Scale-dependent associations of predators and prey: constraints imposed by flightlessness of common murres

We examined the behavioural interactions of a mobile marine predator, the common murre Uria aalge, and its prey in a cold ocean regime in the context of shifting foraging constraints of the predator during various periods of the year. To do this, we explored the scale-dependent distribution patterns of murres in relation to their prey under 2 mobility regimes: (1) during the pre-breeding period when murres can fly and (2) during the post-breeding or moulting period when murres are flightless and are accompanied by flightless juveniles. In addition, we documented for the first time the relative contributions of different prey types in the diets of murres during moult using stable isotopic analysis. Flightless adult and juvenile murres were concentrated on a meso-scale (>100 km) in an area (nursery area) where significantly greater amounts of non-capelin, mainly crustaceans, were located in closely spaced aggregations. Accordingly, flightless murres had variable proportions of fish (capelin Mallotus villosus) and crustaceans in their diet, whereas breeding murres exclusively ate capelin. At fine- and coarse-scales (0.1 to 100 km), murres that could fly during pre-breeding tracked prey at smaller spatial scales (3 to 4 km) than flightless murres (6 to 9 km). We hypothesize that varying mobility regimes and energetic demands (activity levels) of murres during these 2 periods resulted in divergent perceptions of the foraging environment and subsequently in varying tracking scales of prey. This has important implications for the scale of study during different periods of the annual cycle of a predator, when varying constraints and trophic levels may interact to produce diverse behavioural responses to prey distribution and densities.