Loss of nesting sites is not a primary factor limiting northern Chimney Swift populations

Abstract

Aerially-foraging insectivorous bird populations have been declining for several decades in North America and habitat loss is hypothesized as a leading cause for the declines. Chimney Swifts (Chaetura pelagica) are a model species to test this hypothesis because nest site use and availability is easily assessed. To determine if nest site availability is a limiting factor for Chimney Swifts, we established a volunteer-based survey to inventory and describe chimneys (n = 928) that were used or unused by swifts. A logistic regression model showed that swifts preferred chimneys with a greater length exposed above the roofline and greater inside area, which were not associated with residential buildings. The average chimney used by swifts extended 2.86 m above the roofline with an internal area of 10,079 cm². The regression model represents the range of nest-site conditions that swifts will tolerate; this was used to build a linear discriminant function (ldf) that had an I-index of 82 % (measure of prediction success). We applied the ldf coefficients to predict chimney occupancy in three southern Ontario communities. Of 366 open chimneys, the ldf classified 139 as suitable but only 24.4 % were occupied by swifts. Given that >75 % of suitable sites were unoccupied, swifts are likely not experiencing competition from habitat saturation. Our results suggest that Chimney Swift populations, and likely other aerially-foraging insectivorous birds, are limited primarily by other processes not measured in this study, such as changes in prey.