© 1993 International Society for Behavioral Ecology
research-article |
Hazel grouse (Bonasa bonasia) pairs during the nonbreeding season: mutual benefits of a cooperative alliance
Department of Zoology, University of Alberta Edmonton, Alberta, T6G 2E9 Canada and Grimsö Wildlife Research Station S-730 91 Riddarhyttan, Sweden
Address reprint requests to him at the Grimsö Wildlife Research Station.
ABSTRACT
During the nonbreeding season (autumn and winter), hazel grouse (Bonasa bonasia) males and females associate as loosely bound pairs, not as strongly bound pairs as previously thought. These pairs could be considered cooperative alliances, with each member gaining both direct and indirect benefits. The most important direct benefit appeared to be mutual vigilance against predators. This benefit was found at two levels: at the level of arboreal feeding sites, two birds could forage faster and farther from cover than single birds, and at the territory scale, the pair was together more in dangerous habitats. By foraging farther from cover, two birds could use about 23% more of the preferred food trees and 9%–10% more of the food available within a territory than a single bird, partially mitigating the cost of having two birds feeding on limited winter food in the same space. Defending a common territory appeared to be a less important direct benefit of the alliance because the members of a pair did not defend the same territory and often associated with neighboring birds of the opposite sex. A future benefit of the alliance was having breeding partners in the spring; this was most beneficial for males, as the sex ratio was male biased. Members of the pair were often apart. At arboreal feeding sites, this separation was perhaps because the most preferred trees, black alders (Alnns glutinosa) with the most staminate catkins, were located in areas with the most cover, where one bird may be safer from predators than two. Members of pairs in more secure habitats were more often with extrapair birds of the opposite sex. This suggested a trade-off; birds in safe habitats may have visited potential breeding partners, but birds in dangerous habitats may have had to remain together, foregoing this option, to increase their survival probability. This social organization appeared to be an adaptation to surviving in a heterogeneous habitat, with some of the winter food located in dense cover and some located in more open and dangerous situations.
Key words: social organization, social behavior, antipredator behavior, foraging behavior, hazel grouse.