Evaluating the Coastal Environment for Marine Birds
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duties are shared fairly equally, so each bird has only a maximum of half the daylight
hours for foraging (about 7 to 8 hours each in summer at this latitude).
The second fundamental constraint is depth of water at each potential feeding
location. We used the bathymetry data in the GIS together with an equation relating
diving efficiency (the proportion of the total dive cycle spent foraging on the seabed
during the 'bottom' phase of each dive (bottom-time)) and dive depth (given in Wanless
et al, in press) to estimate the predicted diving efficiency of a shag for any given
polygon in the study area. Results from the model indicated that most of the area was
potentially suitable feeding habitat for shags. However, because diving efficiency
declined with depth and depth changes with distance from the coast, polygons with the
highest diving efficiencies tended to be located close inshore, while offshore areas
tended to have relatively low efficiencies.