Foraging-patch selection may be influenced by prey density, prey availability, and the energetic costs associated with searching for and capturing prey. Although prey density influences profitability, prey availability may be influenced by habitat and environmental characteristics. We made in situ observations of Snail Kites (Rostrhamus sociabilis) foraging on their exclusive prey, apple snails (Pomacea paludosa), in selected habitats and environmental conditions. We measured the time to capture a snail at 10 sites on Florida lakes (two sites sampled twice over different years for 12 data points total) and five sites in wetlands of the Everglades. Capture times had an inverse asymptotic relationship to snail density. As snail density increased from ∼0.1 to 0.2m-2, capture times declined from ∼600 to 200 sec; capture times leveled off to ∼75 sec between 0.4 and 1.4 snails m-2. Densities of emergent plant stems and water depths, which may influence capture times, were controlled for by site selection; analyses verified these factors had no effects. Snail Kites were significantly more likely to abandon a foraging bout if snail density was <0.2m-2. We interpret the results in the context of foraging for inconspicuous prey. Our data and other reports indicate ∼0.1–0.2 snails m-2 as an approximate threshold below which flight costs increase dramatically and fewer kites tend to forage. Our observations contribute to the limited information available on what constitutes prey density sufficient for the Snail Kite, and we discuss associated conservation implications for this endangered dietary specialist.
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Vol. 114 • No. 4