Relationship of body size to time of nesting and mate preference in the Ring-billed Gull (Larus delawarensis)

Abstract

The first part of this thesis investigated the relationship between time of nesting and body size in Ring-billed Gulls (Larus delawarensis). There were no significant differences in skeletal measurements among birds from early, peak, post-peak, and late nesting periods. Discriminant function analysis failed to classify individuals by overall body size into groups defined by the periods in which they initiated nesting activities. Body condition was significantly lower in late-breeding birds compared to those from other nesting periods. Although there was a direct relationship between timing of nesting and body condition, no such relationship held with body size or individual skeletal variables. The second part of this thesis examined the relationship between body size and mate preference in Ring-billed Gulls. A total of 16 skeletal measurements were taken on each bird from 108 mated pairs of gulls. Variance in skull and bill measures was significantly lower between mates than in randomly generated pairs. However, correlation coefficients for all single skeletal characters between mates were low. High correlations were found between mates in body condition and fresh weight but not in random pairs. A significant positive correlation existed between overall body size within mated pairs but not within random pairs. The results indicate that Ring-billed Gulls mate assortatively on the basis of body condition and to a lesser degree by overall body size. I postulate, however, that this assortative mating is not the result of active choice of similar-sized or quality mates. Ring-bill probably attempt to obtain the best quality mate possible, but because of the presence of higher quality competitors, actually obtain mates of comparable size and quality. This type of mate choice would appear as assortative mating in studies undertaken after mate selection was completed.