Cool story from Cornell University:
common, chickadee-sized Australian bird is one of the most prolific
cheaters in the avian world — and new research suggests that choices
made by straying females may actually be keeping the species from
diverging into two.
The research, by Daniel Baldassarre and Michael
Webster of the Cornell Lab of Ornithology, will be published online on Oct. 2 in the journal Proceedings of the Royal Society B.
The researchers tested how female fairywrens responded to two different
types of males — a scarlet-backed form that occurs in northwestern
Australia and a more flame-orange form in eastern Australia. The two
forms were once geographically separated, but now occur together in
northeastern Australia. The scarlet-backed form is steadily making
inroads into the range of the orange-backed form, and the researchers
wanted to learn why.
“They had all the building blocks to get going on the classic
speciation process,” said Baldassarre. “But then they came back into
contact too early, and they’re still able to mate with each other.
Interestingly, we found that happens only when they’re deciding who to
Working in an area of Queensland where only orange-backed forms occur,
the researchers used nontoxic red markers to turn orange-backed males
into scarlet-backed males. Then they watched as the birds paired off and
They found that females chose to form social bonds with orange-backed
and scarlet-backed males equally.
But regardless of which form the
females paired with, they overwhelmingly chose scarlet-backed males to
cheat with. DNA tests revealed that because of these dalliances,
scarlet-backed males fathered more than double the number of young than
And that level of gene flow is more than enough to
keep the two forms from continuing on the path to becoming different
species, Baldassarre said.
Many birds cheat on their mates, but fairywrens display some of the
highest rates of extrapair paternity in the bird world.
three-egg clutch has about a 75 percent chance of containing eggs from
at least two different fathers. In some cases, a female’s social mate
fathers no eggs in his nest at all (though he may have young in other
nests). Males even perform special displays for new females — carrying a
bright-red flower petal in their bill — that they don’t do for their
The finding points to the growing understanding that female animals, by
choosing their mates, can exert a strong force on the evolution of a
species. And in red-backed fairywrens, that force is strongest in
“Some males will get 10 extrapair young and others
will get zero, so the females’ choices really matter there,” Baldassarre
said. “But they’ll both probably have two within-pair young, so there’s
not as much at stake.”
All that infidelity gives a female fairywren a second opportunity to
make a choice, Baldassarre said. “With a social mate, a female is
getting a territory and a mate that’s going to preen her and help feed
When she chooses an extrapair mate, she’s going to see that
guy for two seconds, get some genes from him, and that’s it. So she
chooses different qualities.”
The researchers chose southern Queensland as their study site to
simulate the arrival of the scarlet-backed form in a new population as
it spreads eastward. They have not yet been able to do the reverse
experiment, looking at how orange-backed males might fare in a region
dominated by the red-backed form.
But results so far suggest that in situations where two closely related
species appear to be diverging, a second look might be warranted. Even
when birds behave socially as if they are reproductively isolated, there
might be more going on than meets the eye.
“There’s a lot of extrapair
mating in birds,” Baldassarre said, “and that might be a hidden avenue
that could work against the speciation process.”
The study was funded in part by the National Science Foundation, Sigma Xi, and Cornell Deparment of Neurobiology and Behavior.