Vane emargination of outer tail feathers improves flight manoeuvrability in streamerless hirundines, Hirundinidae

Abstract

Recent studies have suggested that the proximal part of the swallow (Hirundo rustica) tail streamer appears to aid turning flight, as expected if streamers evolved initially purely through natural selection for enhanced manoeuvrability. However, the evolution of slender aerodynamically advantageous streamers is also predicted by an alternative hypothesis, which suggests that such a trait could develop primarily to ameliorate the aerodynamic cost of a long size-dimorphic tail. To distinguish between these hypotheses, we have investigated for the effect on manoeuvrability of trimming the tips of the outer tail feathers into short streamers, without lengthening these feathers, in two streamerless hirundine species-the house martin (Delichon urbica) and the sand martin (Riparia riparia). This allowed us to examine the aerodynamic costs and benefits of streamers at an early evolutionary stage that predates elongation of the outermost tail feathers through female choice. We showed that such initial streamers enhance manoeuvrability in streamerless hirundines, confirming the findings of recent studies. However, in contrast to these studies, we showed that improved manoeuvrability resulting from streamers could arise before the outermost tail feathers have become elongated (e.g. owing to female choice). The occurrence of such an aerodynamic advantage depends on the ancestral shape of a forked tail. This provides support for the hypothesis that streamers, like those in the barn swallow, might evolve initially purely through natural selection for enhanced manoeuvrability.

No short-term effects of geolocators on flight performance of an aerial insectivorous bird, the Barn Swallow (Hirundo rustica)

“…flight time and number of strings hit, were only weakly correlated in both this (r = 0.16, n = 66 tests) and previous samples of flight trials (Matyjasiak et al 2004(Matyjasiak et al , 2009. Similarly, maximum acceleration and maximum velocity were weakly correlated (r = 0.20, n = 60 tests).…”

“…Breeding male Barn Swallows were fitted with geolocators, which were deployed using a leg-loop harness. Short-term flight performance of tagged and control birds was measured in a standardized manner in flight tunnels (Rowe et al 2001;Bowlin and Winkler 2004;Matyjasiak et al 2004Matyjasiak et al , 2009Bro-Jørgensen et al 2007), using a robust experimental design in which the withinindividual change in flight performance was compared between birds that were equipped with a geolocator (after allowing for a period of acclimation) and control birds (that were also tested twice) without geolocators. We predicted that geolocator birds would suffer a decline in flight performance traits in the post-versus pre-deployment trial, whereas this change was not expected among controls.…”

Flight costs of long, sexually selected tails in hummingbirds

“…Carpenter et al 1993). Most theoretical and behavioural studies of the role of the avian tail in flight have focused on aerial insectivores, such as swallows ( Norberg 1994;Evans 1998;Matyjasiak et al 1999Matyjasiak et al , 2000Matyjasiak et al , 2004Park et al 2001;Rowe et al 2001). These taxa may be particularly sensitive to small tail-associated changes in body drag, given their high flight speeds maintained over a substantial portion of the day.…”

“…Long tails in birds are thought to degrade flight performance by increasing body drag (Evans & Thomas 1992;Balmford et al 1993;Thomas 1993;Norberg 1995) or by affecting foraging rates and aerial manoeuvrability (Evans 1998;Matyjasiak et al 1999Matyjasiak et al , 2000Matyjasiak et al , 2004Park et al 2000;Rowe et al 2001). For the purposes of quantifying how changes in tail morphology influence drag, the tail has been modelled as an isolated flat plate in free-stream flow (Evans & Thomas 1992;Balmford et al 1993;Thomas 1993;Norberg 1995).…”

How the Adaptation Got its Start

“…The existence of feathers is likely an adaptation for a function like regulating body temperature. Later, they became exapted for flight, and in tail feathers, they were exapted to play a role in aiding the bird's maneuverability as it flies (Matyjasiak et al 2004). And still later, in the barn swallow's ancestors, that basic tail feather structure was exapted again for attracting the opposite sex.…”