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.

[1]  Patricia Parker Rabenold,et al.  Recruitment to food in black vultures: evidence for following from communal roosts , 1987, Animal Behaviour.

[2]  K. H. Skarphéðinsson.  Breeding biology, movements, and persecution of ravens in Iceland , 1990 .

[3]  B. Galef,et al.  Transfer of information concerning distant foods: A laboratory investigation of the ‘information-centre’ hypothesis , 1983, Animal Behaviour.

[4]  L. Lyon,et al.  Seasonal Patterns in Roosting Flocks of Starlings and Common Grackles , 1983 .

[5]  E. Greene.  Individuals in an osprey colony discriminate between high and low quality information , 1987, Nature.

[6]  B. Heinrich.  Ravens in Winter , 1989 .

[7]  Bennett G. Galef.  Information centres of norway rats: sites for information exchange and information parasitism , 1991, Animal Behaviour.

[8]  C. R. Brown,et al.  Cliff Swallow Colonies as Information Centers , 1986, Science.

[9]  D. Gori.  Colony-facilitated foraging in Yellow-headed Blackbirds: experimental evidence for information transfer , 1988 .

[10]  E. Batschelet.  Circular statistics in biology , 1981 .

[11]  A. Zahavi.  The evolution of communal roosts as information centers and the pitfall of group selection: a rejoinder to Richner and Heeb , 1996 .

[12]  Common Ravens Are Attracted By Appeasement Calls Of Food Discoverers When Attacked , 1993 .

[13]  Roost Attendance and Aggression in Black Vultures , 1987 .

[14]  Gerald S. Wilkinson,et al.  Information transfer at evening bat colonies , 1992, Animal Behaviour.

[15]  H. Richner,et al.  Evolution of avian roosting behaviour: a test of the information centre hypothesis and of a critical assumption , 1991, Animal Behaviour.

[16]  Charles R. Brown,et al.  Food-sharing signals among socially foraging cliff swallows , 1991, Animal Behaviour.

[17]  H. Richner,et al.  Communal life: honest signaling and the recruitment center hypothesis , 1996 .

[18]  Donald F. Caccamise,et al.  Avian Communal Roosting: Implications of Diurnal Activity Centers , 1986, The American Naturalist.

[19]  E. C. Waltz.  Resource Characteristics and the Evolution of Information Centers , 1982, The American Naturalist.

[20]  P. Stouffer,et al.  Roosting and diurnal movements of radio-tagged American Crows , 1991 .

[21]  S. Tamm,et al.  Do Roosts Serve as "Information Centers" for Crows and Ravens? , 1980, The American Naturalist.

[22]  B. Heinrich,et al.  Foraging by common ravens in the presence and absence of territory holders: an experimental analysis of social foraging , 1991, Animal Behaviour.

[23]  D. Mock,et al.  Falsifiability and the Information Centre Hypothesis , 1988 .