Influence of introduced peregrine falcons on the distribution of red knots within a spring staging site.

Predator recovery driven by single-species management approaches may lead to conservation conflicts between recovered predators and prey species of conservation concern. As part of an aggressive recovery plan, the Eastern Peregrine Falcon Recovery Team released (1975-1985) 307 captive-reared peregrine falcons (Falco peregrinus) and successfully established a breeding population within the mid-Atlantic Coastal Plain, a physiographic region with no historic breeding population and a critical spring staging area for migratory shorebirds. We examined the influence of resident falcons on the distribution of foraging red knots during spring migration. We conducted weekly aerial surveys (2006-2009) along the Virginia barrier islands during the spring staging period (25 April- 6 June) to map foraging red knots (Calidris canutus) and evaluated the influence of proximity (0-3, 3-6, >6 km) of beaches to active peregrine falcon nests on knot density (birds/km). Accumulated use of beaches throughout the season by red knots was significantly influenced by proximity of beaches to active falcon nests such that mean density was more than 6 fold higher on beaches that were >6 km compared to beaches that were only 0-3 km from active eyries. Whether or not an eyrie was used in a given year had a significant influence on the use of associated close (0-3 km) beaches. From 6.5 to 64 fold more knots used beaches when associated eyries were not active compared to when they were active depending on the specific site. Historically, red knots and other migratory shorebirds would have enjoyed a peregrine-free zone within this critical staging site. The establishment of a dense breeding population of falcons within the area represents a new hazard for the knot population.