Nesting, nurture and food
John Hale, of Dunedin, asked:
Why do some birds establish territory, and build nests at distance from one another, while other birds nest very close together? I’m thinking of blackbirds as opposed to gannets.
Yolanda van Heezik, a zoologist at the University of Otago, responded:
It’s all about the food, mostly, and particularly during the breeding season when adults have to feed their nestlings. Comparing blackbirds with gannets is interesting. Blackbirds search for their food in an area around their nest, but gannets have to fly out to sea to find food, usually far away from their nest. This means there is no point in gannets defending a territory, except for the small area immediately around their nest.
Almost all seabirds live in colonies, which can be very large and densely packed. By nesting close together with many others, seabirds can defend against predation, because a group is more vigilant and better able to defend against attack than a single bird. Birds in the colony can also use information from birds flying back from foraging trips to guide them to the best sources of food, a real bonus when food patches in the ocean are hard to find.
Downsides of living in a colony include greater disease risks, and if the same site is used again and again there could be a buildup of parasites over time. Large groups of birds are also more conspicuous to predators, and the close proximity of nests creates more opportunities for cuckoldry. Coloniality in birds is likely to have evolved at least 20 times independently, suggesting that the benefits must outweigh or at least balance, the costs.
In birds such as blackbirds, where food must be gathered from the area immediately around the nest, the purpose of the territory is to protect this food supply. This results in greater nest spacing. Different species that eat different foods and that don’t directly compete for food or nest sites can be found nesting quite close together.
How vigorously a territory is defended depends on the richness of food resources. It costs energy and time to defend a territory, and noisy activity can expose a bird to predators, so for such behaviour to have evolved means the benefits outweigh the costs, and result in higher survival and reproductive success.
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