Adapting to Life on the Wing: Bird Louse Evolution

Adapting to Life on the Wing: Bird Louse Evolution

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My name is Kevin Johnson and I’m an ornithologist
with the Illinois Natural History Survey – part of the Prairie Research Institute – at the
University of Illinois. One of my specialties is the co-evolution
of birds and bird lice. More than 3,000 species of birds are known
to have feather lice and that’s probably an underestimate because many species of birds
have yet to be examined for these parasites. Most birds have only two basic ways of defending
themselves from lice. They can try to remove the lice with their bills when they preen.
Or, if the lice are on their heads, the birds can try to scratch them off with their feet. Here’s the basic question we were asking
in this study: Did feather lice first specialize to live on the wings, heads or bodies of birds
– or did the lice first choose a type of host – a duck or parrot or songbird for
example – and then later learn to live on a specific part of that host, say on the head,
or wing? If you look at all the lice from all the birds
we studied, all the head lice look very similar to one another. Their heads are kind of triangular,
and have a groove in the underside. Head lice slide a single feather barb through this groove
and grip the feather with their mouthparts to resist being removed by scratching. All the wing lice, too, look a lot a like.
They are narrow and can insert themselves between two feather barbs in the wings. This
helps them avoid being crushed or removed by preening. The body lice are round and either burrow
into the downy parts of the feathers or drop from feather to feather to escape preening. The lice that we call generalists, which move
all over the bird’s body, also look a lot alike. They run through the birds’ feathers
to escape from preening. So when you look at all the head lice that
live on all these different types of birds, or if you just look at the wing lice, you
might think you’re looking at closely related species. You might think that they all evolved
from a common wing louse ancestor. But you’d be wrong. And when you look at all the different types
of lice that inhabit the same group of birds, say ducks, for example, you might say to yourself,
“these lice are all so different, they can’t be closely related.” But, again, you would
be wrong. Our analysis found that if you look at all
the lice that live on one type of bird, the lice that live on ducks, for example, those
lice – the head and wing lice – are more closely related to one another than they are
to those same kinds of lice other types of birds. The head and body and wing lice that
live on parrots are more closely related to one another than to similar looking lice on
other birds. This is true for many of the lice we studied. This indicates that way back in their earliest
encounters with birds, the lice first landed on a particular type of bird, then radiated
– adapted – to take advantage of different habitats on that bird. Our finding is important because it’s an
example of what we call convergent evolution. Different members of the same group, in this
case feather lice, independently arrived at the same or very similar solutions to the
same problems of survival, in this case avoiding getting preened off by the bird. Since lice
can’t fly, if they get removed from a bird they usually end up dead. The changes in their
body shape and behavior were evolutionary adaptations that occurred over many generations
and gave them a survival advantage. And because each of the different habitats on the bird
– the head, the wings, the body – is fairly simple with few options for the lice, the
lice found the same, or at least similar, solutions to the problem of surviving there.

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