica), and negatively related to small and large (2.5–8.0 cm) stem abundance. Abundance of Gray Catbirds (Dumetella carolinensis) was positively related to fruiting shrub species richness. Abundance of Song Sparrows (Melospiza melodia) was positively related to gray dogwood (Cornus racemosa) fruit abundance, and negatively related to small stem abundance and bella honeysuckle (Lonicera × bella) fruit abundance. Fecal analysis suggested that birds as a whole disproportionately consumed fruit species relative to availability, and consumed more bella honeysuckle and common buckthorn than gray dogwood. Fewer fruits remained on open branches than enclosed branches, suggesting birds actively consumed fruits at our sites. Our results suggest that fall migrant and resident songbirds as a whole benefit more from mature shrublands that contain a diversity of fruiting shrub species than from early successional forests. While these characteristics may benefit the suite of fall migrants and residents at our location, appropriate habitat management may differ for other locations or migratory species.