Biology Reference
In-Depth Information
The ruddy turnstone
Arenaria interpres
provides an example of variable foraging
techniques which fits this hypothesis, though there are several techniques involved
rather than just two. This shorebird feeds in small wintering flocks on rocky shores
where it searches for small invertebrates, especially crustaceans and molluscs. Philip
Whitfield (1990) studied colour-banded birds in south-east Scotland and discovered
that they used six distinct techniques: routing (flicking or bulldozing seaweed to expose
prey), stone turning, digging, probing, surface pecking and hammering barnacles.
Individuals showed varying degrees of specialization and their predilections were
maintained over two successive winters. Dominant individuals tended to rout and often
displaced subordinates from good seaweed patches. Subordinates sometimes stole food
exposed by the dominants. When dominants were removed temporarily (and kept in
aviaries), some subordinates increased their use of routing. However, others persisted
with their own favourite techniques.
Variation could also be maintained in a population even if there was no difference in
competitive ability between individuals. Imagine individuals were free to choose between
producer and scrounger. If most of the population were producers, then scroungers might
do best (plenty of food to steal). On the other hand, if most were scroungers then there
would be intense competition for stolen food and a producer might do best. This would lead
to frequency dependent selection and, in theory, the result would be a stable mixture of
producers and scroungers, where each behaviour enjoyed the same pay-off (Fig. 5.9b).
Kieron Mottley and Luc-Alain Giraldeau (2000) performed aviary experiments with
captive flocks of spice finches,
Lonchura punctulata
, to test whether producers and
scroungers would reach this predicted stable equilibrium frequency when individuals
were free to choose which behaviour to adopt. They studied flocks of six birds. Each
aviary was divided into two sections (Fig. 5.10a). On the 'producer' side individuals had
access to a string next to each perch. By pulling on the string, a producer released seeds
into a dish on the scrounger side opposite. The producer could feed on the seeds by
stretching its neck through a small hole in the division between the compartments.
Individuals on the scrounger side had no string, so they searched for patches made
available by the producers. Two treatments were tested: scroungers could gain easy
access to the seeds (dish uncovered) and only partial access (dish covered).
In the first part of the experiment, birds were unable to move between the two sides of
the aviary and the numbers of the flock on the producer and scrounger sides were
varied. As predicted, scroungers did better when there were more producers and the
predicted equilibrium frequency of scroungers was lower when scroungers found it
harder to access the food (Fig. 5.10b).
In the second part of the experiment, all six birds were given free access to both sides
of the aviary. The numbers choosing the producer versus scrounger sides converged on
the predicted stable frequencies after a few days of testing with each treatment
(Fig. 5.10c). Therefore, variability in a population can come about because of frequency
dependent pay-offs from different choices.
Producers could
do best, with
scroungers
'making the best
of a bad job'
Or, there could be
a stable mixture
with producers
and scroungers
doing equally
well
An experimental
test with spice
finches
Alternative mating strategies and tactics
The two hypotheses in Fig. 5.9 can also help us to explain why individuals within a population
often vary in the way they compete for mates. It is useful to distinguish two terms:
