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a search image of a prey species such that they concentrate on one prey type while

ignoring another. As the rare prey (A) becomes more common, birds (such as chick-

adees (Parusspecies) searching for insects in conifers) will accidentally come across

A often enough to learn a new search image and switch their searching to this species.

In practice, it is often difficult to determine whether there is a Type II or III response

because the differences occur at low densities of prey and measurements are usually

imprecise. The most robust evidence comes from determining whether predators ignore

prey until there is a sizeable prey density available: that would indicate a Type III

response.

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250

200

150

100

50

0

3

2

1

0

0 500 1000 1500 2000

0 1020304050

Density of Microtus (number / km^2 )

Biomass available (g)

Biomass removed (g)

“Kill” rate

(a)

(b)

Fig. 10.4The Type II
functional response of:
(a) European kestrel
feeding at different
densities of voles
(Microtusspecies).
“Kill” rate is voles eaten
per predator per
breeding season. (After
Korpimäki and Norrdahl
1991.) (b) Bank voles
feeding on willow
shoots. (After Lundberg
1988.)

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

0 20 40 60 80 100 120 140

Chick density (number / km^2 )

Grouse chicks eaten / h

Fig. 10.5The Type III
functional response of
hen harriers feeding on
red grouse chicks in
Britain. (After Redpath
and Thirgood 1999,
with permission.)