![]() Out of 21 studies using experimental manipulations of stress, seven showed increasing asymmetry with stress, seven did not show any difference and seven showed an increase only with a specific type of stress or in a specific trait( Bjorksten et al., 2000). The effects of stress on asymmetry are, however, not always clearcut. For example, after food deprivation in starlings Sturnus vulgaris, deprived birds had significantly higher levels of asymmetry in their primary feathers( Swaddle and Witter, 1994). The most commonly observed asymmetry in birds occurs in their plumage, particularly during moult. ![]() These environmental stressors include disease, pollution, parasites and food deprivation. Genetic stress occurs during development due to problems such as inbreeding or mutation( Balmford et al., 1993), while environmental stresses may affect individuals at any life stage( Putman and Sullivan, 2000). It is thought that the degree of asymmetry observed between different individuals within a single population reflects the degree of stress encountered by each individual, either directly as an effect of the environment or previously during development. Small differences in the size of bilaterally paired structures, or asymmetry, are very prevalent in nature, occurring in structures as widely different as the fins of Siberian sturgeon Acipenser baeri( Ruben, 1992) to the horns of beetles Onthophagus taurus( Moller, 1992a) or the antlers for fallow deer Dama dama ( Putman and Sullivan, 2000). ![]() There was, however, a significantly increased flight cost when the wing span was reduced without causing asymmetry (increase of 0.45 W paired t-test T=2.3, P=0.03). There was a slight increase in flight cost with both of the asymmetry manipulations (0.5 cm,increase of 0.04 W 1.0 cm, increase of 0.12 W), neither of which reached statistical significance. The mean flight cost in the pre-manipulated birds was 1.90☐.1 W. In this case wing amplitude did not change and wing upstroke slightly decreased, causing an increased wing beat frequency. When the wing area was reduced while maintaining symmetry, birds flew with slower flight speed. They also increased the left wing amplitude and decreased the right up- and downstroke durations to counteract the changes in wing shape, which meant that they had an increase in wing beat frequency. When birds were manipulated to become asymmetric they maintained flight speed. In a separate `control' group ( N=7),approximately 0.25 cm was trimmed off the primary feathers of both wings, to produce the same reduction in wing span as 0.5 cm trimmed from one wing, while maintaining symmetry. In 10 individuals, the primary feathers on the right wing were trimmed first, by 0.5 cm, and then by an additional 0.5 cm in six of these individuals. In addition,simultaneous high-speed video footage enabled differences in flight kinematics such as flight speed, wing amplitude, up- and downstroke duration and wing beat frequency to be examined. ![]() ![]() In this study the 13C-labelled bicarbonate technique was used to measure the energy expended during the flight of zebra finches Taeniopygia guttata,prior to and after experimental manipulation to generate asymmetry and a change in wing span by trimming the primary feathers. Wing asymmetry may have an effect on the kinematics of flight, with knock-on effects for the energetic cost of flying. Asymmetry is a difference in the sizes of bilaterally paired structures. ![]()
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