There are clear advantages to living in cities: safety, ready availability of infrastructure, plenty of company etc. Nevertheless, a large number of people eschew them for the benefits of country life, such as clean air and lots of space. Many species of animals, and particularly birds, face the same choice between living in large groups or remaining in smaller ones, thereby avoiding disadvantages of larger colonies such as the increased risk of disease and increased aggression from neighbours. What causes different individuals of a particular species to take the decisions they do?
It's all in the genes
One possible explanation is that animals and birds might be genetically influenced to breed in smaller or larger colonies. This idea gained widespread acceptance in 2000, when Charles and Mary Brown reported the results of a field experiment of unprecedented scope. The Browns worked on Cliff Swallows, a species of American swallow that breeds in colonies of variable size. Some individuals appear to be more resistant to nest parasites so can breed in larger, more parasite-infested colonies where they can achieve higher foraging efficiencies by following their many neighbours to the insect swarms on which they feed. The scientists cross-fostered an astounding 2,000 nestlings from nests in small colonies to nests in large colonies and vice versa and recaptured over 700 of the birds when they returned to breed in the following years. Analysis of the data showed convincingly that swallows hatched in larger colonies (but reared in smaller ones) chose to breed in large colonies, whereas the reverse was true for swallows hatched in small colonies. In other words, the birds' choice is determined by genetic factors.
Or is it?
Appealing though the conclusion might be, something does not sound quite right. How would a genetic component of the choice of group size be selected and maintained throughout
|Contact: Richard Wagner|
University of Veterinary Medicine -- Vienna