Biology Reference
In-Depth Information
BOX 4.1
SEARCH IMAGES
Luc Tinbergen (1960) studied the feeding behaviour of birds in Dutch
pinewoods. He discovered that they did not eat certain insects when they first
appeared in the spring but then suddenly started to include them in their diet.
He suggested that the sudden change was due to an improvement in the birds'
ability to see the cryptic insects, a process he called 'adopting a specific
searching image'. There are, however, other hypotheses which could explain
Tinbergen's observations. For example, the birds could have seen the insects all
along but only decided to include them in their diet when their abundance
increased sufficiently to make it profitable to search for them (Royama, 1970).
Alternatively, the birds may have been reluctant at first to eat novel prey or have
improved in their ability to capture the prey.
Experiments by Marian Dawkins (1971) eliminated these alternative
explanations and showed that predators can indeed undergo changes in their
ability to see cryptic prey. Her predator was the domestic chick and her prey
were coloured rice grains. The clever design of the experiment was to keep
the prey the same (therefore handling, acceptability constant) and to vary the
background. Two examples are shown in Fig. B4.1.1. Chick (a) was presented
with orange grains on a green background (green line) and on an orange
background (yellow line). Chick (b) had green grains on an orange background
(green line) and on a green background (yellow line). The two tests were run
separately for each chick. In both cases the chicks found the prey quicker on a
conspicuous background. On the cryptic background the chicks mainly pecked
at background stones at first but after 3-4 min they eventually started to find
the grain and by the end of the trial they had 'got their eye in' and were eating
the cryptic prey at the same rate as when it was on the
conspicuous background.
(a)
(b)
100
100
3
6
9
12
3
6
9
12
Time (min)
Time (min)
Fig. B4.1.1
