Field tests of micropatch and prey-size selection by Snail Kites Rostrhamus sociabilis

Six free-living Snail Kites Rostrhamus sociabilis were tested for preference by density for micropatches and size of apple snails Pomacea dolioides in simultaneous choice situations. Two hypotheses were tested: kites respond differentially to varying densities of larger versus smaller size classes of snails, foraging where the expected yield is greater; and kites are capable of selecting the larger size classes from a variety of available snail size classes. Kites preferred to forage in micropatches with high prey density. Variation among kites with respect to prey-size selection was very small. In Experiments I and III there were significant inverse correlations between kite choice and snail size, but no relationship was demonstrated in the other test. Significant differences were found between mean individual weights of snails taken by kites and those not taken in Experiments I and III. No differences were observed in Experiment II. Selectivity indices show that the larger size classes were taken above their availability, although no statistical relationship existed between kite choice and snail abundance. These results corroborate the two hypotheses and suggest that kites use hierarchical decision making by employing density of snails to select appropriate feeding locations before selecting the largest individuals from those locations.