A Vision in White

Just a beautiful shot from the Spring Bird Count:

Great Egret (Ardea alba), Wadsworth Prairie FP, Lake Co, IL 5/10/2014

 This is a Great Egret (Ardea alba). Great Egrets are the second largest herons in North America, after the Great Blue Heron (A. herodia). (Egret and heron are different names for birds in the same family -- generally speaking, we use egret for the white ones and heron for the rest.) They are also found across South America, much of Africa, Europe, and parts of Asia.

As with most herons, they eat primarily fish and amphibians. This means that they are quite sensitive to water levels, especially near breeding colonies. Herring, et al. found that nest success rates in the Everglades varied with food availability, although unlike White Ibis (Eudocimus albus) they didn't adjust their clutch sizes to compensate. (1) Trocki & Paton found that the size of salt marsh patches in Rhode Island was a good predictor for how many egrets were to be found in a patch. (2) I'm not sure if they assessed food availability as a function of patch size, though. Wiggins found that egrets that fed in the company of others caught more fish than those that fed on their own, but they usually caught smaller fish, resulting in an equal amount of energy intake. On the other hand, they argued that the energy solitary birds spent chasing other egrets probably meant that solitary feeding was overall less successful. (3) It seems to me that the bird's choice of strategy is probably dependent upon the nature of the fish available wherever they happen to be feeding. So a shift in water levels could affect the available area to feed in, the concentration of various sizes of fish, and the resulting social structure and success rates of the egrets.

(1) Herring, G., Gawlik, D. E., Cook, M. I., & Beerens, J. M. (2010). Sensitivity of nesting great egrets (Ardea alba) and white ibises (Eudocimus albus) to reduced prey availability. The Auk, 127(3), 660-670.

 

(2) Trocki, C. L., & Paton, P. W. (2006). Assessing habitat selection by foraging egrets in salt marshes at multiple spatial scales. Wetlands, 26(2), 307-312.

 

 (3) Wiggins, D. A. (1991). Foraging success and aggression in solitary and group-feeding Great Egrets (Casmerodius albus). Colonial Waterbirds, 14(2), 176-179.