Raven roosts are mobile information centres

Vagrant, non-breeding common ravens, Corvus corax, inhabiting the forested mountains of Maine are specialized to feed on rich but ephemeral carcasses of large mammals during the harsh winter months. The foraging and roosting behaviour of free-ranging ravens were studied during the winters of 1988–1990. Ravens quickly assembled at carcasses, and into communal roosts. Six lines of evidence indicate that these roosts function as information centres. (1) Roosts comprised both knowledgeable and naive foragers. (2) Departures from roosts were highly synchronized, with most members departing in one direction. (3) Direction of departure often changed from day to day. (4) Birds made naive of food sources (by being withheld from the wild and then allowed to join roosts) followed roost-mates to new feeding sites, whereas control birds held and released outside of roosts rarely found the local food bonanzas. (5) Birds made knowledgeable of food sources (by being released at new carcasses) joined roosts and led roost-mates to the food on three of 20 occasions. (6) The same individuals switched leader and follower roles depending upon their knowledge of feeding opportunities. Although ravens may form roosts at traditional areas (near stable food sources) that are used for many years, the ravens in Maine frequently shifted roost sites to be near newly discovered carcasses. Information exchange at roosts principally occurred on the night of, or the night before, the roost shift. Social soaring displays assembled birds from a wide area and were associated with mass movements to new roosts formed at nearby food.