Osmoregulation in Nestling Glaucous-Winged Gulls

Abstract

-Blood samples from 32 nestling Glaucous-winged Gulls (Larus glaucescens) ranging in age from 1 to 41 days were collected at a nesting site near Vancouver, B.C. The hematocrit was positively correlated, the plasma sodium and chloride concentrations negatively correlated, and the plasma potassium and osmotic concentrations uncorrelated with age. Salt gland secretion was elicited in 23 young gulls by intravenous injection of I M NaC1. The concentration and the volume of secretion increased with age. Sodium secretion was positively correlated with the size of the salt gland. The secretion of potassium and minimum maintained concentrations of sodium were positively correlated with age. In the first hour of secretion, hatchlings eliminated 39% of the injected fluid containing 24% of the injected sodium; in contrast, the oldest nestlings removed 77% of the fluid and 55% of the sodium. Cloacal fluid voided by older salt-loaded birds was less concentrated than that of younger ones. Organ weights were determined in 20 nestlings. Kidney weight was 2-3% of the body weight regardless of age, but the relative weights of the Harderian, salt, and adrenal glands decreased with age. From the time they begin to feed, young marine birds consume foods that are hyperosmotic to their body fluids and must, therefore, eliminate excess solutes (mainly sodium and chloride) in order to remain in osmotic balance. Adult birds of marine species eliminate excess sodium chloride by means of the salt glands (Peaker and Linzell 1975), the secretion of which has roughly twice the concentration of sea water. Thus, they can drink sea water or eat salt-rich foods and still maintain osmotic balance. Adult birds also have other osmoregulatory options: e.g., they can select food items and so, theoretically at least, regulate their salt intake; those living along the coast also have access to fresh water. In contrast, nestlings do not have these choices although their parents could select food of low osmotic concentration or extract salts from it before feeding it to them. However, my observations of the food (often whole small fish or mussels) regurgitated by nestlings suggest that the adults do neither of these things. Nestlings do not ordinarily have access to water, although they have been reported to eat grass, which may provide them with free water (Ensor and Phillips 1972). They may also use dew or the parents may bring them fresh water, although neither of these possibilities seems to have been substantiated. Hence, nestling marine birds probably rely heavily on extrarenal secretion to excrete excess salt. Little has been reported about the rate of maturation of the secretory process or the factors that affect it in such nestlings. Douglas (1968) examined the size and secretory capacity of salt glands from Ad6lie Penguin chicks (Py oscelis adeliae). He found that the gland's weight and secretory rate were linearly correlat d with body size, but that the chloride concentration of the secretion was constant hroughout the growth period. He noted secretion in pipping chicks and suggested that ev n at this age, they were capable of handling the excess salt in their diet of marine inverteb es. Hughes (1968) also found that the salt gland of Common Tern chicks (Sterna hirundo) enlarged in linear fashion with body size, even though older birds were less efficient at excreting imposed salt loads than younger ones. This was due to a decrease in the volume secreted by the salt gland, not in the concentration of the secretion. However, some of the birds in that study were salt-loaded more than once during their growth period and this may have influenced their responses. Ensor and Phillips (1972) compared the secretory capacities of salt glands from 4to 5and 15to 16week-old chicks in two mixed populations of gulls (Larus argentatus and L. fuscus) that had been raised either on a marine island or an inland freshwater site. The younger birds from the marine environment were smaller than their freshwater counterparts and had larger salt glands, nearly twice as large on a weight-specific basis. The secretion of their glands was twice as concentrated in sodium and was produced at three times the rate of that from freshwater birds. These differences disappeared by the time the birds were 15-16 weeks old. In the present study, I examined the blood of birds at representative stages of the entire maturation period for age-related changes in hematocrit, electrolyte concentration, and os-