Social and Metabolic Costs of Flock-Switching Vary with Dominance Status in Dark-Eyed Juncos, Junco Hyemalis
Striegle, Dennis A.
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Individual dominance status in flocks of wintering birds is reported to influence an individual's access to limited food resources. The question of why subordinate birds remain in a flock, despite lowered access to resources, may be explained by significant costs of re-establishing relationships with unfamiliar individuals. This study investigated possible costs experienced by both dominant and subordinate individuals that leave flocks of familiar birds and join unfamiliar groups. In 18 experimental flocks of juncos, a dominant or subordinate member was assessed for changes in both dominance behavior and metabolic rate as it was switched from its familiar flock to a corresponding unfamiliar flock over a 2-day period. When switched to an unfamiliar flock, dominant birds displayed a significant decrease in their ability to displace other flock members, while subordinate birds showed no change in ability to dominate. Through analysis of oxygen consumption, changes in metabolic rate were ascertained for each subject. Dominant individuals displayed a significant increase in metabolic rate after flock-switching, while no significant change was seen for birds of subordinate rank. Individuals of lowest dominance rank in flocks of juncos incur fewer social barriers, when flock-switching, than do birds of highest rank. Because costs of flock-switching for dominant birds are relatively high, and there are virtually no gains for subordinate birds, continued association with familiar individuals provides the greatest benefits for all flocked birds. In wild populations, factors such as resource scarcity and wide dispersal of conspecifics may have a significant impact on the benefits of aggregation.