WHY ARE AMERICAN KESTREL (FALCO SPARVERIUS) POPULATIONS DECLINING IN NORTH AMERICA? EVIDENCE FROM NEST-BOX PROGRAMS

Declines in American Kestrel (Falco sparverius) populations are widely reported, and Breeding Bird Survey (BBS) data Suggest that the North American population declined significantly from 1984 to 2007. Potential causes include the spread of West. Nile virus (WNV), increases in populations of Cooper's Hawks (Accipiter cooperii), and loss of suitable habitat. We examined trends in the numbers of both migratory and resident kestrel populations that Use nest boxes in eight study areas in Florida, Georgia, Virginia and Maryland, New Jersey, Massachusetts, Pennsylvania, Saskatchewan, and the Yukon Territory, 1984-2007. All eight populations underwent significant declines; the mean annual decline in nest-box occupancy rate was 3.0% and ranged from 0.6% in Pennsylvania to 4.7% in New Jersey. Except for the most recent nest-box program, established in 1995 and declining since 2002, all nest-box populations began to experience declines before WNV arrived in North America in 1999. To test whether changes in kestrel population densities generally are associated with the opposite trend in Cooper's Hawks, we examined the 42 BBS physiographic regions for which trends for both species were available. No significant correlations were detected for the period 1966-2007, or for 1980-2007, more closely concurrent with Our nest-box data. Christmas Bird Count data front 1959 through 1988 also failed to demonstrate a significant correlation. Finally, the habitat within our study areas still appears suitable, and die remaining kestrels appear healthy and have high reproductive success. Thus, the principal cause of die decline probably lies elsewhere, perhaps on the wintering grounds or along migration routes. Further, for both migratory and resident. populations, the decline in nest-box occupancy may reflect regional declines, which would reduce the number of individuals available for replacing breeding birds that have died or dispersed.