Biology Reference
In-Depth Information
BOX 5.1
THE ECONOMICS OF TERRITORY DEFENCE IN THE
GOLDEN-WINGED SUNBIRD (GILL & WOLF, 1975)
(a) The metabolic cost of various activities was measured in the laboratory:
Foraging for nectar
1000 cal/h
Sitting on a perch
400 cal/h
Territory defence
3000 cal/h
(b) Field studies showed that territorial birds need to spend
less time per day foraging to meet their daily energy
requirements when the flowers contain more nectar:
Nectar per flower (
μ
l)
Time need to forage (h)
1
8
2
4
3
2.7
(c) By defending a territory a bird excludes other nectar
consumers and, therefore, increases the amount of
nectar available in each flower. The bird therefore
saves foraging time because it can satisfy its energy
demands more rapidly. It spends the spare time
sitting on a perch, which uses less energy than foraging. For example, if
defence results in an increase in the nectar level from 2 to 3
l per flower, the
bird saves 1.3 h per day foraging time (from b). Its' net energy saving is:
μ
(1000
×
1.3)
−
(400
×
1.3)
=
780 cal
foraging
resting
(d) But this saving has to be weighed against the cost of defence. Measurements
in the field show that the birds spend about 0.28 h per day on defence. This
time could otherwise be spent sitting, so the extra cost of defence is:
(3000
×
0.28)
−
(400
×
0.28)
=
728 cal
In other words, the sunbirds make an energetic profit when the nectar levels
are raised from 2 to 3
l as a result of defence. Gill and Wolf found that most of
their sunbirds were territorial when the flowers were economically defendable.
μ
per flower would save the birds less than 0.5 h of foraging time while, as shown in Box 5.1,
an increase from 1 to 2
l saves four hours. Thus, when nectar levels are already high,
territorial exclusion of nectar thieves does not pay for itself in savings of foraging time.
μ
Shared resource defence
Sometimes the economics of resource competition may favour shared defence. An example
is the winter feeding territories of the pied wagtail (
Motacilla alba
) along stretches of the
River Thames near Oxford, UK (Davies & Houston, 1981). Here, pied wagtails feed on
insects washed up by the river onto the bank. After a bird has foraged in a particular stretch
and depleted the insects, the numbers gradually build up as new insects are washed ashore.
