Food, feathers and fluctuating asymmetries

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Abstract
Nutritional, or energetic, stress has been implicated as a causal factor in the inter-individual differences in levels of fluctuating asymmetry in the elongated tails of male swallows (Hirundo rustica). However, there has been no direct experimental test of this hypothesis. We report results from an experiment that has investigated the effects of sequential food deprivations on levels of fluctuating asymmetry in primary feathers and chest plumage of the moulting female European starling (Sturnus vulgaris). Birds were housed individually and taken through their full moult by means of photoperiodic manipulation. During this period, some of the birds experienced food deprivations. At regular intervals, we recorded the length of every primary feather, quantified the chest plumage via video-image analysis, and recorded body mass and subcutaneous fat score measurements. We found that increasing levels of nutritional or energetic stress caused larger development asymmetries, which would appear to have increased fitness costs. Primary feather asymmetry showed a negative relation with amount of subcutaneous fat stored during moult and a positive relation with social dominance. This is the first study to show, directly, the importance of energetic stress in the production of feather and plumage asymmetries. Low levels of feather asymmetries were also associated with an indicator of reproductive status (chest `spottiness'). These findings support the recent literature, suggesting that degree of fluctuating asymmetry, even in traits under stabilizing selection, may reveal aspects of individual fitness value.