Avian predation upon lizards and frogs in a neotropical forest understorey

Although tropical forest birds are known to prey upon small lizards and frogs, no study has documented the attributes of vertebrate-eating birds or whether birds prey opportunistically on the different elements of the herpetofauna within tropical communities. This study is based on a 14-mo investigation on avian diet, supplemented with a 3-y census of frogs and a 1-y census of lizards in a humid forest of central Panama. From 91 bird species, 1086 regurgitates were collected, in which were found 75 lizards and 53 frogs. Over 50% of the common, primarily insectivorous bird species preyed upon lizards or frogs, with a mean frequency of 0.26 prey/sample. These birds (22 species, nine families) foraged on various substrates from different strata of the forest, fed on invertebrates averaging from 3.3 to 17.2 mm in length, weighed from 11 to 195 g, and had bill lengths that varied from 12.2 to 49.8 mm. Based on a logistic regression analysis, intensity of foraging at army-ant swarms was the variable that best explained the likelihood that a bird species preyed upon lizards, leading to a classification that was 91% correct. In contrast, bill length and body length classified correctly 88% of the frog-eating birds, which showed a fairly constant 1:7 bill length/body length ratio (as opposed to a mean but highly variable 1:10 ratio in other species). A multiple regression analysis showed that seasonal variation in intensity of lizard predation was positively related to arthropod abundance except during the breeding season when lizard intake decreased, presumably because nesting birds did not follow ant swarms. Intensity of frog predation correlated with frog abundance over time, the latter being inversely related to arthropod availability. Ninety-seven per cent of all lizards and frogs identified in the diet samples (n = 105) were from two genera, Anolis and Eleutheradactylus, respectively. Prey size distribution in the regurgitates suggested an optimal prey size of 33.5 mm snout-vent length (SVL) for lizards and 14.5 mm SVL for frogs. Birds preyed opportunistically on the different Anolis species, but almost exclusively upon juvenile individuals. Abundances of the different Eleutherodactylus species correlated with their predation rates, but these frogs represented only 10% of all the frogs observed during the censuses. The two most common local anurans, Colostethus flotator and Bufo typhonius, were not taken by any bird species.