Experimental evidence that migrants adjust usage at a stopover site to trade off food and danger

Rich habitats, intensive feeding, and large fuel deposits areassumed to improve the capability for long-distance migrationby birds but may also heighten their vulnerability or exposureto predators. Studies of habitat use by migrants have emphasizedthe importance of feeding, and relatively few studies have consideredhow migrants manage the dangers inherent in acquiring and storingfuel. Migrant western sandpipers (Calidris mauri) stop overon coastal mudflats characterized by a strong feeding–dangergradient, with both food and danger decreasing with distancefrom the shoreline. We experimentally manipulated danger byadding obstructive cover and measured sandpiper usage alongthis gradient. We compared sandpiper usage along a transectextending 100 m on either side of the obstruction with thaton matched control transects without obstructions. The droppingdensity accumulated during a low-tide period provided a sensitivemeasure of sandpiper usage. Mean usage on control transectswas 2.3 droppings/m² and was lower by 1.5 droppings/m² (65%)on treatment transects. Usage did not differ between controland treatment transects at the furthest distances from the obstruction,the difference increased with proximity to the obstruction,and was greater by on average 0.9 droppings/m² on the oceanwardside (low food abundance) than on the shoreward side (high foodabundance). All these findings were predicted by danger managementtheory. Our study provides experimental evidence that migrantbirds are sensitive to danger on stopover and has implicationsfor understanding migration strategies.