Biology Reference
In-Depth Information
1999; Dall, 2002). Any additional advantage of group foraging would, of course,
further enhance communal roosting or coloniality.
Two studies of ravens, Corvus corax , provide strong evidence that communal roosts act
as information centres in the manner envisaged by Ward and Zahavi (1973); namely,
successful foragers actively share information about the location of good feeding sites. In
Maine, USA, Bernd Heinrich and colleagues found that a territorial pair of ravens could
defend a mammal carcass against intrusions from one to two juveniles, but gave way if
there was a gang of six or more juveniles. Upon discovering a large, defended carcass
juveniles called loudly to recruit others (Heinrich et al ., 1993). However, the most effective
recruitment to carcasses occurred at the juvenile communal roost sites. This was
demonstrated experimentally by keeping some birds in captivity for a short time so they
were unaware of the location of food in the wild. When these naïve birds were released at
roosts, they followed knowledgeable birds to carcasses. By contrast, when naïve birds
were released away from roosts they were rarely sighted at the carcasses (Marzluff et al .,
1996). Intriguingly, knowledgeable juveniles initiated the pre-roost soaring displays at
dusk, and also initiated the departure of feeding groups from the roost the following
dawn, suggesting that they actively advertised their discovery of the food sources.
Jonathan Wright and colleagues (2003) studied a large winter roost of up to 1500
ravens (mainly unpaired juveniles) on the isle of Anglesey, North Wales, UK. At distances
of 2-30 km from the roost, they put out sheep and hare carcasses embedded with small,
colour-coded plastic beads. The ravens ingested these beads at the carcass and then
regurgitated them in pellets back at the roost. Beads from each carcass tended to appear
at specific sites within the roost, showing that birds which fed together also slept
together. Aggregations of beads at the roost grew daily, with an increasing radius
centred on the first beads from a carcass, showing that there was an increasing number
of naïve birds joining the feeding and roosting group. Furthermore, groups of birds
would leave the roost together led by the bird that roosted centrally in the aggregation,
and they flew directly towards a specific carcass.
Laboratory studies have also shown that naïve individuals can learn from experienced
individuals about the location of food. Geoff Galef and Stephen Wigmore (1983) trained
rats ( Rattus norvegicus ) to search for food in a three-arm maze. Each arm had food with
a different flavour, cocoa in one, cinnamon in another and cheese in the third. In the
first part of the experiment the rats learned that on any particular day only one of the
three sites contained food, but the site was unpredictable. Then on the days of the actual
experiment each of the seven test rats was allowed to sniff a 'demonstrator' rat in a
neighbouring cage. The demonstrator had been allowed to feed on whatever randomly
chosen food was available for that day, and four of the seven test rats, having sniffed the
demonstrator, went to the correct site on their first choice of the day. 'Sniff ' is the
operative word, because other experiments showed that the cue the test rat picks up
from the demonstrator is the smell of the food it has eaten.
Learning about potential food sources in a more direct way, by seeing others exploiting
them, is important in both flocks of birds and shoals of fish (Krebs et al ., 1972; Laland
& Williams, 1997). In some cases, the decision to recruit others to a feeding site depends
on the costs of food sharing. In experiments with house sparrows, Passer domesticus ,
Mark Elgar (1986a) found that birds who discovered a food source would give
recruitment 'chirrup' calls to attract others if the food was divisible (bread crumbs), but
not if the food was indivisible (same amount of bread but in one piece). Therefore, there
is a trade-off between flock benefits (safety from predators) and costs (food sharing).
Information
transfer in raven
roosts
In rats,
information is
transferred by
smell
Information
transfer may be
tempered by the
costs of food
sharing
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