The conservation of wading birds is increasingly dependent upon determining the ability of human-modified habitats to serve as surrogates for natural habitats. The suitability of commercial shrimp farms and seminatural estuarine mudflats in Northwest Ecuador as foraging habitat for wading birds was assessed by observing the foraging efficiency, diet and behavior of Great (Ardea alba) and Snowy (Egretta thula) Egrets, censusing wading birds, and measuring prey availability (i.e. prey density, standing crop, water depth and diversity). Mean annual wading bird density was greatest in shrimp ponds, but was greater on mudflats during the breeding season when energy demands are most critical. Foraging efficiency (capture/strike) was greater on average in shrimp ponds than on mudflats, and both Great and Snowy Egrets captured significantly larger prey items (in both length and mass) there than on mudflats. However, intake rates (g/minute) were significantly greater on seminatural mudflats than in shrimp ponds during the breeding season. Prey standing crop, mass of individual prey items and water depths were all significantly greater in shrimp ponds than on mudflats, while the opposite was true for prey density and diversity. Shrimp ponds provide important foraging habitat for Great and Snowy Egrets, particularly during harvest events and during the non-breeding season, while four other wading bird species (Tricolored Heron (Egretta tricolor), Little Blue Heron (Egretta caerulea), Yellow-crowned Night Heron (Nyctanassa violacea) and Striated Heron (Butorides striatus)) were precluded from foraging in most ponds due in part to deeper water (>30 cm), pond bathymetry, larger prey items and differing foraging behaviors. While important to Great and Snowy Egrets, shrimp ponds do not appear to provide suitable foraging habitat for all other diurnal wading bird species within the local community, and do not likely serve as complete ecological substitutes for natural foraging habitat throughout coastal Ecuador.
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Vol. 32 • No. 2