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

Rich habitats, intensive feeding, and large fuel deposits are assumed to improve the capability for long-distance migration by birds but may also heighten their vulnerability or exposure to predators. Studies of habitat use by migrants have emphasized the importance of feeding, and relatively few studies have considered how migrants manage the dangers inherent in acquiring and storing fuel. Migrant western sandpipers (Calidris mauri) stop over on coastal mudflats characterized by a strong feeding-danger gradient, with both food and danger decreasing with distance from the shoreline. We experimentally manipulated danger by adding obstructive cover and measured sandpiper usage along this gradient. We compared sandpiper usage along a transect extending 100 m on either side of the obstruction with that on matched control transects without obstructions. The dropping density accumulated during a low-tide period provided a sensitive measure of sandpiper usage. Mean usage on control transects was 2.3 droppings/m(2) and was lower by 1.5 droppings/m(2) (65%) on treatment transects. Usage did not differ between control and 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/m(2) on the oceanward side (low food abundance) than on the shoreward side (high food abundance). All these findings were predicted by danger management theory. Our study provides experimental evidence that migrant birds are sensitive to danger on stopover and has implications for understanding migration strategies.