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Birds of a colony eat together
Northern gannet at Bempton Cliffs.
[Image © Tom Warlow]
Animals trying to determine where their feeding territory ends and the feeding territory of a competing animal nearby begins may be influenced just as much by signals they share with others in their colony as by spats and squabbles with members of competing colonies, reveals a new study in Science.
Bats, bees and seals are a few examples of central-place foragers, animals that gather food in a specific territory, or patch, after leaving a central home site. Central-place foragers return to the same patch again and again for food. Resources of such animals are limited by animals nearby who forage in the same or overlapping space.
Scientists know that some eusocial insects, like ants, defend their foraging territories, but such defense has not been observed in most other species that establish colonies. For these animals, what shapes the boundaries of a feeding territory is unclear, though it may be linked to the distance from the colony.
Seeking to understand what else might determine the boundaries of foraging territories for central-place foragers, Ewan D. Wakefield from England's University of Leeds and colleagues used satellite technology to track over 180 gannets, black and white seabirds, in 12 neighboring colonies in the United Kingdom.
The researchers used the satellite data to determine how far birds from different colonies went to get food. They discovered that the feeding territories of birds from different colonies didn't overlap much, which was surprising, given how close the 12 colonies were.
The researchers propose that, when populations are so dense, animals from competing colonies that might have foraged together once do not continue to; instead, aware that food is harder to come by, they respond by feeding in very specific colony-specific areas.
The authors suggest that birds don't just do this because they feel pressured by competition from the many colonies nearby, but rather, because they share information about their feeding sites with others in their colony as they go to and from the home breeding area. They share this information via visual and audible clues.
The results of this study force scientists to reconsider longstanding concepts in ecology that suggest territoriality is the main factor influencing how central-place foragers determine the location and size of feeding patches. Indeed, the exchange of special cues within colonies may also play a role.