Data from: Morphology of western and semipalmated Sandpipers 1978-2020

Increasing predation danger can select for safety-enhancing modifications
to prey morphology. We document multi-decade wing lengthening of a Pacific
flyway migrant, the western sandpiper (Calidris mauri), and contrast this
with contemporaneous wing shortening of the closely related semipalmated
sandpiper (C. pusilla) on the Atlantic flyway. We measured > 12,000
southbound western sandpipers captured 1978–2020 at a major stopover site
in British Columbia. Wing length increased at 0.074 mm y-1 (SE = 0.017; p
< 0.0003) for adults, and 0.087 mm y-1 (SE = 0.029; p <
0.007) for juveniles. These rates are of similarly large magnitude (4–5%
overall), but opposite in direction, to the rate we previously reported
for semipalmated sandpiper adults (-0.103 mm y-1). In both species, the
change is specific to wings rather than being part of general body size
change. We interpret both trends as responses to the ongoing strong
increase of peregrine falcon (Falco peregrinus) populations since the
mid-1970s, an important predator encountered by these species in
contrasting ways during migration. Western sandpiper and peregrine
migrations have temporal and spatial overlap. Longer wings enhance
migratory speed and efficiency, enabling western sandpipers to decrease
overlap by advancing to safer zones ahead of falcon passage. In contrast,
semipalmated sandpipers primarily encounter peregrines as residents at
migratory staging sites. Shorter wings improve acceleration and agility,
helping migrants to escape attacks. Juvenile western sandpiper wing length
also shows a component additive to the lengthening trend, shifting between
years at 0.055 mm d-1 with the highly variable snowmelt date, with wings
shorter following early springs. On the Pacific flyway, the timing of
peregrine southward passage advances with snowmelt, increasing the
relative exposure of juveniles to post-migratory resident peregrines. We
interpret this annual wing length adjustment as an induced defense, made
possible because snowmelt timing is a reliable cue to danger in the
upcoming migration.