Habitat use and breeding performance in an inshore foraging seabird, the black guillemot Cepphus grylle

• Main Author: Sawyer, Thomas R.
• Format: Thesis
• Language: unknown
• Published: University of Glasgow 1999
• Subjects: Ecology.; Black guillemot; Cepphus grylle; common guillemot
• Online Access: https://dx.doi.org/10.5525/gla.thesis.80904; http://theses.gla.ac.uk/id/eprint/80904

The choice of an appropriate habitat in which to breed successfully is a capacity likely to be subject to strong selection pressures. Studies of habitat utilisation therefore allow us to identify factors determining breeding success. In addition to their intrinsic interest, understanding of these factors plays a crucial role in developing appropriate conservation measures for species and habitat management. In marine birds however, the focus of attention has generally been only on the nesting habitat, and there is relatively little information on foraging habitats. This thesis focuses more on the latter, in an inshore foraging seabird, the Black Guillemot. A colony of Black Guillemots, Cepphus grylle, was studied over two breeding seasons and data were collected on their egg laying, chick rearing and foraging behaviour. Birds were found to lay either one or two eggs, the two eggs being hatched either synchronously or asynchronously. Single eggs were found to be larger the later in the season they were laid. In two-egg clutches the second laid egg was smaller and had reduced survival. Eggs laid earlier in the season had higher fledging success in two egg clutches. Chicks hatched from larger single eggs fledged at an earlier age than those hatched from smaller eggs. Adults providing for chicks brought a variety of food to the nest but a relatively high proportion of the predominant prey item, the butterfish, Pholis gunnellis, in the chick diet was associated with a high growth rate of the second chick. Adult birds catching more of this prey type travelled further to forage and made shorter dives at the foraging site. The overall foraging distances for this colony, whilst at all times being close inshore, were higher than previously reported for inshore foragers and comparable with pelagic foraging seabird species such as the Common Guillemot in nearby colonies. Black Guillemots showed distinct preferences for certain habitat types available in the heterogenous inshore waters at this site and their density on the water was linked to the availability of butterfish. The distribution on the water of foraging birds was consistent across both years of study but showed a degree of seasonal variation, possibly linked to the different demands of chick rearing and provisioning for adults in order to increase body condition for survival in winter. Particularly important habitats were certain distinct types of kelp, beds and these should be the focus of habitat conservation measures.