Competition and Facilitation Influence Central Place Foraging Ecology in a Colonial Marine Predator

Abstract

Coloniality is strongly shaped by aspects of social foraging behaviour. For example, colonies may be important sources of information, while food competition may increase foraging efforts and limit colony size. Understanding foraging ecology considering these apparent trade-offs is required to develop a better understanding of colonial living. We combined animal-borne GPS, cameras and dive recorders to study social foraging in breeding adult northern gannets Morus bassanus-a wide-ranging colonial seabird. We first tested for indirect evidence of prey depletion around the colony by estimating dive location, depth and duration. Next, we tested for sociality during different behaviours (commuting, foraging and resting) and distance from the colony. Finally, we quantified flocks of inbound and outbound birds to compare social foraging between outbound and inbound legs of the commute. Dive probability and depth (n = 46 individuals; n = 1590 dives) increased with distance from the colony, creating dive clusters at ~100 and 180 km consistent with conspecific prey depletion. Camera stills (n = 8 individuals; n = 7495 images) show gannets are highly social, but this varied among behaviours. Sociality was highest during foraging and commuting; especially inbound and social foraging was more likely far from the colony. Gannets were equally likely to be solitary or social when leaving the colony but returning birds were more likely in larger flocks. In summary, despite experiencing intraspecific competition for food, gannets engage in dynamic, context-dependent social foraging associations. Conspecifics aggregated far from the colony possibly because of a prey depletion halo closer to home, but this provided potential benefits via local enhancement and by returning to the colony in flocks. Our results therefore illustrate how competition may, paradoxically, facilitate some aspects of group foraging in colonial animals.

Keywords: Ashmole's halo; animal movement; biologging; foraging ecology; information use; seabird; social interactions.

FIGURE 1

Dive behaviour of adult gannets from Grassholm equipped with combined deployments of GPS loggers and TDRs. (a) GPS tracks from complete foraging trips (purple) annotated with dive locations (orange) and the colony location marked with a red diamond. Inset map shows the study area (red box) in relation to the British Isles. (b) The core foraging distribution (50% KDE) based on dive locations recorded from TDRs. Concentric circles around the colony show 50, 100, 150 and 200 km radii with the colony marked with a red diamond. Smoothing parameter (h) = 10 km and grid cell size = 1 km. (c) Frequency histogram of dive locations as a function of distance from the colony with dives allocated into 5 km bins split by inbound and outbound foraging trip legs. Vertical dashed lines represent 50, 100, 150 and 200 km distance bands from the colony as in (b).

FIGURE 2

Dive locations by individual vary as a function of distance from the distal trip location. Negative values on the x‐axis represent the outbound portion, and positive values the inbound leg. Dives are allocated into 5 km bins.

FIGURE 3

Estimated smooths from a GAMM for the effect of distance from the colony on dive probability of breeding adult gannets for the (a) outbound and (b) inbound legs of a foraging trip. Purple shaded areas represent 95% confidence intervals.

FIGURE 4

Example images of conspecific social interactions from gannet‐borne cameras. Images from an adult breeder with a rear‐facing camera while on a foraging trip showing; (a) followed by a lone adult, (b) followed by an adult and immature, (c) followed by a flock (7 and 8 individuals) of adults, (d) resting on the water with an adult, (e) diving with a conspecific and (f) group foraging.

FIGURE 5

Social behaviours from rear‐facing cameras on foraging gannets. Data are from birds with a full set of GPS fixes and digital images at 1‐min intervals throughout at least one complete foraging trip leg (inbound or outbound) from Grassholm, UK, in 2011 (n = 8 individuals; n = 7495 images). Location points are coloured by social interactions from cameras and behaviours from speed/turning angle: white = camera off, grey = nighttime, blue = resting on the water, yellow = solo travel, orange = travelling with conspecifics, light purple = foraging alone and dark purple = foraging with conspecifics. Arrows represent direction of travel and red diamond the colony location. G149 and G150 did not have a complete set of images for the inbound flight.

FIGURE 6

Estimated smooths from a GAMM for the effect of distance from the colony on social foraging probability (a) inbound and (b) outbound legs of a foraging trip. Purple shaded areas represent 95% confidence intervals.