Do Common Ravens Yell because They Want to Attract Others?

The formation of groups at food bonanzas results from a variety of mechanism, which include recruitment by signalling and information parasitism. Recruitment is distinguished from information parasitism on functional grounds: attraction of a crowd is termed recruitment if the signaler's fitness is enhanced by the attraction of others but termed parasitism if the signaler's fitness is reduced by the attraction of others. We here show, however, that in Common Ravens, Corvus corax, the proximate reasons for giving recruitment signals are probably other than for attracting a crowd. In the forests of the northeastern United States, non-breeding, vagrant ravens commonly aggregate in large numbers at carcasses where they neutralize the defense of territorial adults. We attempted to mimic this situation with a captive flock of juveniles and a pair of resident adults in order to determine the proximate factors triggering "yells", vocalizations which attract nearby ravens to large animal carcasses. Our experiments indicate that yells are given primarily by hungry birds. However, yelling is strongly modified by status. Within the vagrant crowd, status is labile. When successive dominants were removed, replacements immediately took their place. Furthermore, when the dominants were re-introduced to the flock they always suffered significant losses of status and ceased yelling. The territorial male has, and constantly maintains, the highest status within (but not necessarily outside) his territory, and here he rarely yells. In sharp contrast, within the vagrant crowd of unmated birds it is the highest-status birds that are the most likely to yell when approaching food. Furthermore, the dominant vagrants (as well as adults) suppress yelling in subordinates. We conclude that ravens yell proximately to advertise their status at food, and that recruitment is only one of several ultimate advantages of the behavior.