Status Signaling in Harris Sparrows: Some Experiments in Deception

Abstract

Abstract(1) Harris Sparrows signal their social dominance status by variations in the amount of black feathering on their crowns and throat. The potential ability of individuals to cheat on such a status signaling system was experimentally investigated by dyeing and bleaching some free-ranging individuals in a flock attracted to a bait station. As discussed in detail, cheating is a theoretically important problem because survival studies have shown subordinates to be much less likely to survive over winter than dominants. If true, this should result in strong directional selection toward the dominant plumage type. (2) Subordinates dyed to mimic the highest ranking birds of the winter hierarchy were, with but one exception, persecuted by the legitimate studlies. The single exception was apparently a bird which had underestimated his status during the fall molt when the appropriate plumage signal is produced. (3) Bleached birds were forced to fight much more for their status, as dramati. cally shown by what was likely the first encounter between a bleached bird and a normal individual of just slightly lower rank. Eventually the bleached birds behaved as though all others around them were perceived as disrespectful, and despotically attacked birds at an abnormally high rate. (4) While these data strongly suggest that cheating is socially controlled, such a social organization raises some complex evolutionary issues. An alternative explanation of the apparent social control of cheating is that the unusual behavior observed in response to my manipulations merely represented normal defense of winter resources. This explanation requires that the rate of winter dominance inter-actions be controlled by (1) the risk of attack, and (2) the importance of resource defense. If this is true, the dyed birds may have appeared sick since no hormonal manipulation accompanied their dyeing; thus they may have been attacked more because perceived risk was down and potential gain high (beat the toughy while he is down). Likewise, the bleached birds may have become unusually aggressive because the closer approach of low ranked birds suggested to them that a much more active defense of resources was required. Experiments distinguishing these hypothesis have not yet been performed.