THE COAT-TAIL EFFECT IN MERGED FLOCKS OF DARK-EYED JUNCOS - SOCIAL-STATUS DEPENDS ON FAMILIARITY

When two groups of social animals combine to form a larger group, new social relationships must be formed. Among dark-eyed juncos, Junco hyemalis hyemalis, it has been reported that most members of one flock attain higher ranks than those of another flock when two groups are combined. A possible mechanism for this effect involves differential treatment of familiar and unfamiliar individuals by the most highly ranked bird in the combined flock: a so-called 'coat-tail effect' (Wiley 1990, Anim. Behav., 40, 587-596). To demonstrate this effect, 34 small flocks of wild-caught juncos were established, and then combined to form 17 hocks in which each bird's social status was determined. In the combined flocks, members of the highest-ranking bird's original flock attained adjacent, and higher, ranks than members of the other flock. In a second experiment, small flocks from which either the highest- or lowest-ranking bird had been removed were combined. In combined flocks lacking the highest-ranked bird from each flock there was no tendency for birds from one group to rank higher than those of the other. In contrast, when low-ranked birds were removed, members of one flock again attained adjacent, and higher, ranks than members of the other, indicating that the presence of the highest-ranked bird was necessary for a coat-tail effect to occur. There is evidence that juncos behaved differentially towards familiar and unfamiliar birds, and that flock members could have assessed the relative ranks of the highest-ranked members of the small flocks, suggesting a probable behavioural mechanism for the coat-tail effect. (C) 1995 The Association for the Study of Animal Behaviour