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

Abstract

Two recent and independent studies both estimate substantially lower survival rates of semipalmated Calidris pusilla than of western C. mauri sandpipers, consistent with the pronounced multi-decade population decline of the former. Migratory danger has climbed steadily for both these long-distance migrants since the mid-1970s as the number of peregrine falcons Falco peregrinus increased. These predators are present on the Pacific flyway as co-migrants and are a natural (though now more abundant) feature of western sandpiper southbound migrations. Adjustments to migratory speed, timing and routing enable them to mitigate the danger. On the Atlantic flyway peregrines were introduced and breeding populations established in the 1980s at key staging areas, creating a novel hazard for southbound semipalmated sandpipers. Adjustments to migratory timing and speed do not aid in eluding such resident predators, and alternative routes are not available. Semipalmated sandpipers as a consequence have few effective defenses to counter this heightened danger, and we hypothesize that migratory mortality has increased, making over-summering (i.e. skipping a breeding season) more advantageous. The risk effects (i.e. the consequent reduction in population growth rate) so generated are substantial, and may be able to account for a large portion of the population decline.

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

“…Juvenile western sandpipers grow wings just before their southward migration, and as predicted (Figure 1 ), the residual (relative to the lengthening inter‐annual trend) annual mean wing length changes in direct relation to snowmelt date (Figure 5 ). Our measures were made at the first major migratory stopover, and there is little exposure to peregrines or other predators along the route prior to this point (Ydenberg et al, 2022 ). The changes in residual wing lengths can therefore not be attributed to direct killing and must have arisen prior to embarking on migration.…”

“…Dates are reported as “day‐of‐year” (e.g., d‐o‐y 220 = August 8). a Described in Ydenberg et al ( 2022 ). The peregrines introduced and now breeding along the Atlantic flyway increased from 0 (1975) to 27 pairs (2010) in the Bay of Fundy (Dekker et al, 2011 ) and even more in Chesapeake Bay (Watts et al, 2015 ).…”

“…Migrant sandpipers require different defenses against co‐migrating versus resident peregrines (Figure 2 ; Ydenberg et al, 2022 ). Temporal overlap with migrating peregrines can be reduced by higher migratory speed (Hope et al, 2011 , 2014 ; Ydenberg, 2022 ; Ydenberg et al, 2007 ), and also by earlier migratory departure (Jamieson et al, 2014 ).…”

Why do avian responses to change in Arctic green‐up vary?

“…Under this scenario, geographic variation in phenological mismatch may be linked to the decoupling of snow melt from peak abundance of arthropods at some breeding sites but not others, due to spatially heterogeneous changes in climate, or possibly due to variation in re‐nesting potential across sites (Grabowski et al., 2013; Kwon et al., 2019). Spatial variation in the demographic characteristics of breeding populations of Semipalmated Sandpipers related to annual life cycle effects such as over‐summering could also contribute to geographic variation in mismatch (Ydenberg et al., 2022). Future studies investigating mechanisms that determine annual schedules across multiple spatial and taxonomic scales will be critical for understanding the consequences of apparent spatial uniformity in phenological responsiveness (Briedis et al., 2016).…”

Gaming the system: how predators induce prey to make themselves more vulnerable

“…Dekker's (1998); (see also Ydenberg et al, 2010) hypothesis that OOF is an effective antipredator tactic is supported by the fact that almost all observed kills were of dunlins not engaged in OOF. To make quantitative estimates we compare 'risk' and 'danger', terms defined by ); Ydenberg et al, 2022; see also Creel et al, 2019 who use the equivalent terms 'realized risk' and 'inherent danger'). Risk is the actual mortality (measured as kills h -1 , in each of the six hourly intervals).…”