Habitat structure, stress and plumage development

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Abstract
Past studies have demonstrated that birds' perceived risk of predation is influenced by habitat structure, notably the availability of a refuge and the location of food. Thus apparently dangerous habitats, such as those without a refuge, may act as stressors to birds. We test the hypothesis that habitat structure influences stress by manipulating cover availability and food location in European starlings, Sturnus vulgaris, housed in large flight aviaries during moult. We use degree of fluctuating asymmetry (FA) and the number of fault bars in primary feathers to measure stress. The manipulations had no effect on the overall timing or rate of moult, or on the final size and shape of the wings. However, birds from aviaries with cover had lower levels of FA and fewer faults than birds from aviaries without cover. Food location also influenced FA, with birds from aviaries with food located near to cover having lower levels of asymmetry; however, this effect was weak and transitory, only influencing the first primary growth during moult. Food location did not influence the number of fault bars. These results indicate that manipulations of habitat structure can influence stress. Although plumage development may be influenced by perceived risk of predation per se, the possible role of other proximate determinants of plumage abnormalities such as changes in feeding patterns, activity levels, and incidence of mechanical damage are also considered.