Foraging energetics of Great Egrets and Snowy Egrets

Abstract

We studied the prey base, foraging behavior, and energetics of Snowy Egrets (Egretta thula) and Great Egrets (Ardea alba) in the Arthur Kill region of New York–New Jersey during the 2002 breeding season. Prey density increased 10-fold, from 0.12 to 12.0 items/m2 between May and August, with the greatest increase occurring in young-of-the-year mummichogs (Fundulus heteroclitus). Despite increases in prey biomass, birds had higher foraging success in May (47%) than in August (28%). This decrease may have been the result of predator confusion and/or foraging inexperience by newly fledged birds. As a result, birds experienced a net gain of 227.0 W in May and 52.5 W in August. Striking at prey was more costly for Great Egrets (4.15 J, 4.88 W/strike) than for Snowy Egrets (0.34 J, 0.30 W/strike). Pursuing prey by walking and running was relatively inexpensive; Snowy Egrets and Great Egrets spent 0.04 W and 0.06 W, respectively. Snowy Egrets averaged 0.13 m/s while foraging, while Great Egrets moved at a third of that speed (0.04 m/s), supporting the notion that Snowy Egrets could disturb prey that then become more accessible to Great Egrets. Snowy Egrets moved faster in August than in May, but Great Egrets maintained or slowed their foraging speed. Sixty-six percent of the variation in Great Egret energy intake was accounted for by four variables (date, steps, obtuse turns, and attacks from other birds), while 39% of the variation in Snowy Egret energy intake was accounted for by three variables (date, tide, and obtuse turns). Snowy Egrets may sample more foraging locations than Great Egrets, the latter species choosing locations where other birds have already found prey.