FACTORS AFFECTING THE EGG SIZE OF RED‐BILLED GULLS LARUS NOVAEHOLLANDIAE SCOPULINUS

Abstract

SUMMARYThe factors influencing the egg size of the Red‐billed Gull Larus novaehollandiae scopulinus were studied at Kaikoura, New Zealand, between 1964 and 1972. In two‐ and three‐egg clutches there was a trend for the eggs to become smaller in the sequence of laying. Length, breadth and volume of eggs of one‐, two‐ and three‐egg clutches declined significantly as the season progressed. The size of eggs from single‐egg clutches tended to be smaller than eggs from two‐egg clutches laid at the same time.There were correlations between the proportions of one‐egg and of three‐egg clutches being laid at a given period and the mean egg volume of two‐egg clutches. When the mean egg volume of two‐egg clutches increased there was a corresponding increase in the proportion of two‐ and three‐egg clutches laid. When the mean egg volume of two‐egg clutches decreased there was an increase in the proportion of single‐egg clutches laid.The egg size of the Red‐billed Gull showed no direct correlation with the abundance or availability of food; the largest eggs were produced early in the season when food was in short supply. In spite of an increase in the food supply in the middle of the breeding season, birds laying at this time produced smaller eggs than birds which laid earlier in the season. However, early breeders which relayed at the peak in food abundance on average produced a larger replacement clutch than originals laid early in the season. It is suggested that the birds nesting early in the season are able to produce the largest eggs because they are the most efficient foragers for food, and those which nest later in the season produce smaller eggs, even at peak food abundance, because of their inefficiency or inexperience. Early breeders laying replacement clutches tended to lay larger eggs and larger clutches than birds which are producing their first clutches at the same time.Two‐year‐old females laid eggs which were significantly shorter than older aged birds while the breadth and volume of the egg increased with the age of the female up to the fifth year. There was a trend for females to lay larger eggs when mated with older rather than younger males. No statistical differences in egg size were detected between females changing or retaining the partner of the previous season. Female body weight and egg volume were positively correlated in females weighing less than 275 g but not for heavier females.It is suggested that the seasonal decline in egg size and clutch size results from a decrease in the availability of food and the ability of the individual to exploit the resource.