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(a)
(c)
40
40 min
Sharing predicted
30
20
(b)
10
20 min
0.05
0.1
0.15
0.2
0.25
0.4
0.6
0.8
1.0
Renewal rate of food
20 min
Fig. 5.8 (a) A pied wagtail territory owner exploits its riverside territory
systematically, feeding along one bank and then back down the other bank (a circuit
of about 40 min) to allow time for prey renewal between successive visits to the
same stretch. (b) Sharing the territory with a satellite brings benefits in terms of help
with defence, but costs in reducing prey renewal times by half when each bird walks
half a circuit behind the other. (c) An owner was predicted to share its territory when
the rate of renewal of food and the asymptotic abundance of food were above the
curve. These combinations represent instances where the owner gains a net benefit
in feeding rate because costs of sharing are outweighed by the benefits. The
observed outcomes are shown as dots, each one representing a single day: solid
dots - satellite accepted, open dots - satellite chased off. From Davies and
Houston (1981).
The territory owner works systematically around its territory and revisits each stretch on
average about once every 40 minutes, which gives time for prey renewal (Fig. 5.8a).
It is not hard to see why territorial defence pays: without exclusive use of the river
bank, a wagtail's strategy for harvesting the renewing food could easily collapse because
of prey depletion by intruders. One might expect, therefore, that wagtails would always
defend exclusive territories, but this is not so. Sometimes the territory owner tolerates a
second bird, a so-called 'satellite'. The two sharers tend to move around the territory out
of phase with one another, so that the average return time to a site is halved to 20
minutes, resulting in a lower feeding rate for the owner (Fig. 5.8b). To counterbalance
this cost there is a benefit of sharing because the satellite chases away intruders. The
owner therefore saves defence time, leaving more time to feed. The net effect of these
costs and benefits on feeding rate depends on the food supply. On days when the rate of
renewal of food is high the cost of sharing is relatively low, and the benefit of sharing is
relatively large because intrusion rates increase with increased food on the territory. By
calculating how these costs and benefits influence an owner's feeding rate it was possible
to predict on which days it would pay a territory owner to share and on which days it
should evict a satellite. The predictions were right for 34 out of 40 days (Fig. 5.8c).
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