Communal roosts as structured information centres in the raven, Corvus corax

Summary

• 1
  Ravens (Corvus corax, L.) feed on rich but ephemeral carcasses of large animals. Non-breeding juveniles forage socially and aggregate in communal winter roosts, which may function as ‘information centres’ regarding food locations.
• 2
  In a large roost in North Wales, regurgitated pellets on the forest floor contained a variety of prey remains, which were more similar for ravens that had roosted close together the same night.
• 3
  Sheep carcasses placed at varying distances from the roost were baited with colour-coded plastic beads. These were ingested and regurgitated in pellets back at the roost in aggregations, the spatial distribution of which consistently reflected the geographical location of bait sites.
• 4
  Aggregations of beads at the roost grew daily with an increasing radius centred upon the first pellet per carcass. This mirrored the linear increase of six birds per day in the size of groups flying between roost and carcass each morning. Rates of recruitment were greater for carcasses closer to the roost.
• 5
  Groups were led by a single bird roosting centrally within the aggregation. When individually identifiable (37·5% of cases), these individuals were dominant at the carcass and were among the minority of birds involved in acrobatic display flights at preroost gatherings.
• 6
  When contrasted with data on two alternative groups of ravens peripheral to the main roost which foraged and roosted collectively, these results provide strong circumstantial evidence for raven roosts as structured information centres. The adaptive basis for competitive recruitment resulting in excessively large group sizes is also discussed.