Biology Reference
In-Depth Information
Number of species in each category
Table 2.2
Social
organization of
weaver bird
species (Ploceinae)
in relation to
habitat and diet
(Crook, 1964;
Lack, 1968).
Pair bond
Sociality
Habitat
Main
food
Monogamous
Polygynous
Solitary
Grouped
territories
Colonial
Forest
Insects
17
0
17
0
1
Savannah
Insects
5
1
4
0
2
Forest
3
0
2
0
1
Insects
+
seeds
Savannah
Insects
+
seeds
1
7
1
0
7
Grassland
Insects
+
1
1
1
0
1
seeds
Savannah
Seeds
2
11
0
1
16
Grassland
Seeds
0
15
0
13
3
(1)
Species living in the forest tended to be insectivorous, solitary feeders, defend large
territories and build cryptic solitary nests. They are monogamous and males and
females have similar plumage.
(2)
Species living in the savannah tended to eat seeds, feed in flocks and nest colonially
in bulky conspicuous nests. They are polygamous and there is sexual dimorphism in
plumage, the males being brightly coloured and the females rather dull.
Predation and
food dispersion
are key selective
forces
Why is the behaviour and morphology of the weaver birds linked to their ecology in
such a striking way? Crook invoked predation and food as the main selective pressures
that have influenced the evolution of social organization. His argument was as
follows:
(1)
In the forest, insect food is dispersed. Therefore, it is best for the birds to feed
solitarily and defend their scattered food resources as a territory. Because the
food is difficult to find, both parents have to feed the young and, therefore, they
stay together as a pair throughout the breeding season. With the male and female
visiting the nest, both must be dull coloured to avoid attracting predators. Cryptic
nests spaced out from those of neighbours decrease their vulnerability to
predation.
(2)
In the savannah, seeds are patchy in distribution and locally superabundant. It is
more efficient to find patches of seeds by being in a group because groups are able to
cover a wider area in their search. Furthermore, the patches contain so much food
