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(a)
(b)
(c)
(d)
1993
1994
1995
1995
1.0
≥
2 helpers
0.8
0.6
0.4
≤
1 helper
0.2
0.0
Low
Medium
High
Low
Medium
High
Low
Medium
High
Low
Medium
High
Territory quality
Fig. 10.6
Sex ratio adjustment in the Seychelles warbler. The offspring sex ratios (proportion male) produced
on different quality territories in (a) 1993, (b) 1994 and (c) 1995. Mothers produced daughters on high-quality
territories and sons on low-quality territories. (d) The data from 1995 are also shown distinguishing between
nests that had either one (solid circles) or more than one (open circles) helper already at the nest. When mothers
already had more than one helper, they produce sons, irrespective of territory quality. From Komdeur
et al
.
(1997). Reprinted with permission from the Nature Publishing Group. Photo © Martijn Hammers.
for food and breeding means that having more than two helpers is disadvantageous.
Consistent with this, on high-quality territories, pairs switched from producing 85%
females when no or one helper was already present to producing 93% males when
two or more helpers were already present. This was also shown experimentally -
when one helper was removed from high-quality territories with two helpers, the
breeding pairs switched from producing 100% males to 83% females.
Komdeur's results had a huge impact when they were first published, crushing the
conventional wisdom that vertebrates such as birds could not manipulate their offspring
sex ratios. In addition, more recent work has shown that the pattern cannot just be the
result of differential mortality. Females of this species normally produce one egg, but
when moved to vacant high-quality territories females produced two eggs, one day apart.
At least some
birds can adjust
their offspring sex
ratio with
remarkable
precision
