Effect of body condition on reproductive performance and secondary sex ratios in ring-billed gulls (Larus delawarensis)
Meathrel, Catherine Elizabeth
Master of Science
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Nesting Ring-billed Gulls (Larus delawarensis) were studied on Granite Island, northern Lake Superior, during the breeding seasons of 1983 and 1984 to determine if there was a relationship between food supply and the time of breeding, body condition, and egg characteristics affecting reproductive success. Eggs laid in 1984 were heavier than those laid in 1983. This may have been related to smaller pre-breeding food abundance in 1983. In each year. Ring-billed Gulls nesting later in the season laid fewer and smaller eggs that had poorer hatching success. Within clutches, third laid eggs were smaller, lighter and produced fewer young than first or second eggs. Chemical analyses of eggs revealed that eggs laid in 1984 contained more albumen, and proportionately more yolk lipid, than eggs laid in 1983, Eggs laid later in each year contained less albumen and fewer nutrients (lipid and protein) than earlier laid eggs. Within early and late clutches, c-eggs had decreased levels of albumen and nutrients. For all eggs analyzed, an average of 35% of fresh egg weight was yolk, with a fresh yolk to albumen ratio of 0.63. Water accounted for 69% of fresh egg weight. Through the laying sequence, approximately 8% of fresh egg weight was yolk lipid. The energy and nutrient content of eggs remained proportionately constant through the laying sequence each year. Egg weight was strongly correlated with egg volume and water content. Chick weight at hatching was positively correlated with fresh egg weight, though this correlation was weak. There was no difference in hatch weights within clutches. Male and female eggs did not differ in size or weight. There was no relationship between chick sex and egg sequence in either year. The secondary sex ratio was skewed in favour of female chicks in 1983. The overall secondary sex ratio did not differ from unity, and the ratio of unisexual and bisexual 3-egg clutches laid early in 1984 did not differ from 1:2:1. In each year, females nesting later in the season had a lower body weight and condition index than earlier nesting females. The indexed body condition of females nesting early in 1983 was smaller than in 1984. Both male and female indexed condition decreased through the breeding season in 1984. The body weight of females was correlated with egg weight but not nutrient content. Egg size and composition were not satisfactory predictors of female body weight and condition. Chemical analyses of post-laying females revealed that late nesters contained proportionately more body water and protein, but less lipid than early nesters. Between years, there were no differences in the body weight or nutrient content of post-laying females. The energy content of the body was most strongly correlated with lipid levels in the body. Both the body weight and indexed condition of early nesting females were correlated with body lipid and energy levels. Females with larger stored nutrient levels (i.e. those nesting earlier each year) laid heavier eggs which contained more nutrients. Pre-breeding adult males and females tended to weigh more and contain more energy per unit body weight than early nesters. I suggest that reduced body condition, and hence egg quality, for those birds nesting in 1983 and late nesters, were primarily related to differences in food abundance and acquisition efficiency during the pre-breeding period.