Western sandpipers have altered migration tactics as peregrine falcon populations have recovered

Abstract

The presence of top predators can affect prey behaviour, morphology and life history, and thereby can produce indirect population consequences greater and further reaching than direct depredation would have alone. Raptor species in the Americas are recovering since restrictions on the use of dichlorodiphenyltrichloroethane (DDT) and the implementation of conservation measures, in effect constituting a hemisphere-wide predator-reintroduction experiment, and profound effects on populations of their prey are to be expected. Here, we document changes in the behaviour of western sandpipers (Calidris mauri ) at migratory stopover sites over two decades. Since 1985, migratory body mass and stopover durations of western sandpipers have fallen steadily at some stopovers in the Strait of Georgia, British Columbia. Comparisons between years, sites and seasons strongly implicate increasing danger from the recovery of peregrine falcons (Falco peregrinus) as a causal factor. A decade-long ongoing steep decline in sandpiper numbers censused on our study site is explained entirely by the shortening stopover duration, rather than fewer individuals using the site. Such behavioural changes are probably general among migratory shorebird species, and may be contributing to the widespread census declines reported in North America.

Optimal bird migration revisited

“…The possible importance 368 of peregrine falcons Falco peregrinus for the migration strategies of arctic-breeding 369 shorebirds in North America has been illuminated and investigated in a series of 370 studies (e.g. Ydenberg et al 2004Pomeroy et al 2006). 371 These authors suggest that the different migration strategies of adults and juveniles 372 have evolved to a large degree as a consequence of differential exposure to the 373 "spatiotemporal predation landscape".…”

Predation in bird populations

“…The same applies to studies that correlate natural variation in predator abundance with prey population changes. For example, it may only take one Peregrine hunting on an estuary for 10,000 Western Sandpipers Calidris mauri, to avoid whole sections of the estuary, or even the estuary entirely (Lank et al 2003;Ydenberg et al 2004). If there were 10 Peregrines, for example, the effect would be the same.…”

Interannual differences in the relative timing of southward migration of male and female western sandpipers (Calidris mauri)

“…Mistnetting began with the onset of adult migration early in July, and continued through the end of the juvenile migration period in late August. Methods are described by Butler et al (1987) and Ydenberg et al (2004). We restricted analyses to years in which at least 20 individuals of a particular age class were captured over at least 10d, admitting 14 years with on average 317 adults, and 18 years with on average 383 juveniles.…”