Biology Reference
In-Depth Information
Improved vigilance for
predators
Groups detect predators sooner
For many predators success depends on
surprise; if the target is alerted too soon
during an attack it has a good chance of
escape. This is true, for example, of
goshawks, Accipiter gentilis , hunting for
wood pigeons, Columba palumbus . The
hawks are less successful when attacking
larger pigeon flocks mainly because the
birds in a larger flock take to the air
sooner, when the hawk is still some
distance away (Figs 6.7a, 6.7b). The most
likely explanation for this is that the
bigger the flock, the more likely it is that
one bird will be alert when the hawk
looms over the horizon (Pulliam, 1973).
Once one pigeon takes off in alarm, the
others follow. Water skaters in larger groups also respond sooner to an approaching model
predator and begin their escape movements when the predator is further away (Fig. 6.7c).
A problem in interpreting these results is that there may be a delay between prey
detecting the predator and their escape response. Therefore, escape reactions are not
necessarily a good measure of detection. However, in general individuals should be safer
in larger groups so if anything there should be a longer delay between detection and
response in larger groups. This would lead to response times increasing with group size,
Fig. 6.6 Group defence. In dense
colonies of guillemots, like this one,
breeding success is higher than in sparse
colonies because of more effective defence
against nest predators such as gulls. From
Birkhead (1977). Photo © T. R. Birkhead.
Many eyes are
better than one
(a)
(b)
(c)
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Number of pigeons in flock
Number of individuals per flotilla
Number of pigeons in flock
Fig. 6.7 (a) Goshawks are less successful when they attack larger flocks of wood pigeons. (b) This is largely
because bigger flocks take flight at greater distances from the hawk. The experiments involved releasing a trained
hawk from a standard distance. From Kenward (1978). (c) Water skaters, Halobates robustus , in larger groups
also respond sooner to an approaching model predator, by agitated movements on the water surface when the
predator is further away. From Treherne and Foster (1980).
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