Kelp gulls (mean body weight of 905.3 k 46.5 g, plasma osmolality of 316.5 k 1.5 mOsm.kg', plasma Na+ of 151.3 rt 0.8 mEq.literl, and plasma K' of 3.1 2 0.2 mEq.liter-l (means 2 SE) were adapted to fresh water and had basal plasma concentrations of arginine vasotocin (AVT) and angiotensin I1 (AII) of 13.6 t 1.1 pg.ml-l and 48.1 f 3.4 pg.ml-', respectively. Adaptation to seawater strengths of 25,50,75, and 100% produced significant elevations of plasma osmolality, electrolytes, and AVT concentrations that plateaued with 50% seawater. Body mass and plasma A11 levels were not significantly altered.
Comparison of salt gland function in birds adapted to fresh water with those adapted to 100% seawater showed some differences in concentrating and secretory capabilities.
The presence of supraorbital salt glands in marine birds (Schmidt-Nielsen, '60) makes it possible for them to survive with seawater as their only drinking fluid. This ability is also likely to involve adaptive changes of renal excretion, particularly of the formation of the osmoregulatory hormones arginine vasotocin (AVT) and angiotensin I1 (AII). In a number of avian species with salt glands, adaptation to salt water elevates plasma osmolality and increases salt gland activity (Deutsch et al., '79; Baudinette et al., '82; Burger and Gochfeld, '84; Roberts and Hughes, '84). However, the Pekin duck is the only one of these in which the effects of saline adaptation on plasma AVT and A11 have been examined (Mohring et al., '80; Gray and Simon, '85). As Pekin ducks are not usually exposed to salt water and their maximum salt gland fluid concentration is sufficient only to match drinking fluid salinities up to 67% of seawater (Simon, '82; Roberts and Hughes, '84), their adjustments in salt and fluid balance and related hormonal changes may differ from those of coastal birds naturally adapted to seawater.
Gulls drink seawater and eat food with a high salt content and yet are able to maintain body mass and plasma osmolality (Hughes, '70, '87). These birds maintain salt gland function and may use them when there is no salt stress, indicating that this pathway may be important, irrespective of diet and habitat (Hughes, '7 0, '7 2).
The present studies were performed on kelp gulls to determine if plasma AVT or A11 levels change during adaptation to seawater and to compare the secretory capacities of salt glands in freshwater and seawater-adapted birds.