Fine-scale foraging behaviour of a medium-ranging marine predator

Citation

Hamer KC, Humphreys E, Magalhaes MC, Garthe S, Hennicke JC, Peters G, Gremillet D, Skov H & Wanless S (2009) Fine-scale foraging behaviour of a medium-ranging marine predator. Journal of Animal Ecology, 78 (4), pp. 880-889. https://doi.org/10.1111/j.1365-2656.2009.01549.x

Abstract
1 Movement patterns of predators should allow them to detect and respond to prey patches at different spatial scales, particularly through the adoption of area-restricted search (ARS) behaviour. Here we use fine-scale movement and activity data combined with first-passage time (FPT) analysis to examine the foraging strategy of northern gannets Morus bassanus in the western North Sea, and to test the following hypotheses: (i) birds adopt a hierarchical foraging strategy characterized by nested ARS behaviour; (ii) the locations and characteristics of ARS zones are strongly influenced by physical oceanography; (iii) the initiation of ARS behaviour is triggered by the detection and pursuit of prey; (iv) ARS behaviour is strongly linked to increased foraging effort, particularly within nested ARS areas. 2 Birds on 13 of 15 foraging trips adopted ARS behaviour at a scale of 9ú1 ñ 1ú9 km, and birds on 10 of these 13 trips adopted a second, nested ARS scale of 1ú5 ñ 0ú8 km, supporting hypothesis 1 above. ARS zones were located 117 ñ 55 km from the colony and over half were within 5 km of a tidal mixing front ~50 km offshore, supporting hypothesis 2 above.3 The initiation of ARS behaviour was usually followed after only a short time interval (typically ~5 min) by the commencement of diving. Gannets do not dive until after they have located prey, and so this pattern strongly suggests that ARS behaviour was triggered by prey detection, supporting hypothesis 3 above. However, ~33% of dives in mixed coastal water and 16% of dives in stratified water were not associated with any detectable ARS behaviour. Hence, while ARS behaviour resulted from the detection and pursuit of prey, encounters with prey species did not inevitably induce ARS behaviour.4 Following the initiation of ARS behaviour, dive rates were almost four times higher within ARS zones than elsewhere and almost three times higher in zones with nested ARS behaviour than in those without, supporting hypothesis 4 above and suggesting that the foraging success of birds was linked to their ability to match the hierarchical distribution of prey.

Keywords
10; 3; ABILITY; activities; Adoption; analysis; AREAS; Behaviour; data; detection; ENCOUNTER; foraging; HYPOTHESIS; location; marine; movement; NORTH-SEA; PATTERN; PATTERNS; RATES; SCALE; Scales; SEA; SEARCH; Spatial; Strategies; STRATEGY; success; time; WATER

Journal
Journal of Animal Ecology: Volume 78, Issue 4