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Young gulls captured from an essentially marine colony with no access to freshwater showed a significantly greater ability to excrete a hypertonic salt load than did birds of approximately the same age captured from an inland colony.

This ability was reflected by higher extrarenal secretion rates from the heavier nasal gland of the “marine” birds. That this increased ability was due to environmental exposure and not to genetic variations was shown by the fact that these differences disappeared when the two groups of birds were maintained on a freshwater diet for four weeks.

The main environmental factor stimulating the development of the nasal gland in the “marine” birds appeared to be non-availability of freshwater, as analysis of the diet of both colonies showed only minor differences.