A short diary this week. I’ll be out until about 10 AM or so. It seems like relatively few people show up in the first hour but my apologies if my response to a comment is delayed. This diary is about birds with forked tails such as the barn swallow.
It seems to me (and I could be mistaken) that, lacking a tail ourselves, we tend to underestimate the importance of tails in other animals. The tails we do notice, and remark upon, tend to be spectacular and showy, such as the tails of skunks or pheasants. So why have a tail? More specifically, why have a tail if you are a bird?
Tails help birds in flight by affecting both lift and maneuverability. Tails are also useful organs for displays. For example, the peafowl is the ‘poster child’ for sexual selection, in large part because of the long and elaborately decorated tail of the male. Closer to home the enlarged and decorated tails of wild turkeys and sage grouse play an important role in courtship displays.
A common idea is that natural selection would oppose sexual selection in these cases with the size of the tail being a compromise between the two. Research by Malte Andersson on the Long-tailed Widowbird in Africa tended to support this idea.
Today’s Dawn Chorus is going to focus on a more subtle form of tail elaboration: the forked or ‘swallow’ tail is the subject of our discussion. Many birds have elongated feathers at the out edges of their tails giving the tail as a whole a forked appearance. North American birds with this type of tail include the barn swallow, several species of tern, swallow-tailed kite, magnificent frigate bird, and most spectacularly the scissor-tailed flycatcher. I was fortunate to see a number of individuals of this species in Texas just over a week ago. Below are two videos. The first has a much better view of the bird but the second has a better shot of the bird in flight.
Below are some of the other fork-tailed birds.
These fork-tailed birds come from a number of different families and are not closely related to one another. What do they have in common?
1) Both males and females have forked tails although in some cases males have longer feathers.
2) Most of these birds catch food while flying. Terns are something of an exception although they use fairly precise maneuvering to set themselves up for dives.
It appears that in barn swallows, where tail length has been studied extensively, that both natural and sexual selection are important influences on tail length. The forked tail appears to help with maneuverability and experimentally reducing or increasing the length of the elongated side feathers does reduce the overall health and condition of the birds. However increasing the male feathers in length does seem to increase the probability of getting a mate.
It seems likely that the forked tail functions in precise movement while flying all of these birds. In some cases the tail ‘forks’ have been co-opted as a mating signal.