Attraction to tropical mixed-species heron flocks: Proximate mechanism and consequences

Summary

• 1. A series of experiments using flocks of model birds on natural foraging grounds in Panama was designed to test whether each species participating in mixed heron flocks was drawn to the flock by resources alone, by the presence of members of its own species, by the presence of other species, or by the white color of flock participants.

• 2. The presence of Snowy Egrets was determined to be the attractive force. Field observations revealed that herons of all participating species increased their foraging success by proximity to Snowy Egrets (Fig. 1). Stomach-content analysis and direct observations revealed complete dietary overlap among all participating species. There was no significant difference in mean foraging rates of any species, except for adult Little Blue Herons, which caught fewer fish than even immature members of their own species. Adult but not immature Little Blue Herons were selectively attacked by other species, and foraged farther from Snowy Egrets in the flocks.

• 3. Benefits of mixed flocking accrue to Great Egrets, Little Blue and Tricolored Herons by increased foraging success when foraging close to Snowy Egrets; Snowy Egrets benefit by using subordinate species as easily supplanted indices of temporally varying food.