Biology Reference
In-Depth Information
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School size of prey
Fig. 6.5
The confusion effect of grouping by prey. The capture success per attack of,
from left to right: squid, cuttlefish, pike and perch, when attacking small prey fish in
singles, groups of six, or groups of twenty. In all cases, capture success declines with
increasing prey group size. From Neill and Cullen (1974).
by forcing the predator to continually switch targets during the pursuit. Milinski (1984)
also found that sticklebacks preferred to attack single water fleas
Daphnia
on the edge
of a swarm, perhaps because these were easier to target.
Indirect evidence for the confusion effect is provided by experiments showing that
predators often target odd-coloured individuals in a group (Ohguchi, 1978; Landeau &
Terborgh, 1986). By concentrating on an individual that looks different from the rest, a
predator may be able to counteract confusion.
Communal defence
Prey are often not just passive victims but may actively defend themselves by attacking or
mobbing a predator and grouping may enhance prey defence. Nesting black-headed
gulls,
Larus ridibundus
, will mob crows who come to their colony in search of eggs and
chicks. In the centre of a dense colony many gulls mob a crow at the same time because
it is close to many nests, and group defence reduces crow success (Kruuk, 1964).
Similarly, breeding success is greater in denser parts of the colonies of guillemots
(= common murres)
Uria aalgae
because the defence of many, closely-packed incubating
birds is more effective in deterring nest predators such as gulls (Fig. 6.6; Birkhead, 1977).
Andersson and Wicklund (1978) demonstrated by experiment the increased
effectiveness of group defence by fieldfares
Turdus pilaris
, a colonial-nesting thrush
which breeds in Scandinavian boreal forests. Fieldfares vigorously mob and defecate on
crows and other predators, and artificial nests placed near fieldfare colonies survived
better than those placed near solitary fieldfare nests.
Communal
mobbing of
predators
