Behavioural ecology of common murre and Atlantic puffin predation on capelin : implications for population biology

Piatt, John F. (1987) Behavioural ecology of common murre and Atlantic puffin predation on capelin : implications for population biology. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Relationships between capelin (Mallotus villosus), common murres (Uria aalge), and Atlantic puffins (Fratercula arctica) were studied at Witless Bay, Newfoundland, during the summers of 1982 to 1984. Capelin occurred inshore from May to August, but abundance peaked during periods of maximal tidal oscillations and increasing water temperatures in late June and early July. From 1982 to 1984, capelin abundance declined by an order of magnitude, but school densities remained similar. Capelin size varied by 30-40% seasonally, and up to 20% annually, partly due to temporal variations in age-class composition of schools. Capelin dominated by weight in diets (4842 items identified) of murres (adult:82.7% and chick:90.8%) and puffins (78.8% and 93.2%), but juvenile cod (Gadus morhua) and sandlance (Ammodytes dubius) were also important prey. Numbers of capelin in bird stomachs declined from 1982 to 1984, and were seasonally correlated with local capelin density; suggesting sigmoidal (Type III) functional responses. Numerical proportions of capelin in diets (from gizzard contents or chick meals) were not correlated with seasonal or annual variations in local capelin density. Variations in size of prey eaten by birds generally reflected natural variations in prey size. Bird diet overlap was extremely high, and diet diversities were low (puffin > murre). Numbers and flock sizes of birds were best correlated with capelin abundance and density, respectively. Murres were best correlated with higher density capelin schools in deeper offshore waters, and formed larger, more aggregated flocks than puffins. Both murres and puffins exhibited sigmoidal (Type III) aggregative responses to capelin density, with murres having a significantly higher prey density threshold. Density thresholds varied with overall capelin density. Murres had a larger maximum foraging range than puffins, and ranges varied annually with capelin abundance. Murre and puffin feeding ecology and behaviour may be best explained within the context of optimal foraging and predation theory. Ecological differences between murres and puffins may be largely due to their differing body sizes (murre > puffin). Implications for population biology of the Atlantic Alcidae are discussed.

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