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

Abstract

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.

Avian wings can lengthen rather than shorten in response to increased migratory predation danger

“…Evolutionary ecology northbound passage of sandpipers (Watts & Truitt, 2021), and their young have fledged and are hunting by the time of sandpiper southward passage (Dekker et al, 2011).…”

“…However, there are now populations of peregrine pairs breeding at each of their major southward migratory staging areas, populations established by the introduction of captive‐bred peregrines in the 1980s (Dekker et al, 2011 ; Watts et al, 2015 ). These peregrines begin breeding before the northbound passage of sandpipers (Watts & Truitt, 2021 ), and their young have fledged and are hunting by the time of sandpiper southward passage (Dekker et al, 2011 ).…”

Danger, risk and anti‐predator behavior in the life history of long‐distance migratory sandpipers

“…But, as part of conservation efforts that began in the mid‐1970s, captive‐raised peregrines were released and induced to breed on the major Atlantic coast staging regions, including the Bay of Fundy (Hope et al 2020), Delaware Bay and Chesapeake Bay (Watts et al 2015), where peregrines have historically bred little or not at all. Scores of pairs now breed in each of these areas, and actively hunt sandpipers during both northward (Watts and Truitt 2021) and (with their young of the year) southward passage periods (Dekker et al 2011). Semipalmated sandpipers launch their long over‐Atlantic flight to South America from these regions, for which a staging period of several weeks is required (Pfister et al 1998).…”