Dietary Salt as a Physiological Constraint in White Ibis Breeding in an Estuary

Abstract

We monitored the growth and metabolism of 28 nestling white ibis (Eudocimus albus) taken at 21 d of age and hand-reared for 3 wk on ad lib. diets of (1) freshwater crayfish (Procambarus clarkii; salt content 535 mosm kg⁻¹; n = 12), (2) brackishwater fiddler crabs (Uca spp.; 1,080 mosm kg⁻¹; n = 12), and (3)freshwater crayfish salt-loaded to a salt content approximating that of fiddler crabs (1,090 mosm kg⁻¹; n = 4). Brackish water (480 mosm kg⁻¹) or fresh tap water was provided to the birds on an ad lib. basis. Nestlings maintained on the unaltered-crayfish diet gained mass at rates similar to those of parent-reared nestlings. Birds on the fiddler-crab and salt-loaded crayfish diets lost approximately 3% of their body massper day until fresh water was substituted for brackish water 7 d into the experiment. Nestling hematocrits, serum osmolality, and serum electrolyte levels indicated that during the first week of the experiment, nestlings on the two high-salt diets were dehydrated and salt loaded. This occurred even though birds on high-salt diets exhibited hypertrophic development of their salt glands. When birds on the high-salt diets were allowed to drink fresh water, all began gaining mass within 2 d, and their serum osmolality and salt returned to levels similar to those of nestlings on the low-salt diet. Our experiments indicate that a physiological constraint acting on nestling ibis is responsible for the fact that ibis breeding in coastal colonies fly long distances inland to secure freshwater prey for their young.