The effects of age on the breeding biology and recruitment of the herring gull (Larus Argentatus)

Abstract

Previous extensive ringing and the cull of Herring Gulls between 1972 and 1974 on the Isle of May by the Nature Conservancy Council facilitated an investigation of the effects of age on the breeding and recruitment of this gull species. Median laying date increased with increasing female age, four year old birds laid 11 days later than the colony median laying date, five year old gulls advanced their laying by six to nine days and laying dates progressively advanced up to the seventh year of life. Clutch size increased with the age of females. For four, five, six and seven year old birds mean clutch sizes were 1.71, 2.18, 2.43 and 2.74 eggs per female. Although clutch size decreased as the season progressed, in any time period the largest clutches were laid by the oldest birds. For all eggs laid regardless of clutch size, egg breadth and volume increased with age of the laying female. The seasonal decline in egg volume was not solely attributable to recruits laying later since four and five year old gulls laid smaller eggs than older birds regardless of the time of laying. Hatching and breeding success both increased with parental age. A large proportion of chicks raised by inexperienced breeders died within the first 10 days after hatching. This component of chick mortality was attributable to inadequate reproductive behaviour by inexperienced birds. Most four and five year old gulls nested at low densities and there was some evidence to suggest that breeding success may decrease at the highest nesting densities. The colony of Herring Gulls on the Isle of May has been increasing at about 13-14% per annum since 1950. This rate of increase is similar to other Herring Gull colonies elsewhere in Britain. Using the survival of gulls ringed as adults prior to the cull, the average annual survival rate is estimated as 0.935 - 0.100 and the survival rate in the first year of life was calculated as 0.83, 0.83 and 0.67 in three successive year classes. Prior to culling, the Isle of May was a dense colony and although some three year old gulls held territory, none bred. Some four year old gulls held territories and bred, although the proportion was low. It is estimated that the mean age of recruitment was 5.00 years and that the mean age of first breeding was 5.25 years, with 55% of birds breeding for the first time when five years old. Some individuals probably do not breed until they are seven years old. There was no evidence of differences in the age of recruitment of male and female Herring Gulls. Calculations based on the life-table of the Herring Gull and the number of marked recruits recovered during the cull lead to the conclusion that approximately 70% of the surviving young did not return to breed at their natal colony. Young reared on the Isle of May have been found nesting up to 250km away. Of these recruits which returned to nest on the Isle of May, 66% nested close to their place of birth. Males showed a greater tendency to return to their natal area than females, and this was evident both amongst those birds nesting on the island and through a higher proportion of females emigrating into other colonies. Clearing experiments suggested that recruitment was influenced by the density of breeding birds. Recruits were attracted to dense, undisturbed areas but they had much greater difficulty in establishing a territory there. A comparison of pre- and post-cull age structure on the island indicated large increases in recruitment pressure from gulls in their third, fourth and fifth years of life as a result of culling. From culled samples of known aged birds it was evident that bodyweight increased with age in both males and females. Body weight differences were correlated with increasing weights of Pectoralis major flight muscle, but not with overt visceral fat storage. Thus the breeding performance of recruits may have had both a physiological as well as a behavioural component. In mid-May recruit gulls had smaller brood patches and males had undeveloped testes in comparison to established breeders. On previously culled areas, recruits had significantly larger brood patches, a result of the greater ease of establishing territories by these birds on partially cleared breeding areas.