The “Incubatory Reserves Constancy” hypothesis asserts that incubation could be a departure from breeding stress that allows for the maintenance or recovery of body reserves after laying effort (females) or territory defense (males) in those species with bi-parental incubation such as gulls. The plasma composition and body mass of incubating Yellow-legged Gulls ( Larus cachinnans ) were analyzed and related to the number of days after egg-laying. Female gulls showed an increase in uric acid and cholesterol levels, whereas males showed only an increase in uric acid values throughout this period. Moreover, females increased while males maintained their body masses. These results could reflect a recovery process after the laying effort supporting the Incubatory Reserves Constancy hypothesis in females. Uric acid and urea levels are positively correlated to body condition in Yellow-legged Gulls, which could be the result of a change in diet composition. This disagrees with recent findings on body composition in incubating gulls and could be related to variations in food availability among populations or years, and could reflect flexibility in the investment devoted by each sex.

Yellow-legged gulls breed in high density areas, and the condition-dependent hypothesis suggests that birds with high physical condition can obtain breeding benefits in high-density areas because they are able to pay off the energetic costs of aggressive behaviour and territory defence. This study and others showed a relationship between aggressiveness or copulation behaviour and nest-density during the pre-laying period in gulls. The link between density and behaviour can be explained by the strong competition for space and mates. Testosterone regulates mate behaviour and can play an important role in the condition-dependent hypothesis. We tested the effects of testosterone implants on male breeding behaviour. In a high-density colony, testosterone-implanted male yellow-legged gulls showed higher aggression and copulation frequencies than controls during the courtship period. In addition, these testosterone-treated birds acquired larger territories than the controls. Thus, a high testosterone level can increase individual fitness in densely populated areas given the benefits derived, on the one hand, from a large territory and, on the other, from an increased copulation rate that would guarantee the male's paternity of the chicks born in his own nest. Since testosterone increases energetic requirements, Only high-quality males would be able to pay off the costs of high levels of testosterone and so obtain the benefits of breeding in high-density areas. Testosterone could therefore provide a proximate mechanism regulating nest density in gull colonies.