Smooth-billed ani (Crotophaga ani) predation on butterflies in Mato Grosso, Brazil: risk decreases with increased group size

J. Burger et M. Gochfeld, Smooth-billed ani (Crotophaga ani) predation on butterflies in Mato Grosso, Brazil: risk decreases with increased group size, BEHAV ECO S, 49(6), 2001, pp. 482-492
Citations number
Categorie Soggetti
Animal Sciences
Journal title
ISSN journal
0340-5443 → ACNP
Year of publication
482 - 492
SICI code
Although most insects are vulnerable to predation by a variety of predators , including birds, there are few direct observational studies in the wild o f avian predation on adult butterflies. We examined the predatory behavior of smooth-billed anis (Crotophaga ani) on butterflies, and the spacing beha vior of the butterflies which were concentrated on a mineral-rich beach on the Cristalino River, in Mate Grosso, Brazil. We studied eight of the most regularly occurring butterfly species which came each morning to engage in "puddling." Most species of butterfly were closely associated with conspeci fics, although nearest-neighbor distance varied among species. The pierids - "yellows" (Aphrissa statira, Phoebis trite), "oranges" (Phoebis argante), and sometimes "whites" (Daptoneura leucadia) - formed very dense groups (o r clusters) of up to 1,000 individuals occasionally joined by a few kite sw allowtails (Eurytides spp.). Most other butterfly species formed small grou ps (e.g., daggerwings, Marpesia spp.) or were dispersed individually and no n-clumped over the beach (e.g., dingy purplewing, Eunica monima). Anis fora ged using two strategies: rapid frontal attack on dense groups of butterfli es (yellows, oranges, whites), and a stealthy approach to solitary butterfl ies (mainly purplewings) or those in small groups. For yellows, the most co mmon butterfly caught by anis, the capture rate reached over 6 per 15 min p er ani, and about 8% of those captured managed to escape. Capture rates wer e much lower for the other species. Time of day, age of the ani (adult or y oung-of-the-year), and total number of each butterfly species present accou nted for variation in the number of each species captured by anis. The numb er of butterflies captured per 15 min increased as the number of butterflie s present increased, but reached a threshold beyond which the capture rate did not increase. The capture rate per individual butterfly (individual ris k) decreased with group size up to a group of 40 individuals and remained l ow with further increases. Thus a butterfly in a group of 100 was no less l ikely to be eaten than one in a group of 40. For individual ani forays into dense groups of pierids, an individual ani was unable to catch more than 1 6, regardless of group size. These data confirm the dilution effect of grou p size for butterflies; each individual yellow or orange was less at risk f rom ani predation when in a group.