The use of energy, fat, and protein reserves by breeding Great Black-backed Gulls

Abstract

Changes in mass (adjusted for body size) and in nutrient reserve levels were examined in adult male and female Great Black-backed Gulls (Larus marinus), from arrival at the colony to 1 week prior to the fledging of young. During the prelaying period, males lost approximately as much of their prebreeding fat reserves as females gained in mass, suggesting that males contributed substantial energy to the reproductive investment of the pair. We did not analyze prebreeding females for fat or protein but, based on the mass-fat correlation, females likely put on fat during prebreeding that was to be allocated for egg formation. However, their overall loss in mass was equal only to the accumulation prior to laying, suggesting that increased body reserves serve breeding directly, as capital costs for clutch formation. In contrast, males did not regain prebreeding mass until prefledging, suggesting that costs associated with prebreeding activity by males were larger than those for females. Once the clutch was completed, both sexes carried comparable reserves; neither sex fully regained prebreeding levels during incubation. Female mass at the end of the chick-rearing period was lower than during or before incubation, while males recovered their prebreeding mass (adjusted for body size).