In
most birds, males are larger than females, but in some
birds, such as many shorebirds and birds of prey, the
reverse is true. No one is certain why there is this
"reversed sexual size dimorphism" in raptors, but a number
of interesting hypotheses have been advanced. All are based
on a well-established correlation this size difference
between the sexes is less pronounced in species that pursue
sluggish prey than in those that pursue birds. Vultures,
whose prey are least agile of all, show little sexual size
difference. Mammal-hunting buteos, such as the Red-tailed
Hawk, evolved males that are somewhat smaller than females,
whereas in bird-hunting accipiters and falcons, females may
be half again as heavy as males. One explanation for the
females' larger size suggests that it protects them from
aggressive males that are well equipped with sharp talons
and beaks, and the killer instincts to go with them.
According to this theory, over evolutionary time, females
have preferred to mate with smaller, safer males -- in fact,
the female may have to be able to dominate the male for
proper pair bonding to occur and for the male to remain in
his key role as food provider to both female and young. Such
a system would involve sexual selection for smaller size in
males. Bird-hunting raptors are assumed to show aggression
most suddenly, and to represent the greatest threat to their
mates, and they are the ones exhibiting the greatest size
difference. In experimental pairings set
up so that male American Kestrels were the larger of the
pair, Cornell ornithologist Tom Cade found that the females
did not suffer from an avian version of wife abuse. The size
difference in kestrels was not very great, however, so this
may not be a definitive test of the hypothesis. It seems
that while sexual selection may play a role, there probably
is more to it. Another hypothesis proposes
that the size differences allow the two sexes to hunt
different prey and thus reduce competition for food.
Competition is thought to be more severe among bird hunters
than among other hawks, since their small agile prey are
able to flee in three dimensions and are thus effectively
scarcer than, say, carrion or ground squirrels. Indeed,
there are data indicating that the hunting success of
bird-chasing raptors is only about half that of raptors
preying on mammals, and only a sixth that of raptors eating
insects. Tom Cade has suggested that, for bird eaters,
available food supply in the nesting territory can become
limiting, making it adaptive for the male to specialize on
small prey and for the female to specialize on large prey.
The male feeds the female and young at the beginning of the
nesting season; the female becomes an active hunter when the
nestlings are larger, and the adults then tend to partition
the prey resource in their territory. But if reducing intersexual
competition for food is the reason for the size difference
in raptors, why aren't males sometimes the larger sex? One
possible reason is that females need to be larger because
they must accumulate reserves in order to produce their
eggs. Another is that females do not forage for a
substantial period while incubating eggs and brooding young.
They avoid the risks of the hunt during that time, but they
must rely on the small male to feed the entire family. Small
fleet prey, aerial or terrestrial, are more abundant than
large sluggish prey, so that over time smaller male
bird-eating raptors would be favored over larger, less agile
ones, because they would be better providers. For species
that take more sluggish prey, however, small males would not
be so advantageous, which might explain the relationship
between prey speed and the male-female size
discrepancy. SEE: Sexual
Selection;
Raptor
Hunting. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.