Aerodynamically speaking, tails are a drag for a bird that needs to gain lift to remain airborne, so tails should be short and flexible to act as rudders in flight. Yet some birds (especially males) sport elongated, ornate, feathery ornaments at the ends of their bodies intended to attract the opposite sex. These long tails are indeed an impediment to flight, but the birds’ ability to fly with such a handicap is a signal to the female of the male’s fitness as a mate.
In other species, both sexes have a forked tail with elongated outer tail feathers and shorter inner tail feathers —the most well known example being the Barn Swallow, a bird famous for its rapid changes of direction in flight as it pursues insects. A number of other bird species exhibit similar tail structures, but you have to wonder how this sort of a tail helps aerial insect foragers?
While on hikes in the Pantanal in Brazil, we had an opportunity to watch Fork-tailed Flycatchers foraging from a favorite perch, as they sallied out and back in tight circles.
Some rapid-fire photos of their predation attempts might help illustrate how elongate outer tail feathers are used to produce the quick direction changes in these aerial foragers. (Successive images of the bird’s flight have been super-imposed on the original image of the bird perching.)
Is that curl in one outer tail feather purposeful or just a result of wind?
So, how do these elongate outer tail feather assist in rapid direction changes in these birds?
In a study of the aerodynamics of flight in Barn Swallows, Åke Norberg noted that the outer tail feathers are drooped below the rest of the tail surface to increase lift and reduce drag in the tail. (This mechanical advantage is similar to the way flaps on the wing of fixed aircraft function to achieve lift.) Norberg further demonstrated that the arc of the central shaft of the outer tail feather and its elongated streamer are adjusted continuously as the bird turns in tight circles, ascends, or descends. Alternate lift/drag differences on the two sides of the tail enable rapid changes in direction. Thus, the longer the tail, the more efficient the flight in complex aerial patterns.
Presumably, the same advantages of tail structure and foraging habits hold for other long-tailed members of this large family of flycatchers (Tyrannidae), like the Scissor-tailed Flycatcher of the southwestern U.S. and the Streamer-tailed Tyrant of the Brazilian Pantanal.
[R. Åke Norberg, 1994. Swallow-tailed streamer is a mechanical device for self-deflection of tail leading edge, enhancing aerodynamic efficiency and flight manoeuverability. Proceeding of the Royal Society B. Vol 257, 22 September 1994.]