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 hye malis hyemalis, it has been reported that most members of one flock at tain higher ranks than those of another flock when two groups are comb ined. A possible mechanism for this effect involves differential treat ment 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 flo cks of wild-caught juncos were established, and then combined to form 17 hocks in which each bird's social status was determined. In the com bined flocks, members of the highest-ranking bird's original flock att ained adjacent, and higher, ranks than members of the other flock. In a second experiment, small flocks from which either the highest- or lo west-ranking bird had been removed were combined. In combined flocks l acking the highest-ranked bird from each flock there was no tendency f or birds from one group to rank higher than those of the other. In con trast, when low-ranked birds were removed, members of one flock again attained adjacent, and higher, ranks than members of the other, indica ting that the presence of the highest-ranked bird was necessary for a coat-tail effect to occur. There is evidence that juncos behaved diffe rentially towards familiar and unfamiliar birds, and that flock member s 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