Biology Reference
In-Depth Information
0.50
0.25
0.00
-0.25
-0.50
full
costs
free
eggs
free
chicks
Treatment
Fig. 1.8
Fitness of female great tits in the experiment of Visser and Lessells (2001).
Fitness was measured as (female survival to next breeding season)
+
(0.5
×
number of
offspring surviving to next breeding season). The logic behind this measure is that each
offspring has only half the female's genes, so has half the 'genetic value' of the female
herself. Female fitness is measured relative to controls (who raised the clutch size they
initially laid) for three experimental groups who each had two extra chicks to raise, but
with varying extra costs (see text). Females given
free
chicks or
free eggs
did better
than controls but females forced to pay the
full costs
of laying and incubation had
lower fitness than controls. From Visser and Lessells (2001).
breeding in southern Sweden. Observed clutch sizes varied from five to eight depending
on feeding conditions in different territories. To test the hypothesis that some females
laid only five eggs because this was the maximum number of young they could raise
efficiently on their particular territories, Högstedt manipulated clutch sizes
experimentally. He found that pairs that had produced large clutches did best with
large broods, while those which had laid small clutches did best with smaller broods
(Fig. 1.9). Variation in clutch size occurred because there was a range of territory
quality and each pair raised a brood size appropriate for its own particular territory.
Experiments have shown similar individual optimization of clutch size in great tits
(Pettifor
et al
., 1988; Tinbergen & Daan, 1990) and collared flycatchers (Gustafsson &
Sutherland, 1988).
