Biology Reference
In-Depth Information
(a)
(b)
1
1
2
3
2
4
5
3
6
4
M , 1st
5
6
2nd
F
E
D
C
B
A
F
E
D
C
B
A
Quality of breeding situation
Fig. 9.10 Female settlement patterns predicted by the polygyny threshold model for
two distributions (a and b) of male territory quality (territories A to F). It is assumed that
the first female does not suffer from the arrival of a second female, so the top line
represents the reproductive success of both monogamous (M) females and the first
females in polygyny, while the bottom line refers to second females in polygyny. The
sequential settlement patterns of six females (1-6) are shown for the six male territories,
assuming that females settle where their expected reproductive success is greatest.
In both cases two males become polygynous (A and B), two monogamous (C and D)
and two remain unmated (E and F). However, settlement patterns and the reproductive
success of monogamous versus polygynous females vary depending on the choices
available. After Altmann et al . (1977) and Davies (1989).
differences in male territory quality seem sufficient for females to cross the polygyny
threshold (Fig. 9.9).
Red-winged blackbirds (Agelaius phoeniceus)
This North American species also breeds in marshes, where females build nests in
emergent vegetation, such as cattails Typha . Males defend territories and attempt to
attract females by singing and displaying their striking red epaulets (Fig. 9.8). Stanislav
Pribil, Jaroslav Picman and William Searcy studied a population in Ontario and tested
four key predictions of the polygyny threshold model by some clever field experiments.
At the start of the breeding season they selected pairs of adjacent male territories such
that each member of a pair was matched in terms of territory quality (emergent
vegetation) and number of newly-arrived females. Each territory of a matched pair was
then given, at random, a different experimental treatment.
An experimental
test of the
polygyny
threshold model
Is there a cost of polygyny? In one experiment, females were removed from 40 pairs
of male territories, so that one territory in each pair had two females and the other
had one female. The removed females were kept in aviaries and then released at the
(i)
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