Studies of Recruitment in the Great Skua

Klomp, Nicholas Ignatius (1991) Studies of Recruitment in the Great Skua. PhD thesis, University of Glasgow.

Abstract

1. Recruitment is defined as the whole process whereby individuals join the breeding group of a population. Various factors which can potentially influence recruitment in seabirds were described and discussed. Great Skuas were considered to be particularly suitable for a study of recruitment in seabirds for several reasons, but mainly because of the tendency for nonbreeders to frequent traditional, fixed club-sites. Some background information on the biology of the Great Skua population on Foula was presented to enable a study of recruitment in Great Skuas to be conducted with a general understanding of the dynamics of the population on Foula. 2. A technique for the accurate determination of the age of adult Great Skuas and other birds was described and validated using twelve known-age birds. The technique, involving counting endosteal lamellae in sections of tibiae taken from dead birds, was also validated independently by three naive observers. Various possible applications for this technique were proposed. 3. Various external measurements were taken of 863 adult Great Skuas by two observers from 1974 to 1990. An analysis of a sample of these measurements revealed that the variability of linear measurements of adult Great Skuas was greater between the two observers than within each observer's repeated measurements of the same birds. However, the measurements taken by the two observers correlated significantly, such that one set of measurements could be transformed to equate to the other set, thus allowing valid comparisons to be made between years, age, sex and breeding status. 4. Adult female Great Skuas were significantly larger than males in all of six morphometric variables measured. Values of some morphometric variables appeared to increase with age, although the variation with age within a sex was generally small compared to differences between sexes. The mean sizes and weights of nonbreeding Great Skuas decreased over the breeding season, and this was mainly attributed to older nonbreeders arriving at the colony earlier in the season than younger birds. However, 4 and 5 year-old nonbreeders trapped relatively early in the season were heavier and heavier for their wing length than later arriving birds of the same age. Also, individual nonbreeders were found to decline in weight over a season. Weight, and weight for wing length,' of breeding Great Skuas were significantly less during 1988-1990 than prior to this period, coinciding with a period of reduced prey availability and increased foraging effort in the late 1980s. 5. Ring recovery data were used to describe the dispersal of Great Skuas from Foula from 1963 to 1990. In general, the dispersal patterns and causes of recoveries had not changed since they were first described by Furness (1978) up to 1977. However, recoveries of 1-4 year-olds had increased since 1977 and recoveries of first-year birds were greater after 1986 than for all previous years combined. 6. Ring recovery data and observations of individually colour-ringed birds provided several lines of evidence suggesting that levels of immigration and emigration in the Great Skua population on Foula were very low. Great Skuas also displayed a high degree of micro-philopatry, generally choosing clubs and breeding sites within areas on the island from which they originated. 7. An analysis' of Great Skua chick-ringing data on Foula from 1975-1990 revealed that median hatching dates differed significantly among years and areas on the island. Relative and calendar hatching date appeared to influence chick weight, survival and dispersal of an individual in its first year, and subsequent adult survival up to six years after hatching. Chicks that hatched earlier had greater chick weight, greater first-year post-fledging survival, dispersed shorter distances in their first year of life and were heavier as 3-6 year-old nonbreeders, compared to chicks that hatched later in the season. These findings were most easily and consistently explained in terms of inheritable parental quality. 8. Diurnal and seasonal variations in numbers of nonbreeding Great Skuas on Foula were monitored during the 1975, 1976, 1988, 1989 and 1990 breeding seasons. The accuracy and use of daytime and night-time counts were compared. Night-time counts represent the total number of nonbreeders attending the colony on a given date, whilst daytime counts were more variable, although consistently timed afternoon counts were able to provide useful diurnal and seasonal comparisons. The mean number of nonbreeders attending the colony between May and August was significantly less in 1988 (732 individuals), 1989 (662 individuals) and 1990 (656 individuals) than in 1975 (approximately 1949 individuals). 9. Numbers of nonbreeders at the colony correlated with seasonal changes in diet that appeared to indicate changes in food availability. A reduced afternoon attendance by nonbreeders in 1989 (40%) and 1990 (38%), compared to 1976 (67%), reflected unusually poor Great Skua breeding seasons on Foula during 1988-1990. (Abstract shortened by ProQuest.).

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Ecology
• Date of Award: 1991
• Depositing User: Enlighten Team
• Unique ID: glathesis:1991-78295
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jan 2020 15:34
• Last Modified: 30 Jan 2020 15:34
• URI: https://theses.gla.ac.uk/id/eprint/78295