The effects of relative fuel load on migration speed and on vulnerability have been investigated, but the effects of seasonal variation in predation danger on the amount of fuel and duration of stopover have not been considered. We analyzed seasonal patterns of stopover residence times for western and semipalmated sandpipers Calidris mauri and C. pusilla on southward migration in relation to the passage of migratory peregrine falcons Falco peregrinus. We predicted that individuals on stopover far in advance of the seasonal arrival of falcons would adjust stopover length and hence relative fuel load to migrate slowly and cautiously. We predicted that individuals on stopover later in the season would increase migratory speed as the arrival of migratory falcons came closer, while individuals on stopover under or behind the passage of falcons would migrate slowly. Adult and juvenile semipalmated and adult western sandpipers migrated prior to seasonal increases in peregrine abundance, and as predicted, the seasonal patterns of their stopover durations are consistent with an increase in the speed of migration as the date of peregrine arrival approached. Juvenile western sandpipers, in contrast, migrating concurrently with falcons, slowed their speed of migration as predator abundance increased. Stopover patterns differ between species due to different relative fuel loads. The results fit predictions made based on a ‘mortality‐minimizing’ migration strategy.

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Peregrine falcons (Falco peregrinus) have undergone a steady hemisphere-wide recovery since the ban on DDT in 1973, resulting in an ongoing increase in the level of danger posed for migrant birds, such as Arctic-breeding sandpipers. We anticipate that in response migrant semipalmated sandpipers (Calidris pusilla) have adjusted migratory behavior, including a shift in stopover site usage toward locations offering greater safety from falcon predation. We assessed semipalmated sandpiper stopover usage within the Atlantic Canada Shorebird Survey dataset. Based on 3,030 surveys (totalling ∼32M birds) made during southward migration, 1974-2017, at 198 stopover locations, we assessed the spatial distribution of site usage in each year (with a "priority matching distribution" index, PMD) in relation to the size (intertidal area) and safety (proportion of a site's intertidal area further than 150 m of the shoreline) of each location. The PMD index value is >1 when usage is concentrated at dangerous locations, 1.0 when usage matches location size, and <1 when usage is concentrated at safer locations. A large majority of migrants were found at the safest sites in all years, however our analysis of the PMD demonstrated that the fraction using safer sites increased over time. In 1974, 80% of birds were found at the safest 20% of the sites, while in 2017, this had increased to 97%. A sensitivity analysis shows that the shift was made specifically toward safer (and not just larger) sites. The shift as measured by a PMD index decline cannot be accounted for by possible biases inherent in the data set. We conclude that the data support the prediction that increasing predator danger has induced a shift by southbound migrant semipalmated sandpipers to safer sites.

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The behavior of migrating birds is governed by time‐, energy‐ and danger‐minimizing strategies. The adjustment of migration speed (i.e. the rate at which distance is covered during a migration) is a behavioral tactic that might contribute to these strategic goals. Shorter stopovers and greater fuel loads increase migration speed, but both require more intensive foraging at stopovers, making migrants more vulnerable to predators. A simple numerical model shows how seasonal alterations in migration speed can lower the exposure of western sandpipers to peregrine falcons, their most important predator. The ‘caution–speed–caution’ pattern of higher migration speed in the mid‐passage period, observed in earlier work, requires that the intensive foraging necessary heightens vulnerability, and that migrants are exposed to both migrant predators as well as predators resident at migratory stopovers.

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2Peregrine falcons (Falco peregrinus) have undergone a steady hemisphere-wide recovery since 3 the ban on DDT in 1972, resulting in an ongoing increase in the level of danger posed for migrant 4 birds, such as Arctic-breeding sandpipers. We anticipate that in response migrant semipalmated 5 sandpipers (Calidris pusilla) have adjusted migratory behaviour, including a shift in stopover site 6 usage towards locations offering greater safety from falcon predation. 7 We assessed semipalmated sandpiper stopover usage within the Atlantic Canada Shorebird 8 Survey dataset. Based on 3,030 surveys (totalling˜32M birds) made during southward migration, 9 1974 -2017, at 198 stopover locations, we assessed the spatial distribution of site usage in each 10 year (with a 'priority matching distribution' index, PMD) index in relation to the size (intertidal 11 area) and danger (proportion of the intertidal area within 150m of the shoreline) of each location. 12 The PMD index value is > 1 when usage is concentrated at dangerous locations, 1.0 when usage 13 matches location size, and < 1 when usage is concentrated at safer locations. 14 A large majority of migrants are found at the safest sties in all years, however our analysis of 15 the PMD demonstrated that the fraction increased over time. In 1974, 80% of birds were found 16 at the safest 20% of the sites, while in 2017, this had increased to 97%. A sensitivity analysis 17 shows that the shift was made specifically towards safer (and not just larger) sites. The shift as 18 measured by a PMD index decline cannot be accounted for by possible biases inherent in the 19 data set. We conclude that the data support the prediction that increasing predator danger has 20 induced a shift by southbound migrant semipalmated sandpipers to safer sites. 21 22 2008) or behaviour. For example, songbirds reduce the amount and volume of singing after hearing a 24 1 Hope et al. Sandpipers shift to safetypredator (e.g. Ellison and Ydenberg, 2018). This occurs because singing makes a male conspicuous and so 25 more vulnerable to attack. Lowering the intensity of singing lowers conspicuousness and reduces the risk. 26 However, this comes at a cost because singing also has benefits (mate attraction, territorial defence) that are 27 presumably also reduced. Many such behavioural responses have been documented (Cooper et al., 2015). 28These responses can create impacts on prey populations, competitors, and other trophic levels, together 29 termed 'risk effects'. Over the past two decades it has become increasingly apparent that risk effects can 30 propagate through entire ecosystems (e.g. Schmitz et al., 2004). The 'risk effects' on prey populations 31 stemming from the large and expanding influence of humans on the planet's oceans, particularly the 32 great reductions in the abundance of top predators are well documented (Madin et al., 2016; Myers and 33 Worm, 2003). Heithaus et al. (2008) documented many novel repercussions of changing marine predator 34 abundances (see also Estes ...