untitled

food supplies. The larger the territory, the greater is the abundance of food.

However, we have already seen that there are diminishing returns, in terms of actual

feeding rate, as prey abundance increases. As a consequence, food benefits deceler-

ate with increasing territory size. Similarly, the time and energy needed to patrol the

perimeter also rise with territory size. Moreover, the larger the territory, the greater

the risk that other individuals will intrude. As a result, costs continue to rise steadily

while benefits show diminishing returns with increasing territory size. The profit

margin is clearly greatest for individuals which hold territories of intermediate size

(Fig. 5.13). Provided that females are attracted to males which hold territories with

sufficient resources successfully to rear offspring, the same sort of logic would pre-

dict that they favor intermediate-sized territories. In short, territory formation can

be viewed as an economic decision, like many of the other behavioral processes described

earlier in this chapter. Natural selection should favor the evolution of territoriality,

if it enhances survival and long-term reproductive success.

Economic models of territory formation predict that territory size should be neg-

atively related to both prey abundance (because it affects diminishing benefits) and

forager abundance (because it affects the cost of defending the area). These predic-

tions have been borne out in several field studies. For example, the size of rufous

hummingbird (Selasphorus rufus) territories is inversely proportional to the abundance

of flowers per unit area (Gass et al. 1976; Kodric-Brown and Brown 1978). Hence,

larger territories hold approximately the same resource abundance as smaller, richer

ones. Similar patterns have been observed in many other species, ranging from

shorebirds (Myers et al. 1979) to roe deer (Capreolus capreolus) (Bobek 1977). Most

convincing have been changes in territory size as a direct consequence of experimental

alteration of either forager abundance (Bobek 1977) or resource levels (Myers et al.

1979). One difficulty with interpreting such experiments, however, is that experi-

mental alteration of food levels often triggers changes in intruder pressure, so these

factors tend to co-vary.

In many cases, individuals are faced with a choice of breeding (perhaps unsuc-

cessfully) in a poor-quality territory or waiting for a vacancy in a better territory.

Such is the case for many passerine birds, in which young birds are often relegated

to poor breeding habitats. We discussed one such example earlier in the chapter: low-

ranking great tits occupy hedgerow territories rather than prime territories in wood-

land (Krebs 1971). Removal of prime territory-holders leads to rapid replacement by

76 Chapter 5

120

100

80

60

10 12 14 16

Territory area

Benefit or cost

Benefit

Cost

Fig. 5.13Cost versus

benefit of defending

territories of different

size. The optimal

solution in this

hypothetical case

would be to defend

an intermediate-sized

territory of around

10.5 units, where there

is maximum difference

between the two curves.

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