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Title: Factors affecting breeding success of the herring gull (Larus argentatus Pont.) at an increasing and a decreasing colony
Author: Armstrong, Roy
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1992
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(1) The breeding success of Herring gulls was studied at a decreasing (Walney Island, Cumbria, in 1989) and an increasing colony (Sanda Island, Argyll, in 1990) to establish whether differences in breeding performance could account for the differences in population dynamics. No differences were found in laying date, hatching success, chick growth rates or fledging success per brood. Differences were found however in the volume of the a and b eggs from clutches of three (larger at Walney), the survival of c chicks (lower at Sanda), although these did not lead to a difference in overall breeding success between the colonies. The differences in population changes at each colony do not appear to be the result of differences in breeding success. (2) At both colonies, clutch size, egg volumes, hatching success, chick survival and fledging success per brood were very high by comparison with earlier studies in Britain. Egg volumes at Walney were the highest yet reported from a colony where no culling has taken place, suggesting that availability of food immediately prior to egg-laying, is very high. (3) The proportion of birds breeding in third year plumage was recorded at both colonies. None were found at Sanda, however the proportion breeding at Walney was the highest recorded at a colony where culling had not taken place. The proportion was similar to that found in colonies where extensive culling had taken place e.g. Isle of May, suggesting that competition for nesting territories and food was low at Walney in 1990. (4) Examination of the diet of incubating adults revealed large differences in the diet at the two colonies. At Sanda, most of the food came from farmland (earthworms and barley), whereas at Walney the main component of the diet was refuse. The main component of chick diet at both colonies was fish. At Sanda these were gathered from behind Norway Lobster boats, while at Walney, they appear to come from Fleetwood fish docks. The second largest component of chick diet at Sanda was sandeels, and at Walney, refuse. Sandeels were caught either directly by the adults around Sanda, or stolen from auks, particularly Razorbills. (5) Changes in chick diet with chick age were studied. Earthworms formed a large part of the diet of small chicks (< 1 week post-hatch at Walney, < 2 weeks post-hatch at Sanda). As chicks grew the composition of their diet changed, with the proportion of sandeels in the diet increasing with age at Sanda, and the proportion of refuse in the diet increasing at Walney. The amount of fish in the diet did not change with chick age at either colony. These changes in diet were found to be independent of seasonal changes in food availability. (6) The availability of the three main components of the diet at Sanda have all increased during the period of population growth. In particular, the Norway Lobster fishery has increased rapidly coincidental to the period of most rapid population expansion, suggesting that the population growth is a result of increased food availability. The availability of refuse at Walney has decreased during the most recent decreases in the gull population, as a result of both a decrease in the absolute volume tipped and more rapid burial of refuse in recent years. (7) The cause of decline at Walney appears high levels of mortality due to infection by Clostridium botidinum, causing botulism. This explains the observed similarities between this study and studies at colonies where extensive culling has been carried out, particularly in the large egg volumes and number of third year birds found breeding. It also explains why the Lesser black-backed gull has declined less rapidly than Herring gulls at Walney as they feed less on refuse. Sanda gulls in spite of sharing common wintering areas with Walney Herring gulls, have not declined as botulism occurs most often in the summer or before dispersal to winter quarters.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available