An Analysis of Physical, Physiological, and Optical Aspects of Avian Coloration with Emphasis on Wood- Warblers
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Journal Article Melanin and the Abrasion Resistance of Feathers Get access Richard H. C. Bonser Richard H. C. Bonser School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 I UC, U.K. Search for other works by this author on: Oxford Academic Google Scholar The Condor, Volume 97, Issue 2, 1 May 1995, Pages 590–591, https://doi.org/10.2307/1369048 Published: 01 May 1995 Article history Received: 22 November 1994 Accepted: 21 December 1994 Published: 01 May 1995
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Three major classes of hypotheses have been developed to account for the colors of animals: (1) Selection is for the physical or chemical properties of the biochromes; (2) selection is for color patterns that enhance the animals' own vision; and (3) selection is for colors that serve as optical signals for other animals.
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We sampled bacteria from the plumage of 1,588 individuals of 83 species of birds. Feather-degrading bacteria, those able to extract energy and nutrients by breaking up β-keratin, were isolated from 134 individuals in 32 species. Nine of 11 samples of feather-degrading (keratinolytic) bacteria were identified as Bacillus licheniformis, one as B. pumilus, and one as a Bacillus of undetermined species. A strong correlation between occurrence of keratinolytic bacilli and the number of birds sampled per species suggests that feather-degrading bacilli are widespread among birds. The bacillus occurred on 6.7 to 10.7% of birds and showed little annual variation. The incidence of birds with feather-degrading bacilli was highest in late fall and winter and lowest in early spring and late summer. The bacilli occurred most frequently on the venter and less commonly on the dorsum and tail. They occurred most frequently on ground-foraging species and least frequently on aerial-foraging species. Regardless of avian species, time of year, or area of the bird from which the bacilli were isolated, the rate at which bacilli degraded feathers was similar. Because bacilli are active only when conditions are warm and humid, we suggest that they degrade feathers during the summer when the bird becomes wet, for example during thunderstorms. Such feather degradation may contribute to the deterioration of feathers and be a selective force in the evolution and timing of molt.
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An Analysis of Physical, Physiological, and Optical Aspects of Avian Coloration with Emphasis on Wood-Warblers Get access John A. Endler John A. Endler Department of Biological Sciences, University of California, Santa Barbara, CA 93106 Search for other works by this author on: Oxford Academic Google Scholar The Condor, Volume 89, Issue 3, 1 August 1987, Pages 680–681, https://doi.org/10.2307/1368665 Published: 01 August 1987
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While exaggerated length and ornamental shapes are confirmed sexually selected tail traits in birds, the signal function of tail markings has received less study. Signal roles for tail markings as amplifiers of length, shape, and feather quality are discussed, and the role of tail markings as feather-quality handicaps is proposed: absence of melanin increases damage and abrasion. Predicted correlations of tail markings with other tail traits are derived for these signal roles. A comparative study of the relationships between these tail traits in an entire avifauna, the western Palearctic, tested the predictions. Tail displays were present in nearly 80% of species, associated with greater long-tailedness, but not all displayed tails had markings or ornamental shape. The incidence of marks across tail shapes and the combinations of marks indicate that tail markings act as handicaps or amplifying handicaps of tail feather quality. Tail elongation and ornamental shapes could act as additional handicaps of feather quality-that is, they could be multipurpose signals. Incorporation of revealing indicators such as feather damage and associated handicap and/or amplifying traits may allow a reduction in the cost of signaling while maintaining signal reliability and, hence, influence sexual selection in complex signaling systems.
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The aims in this paper are first to review theories of the evolution of bird coloration and, in some cases, partly revise and extend them, secondly to analyse the coloration of all the birds of a given geographical region using multiple regression, and thirdly on the basis of this analysis to evaluate the various theories. Theories . There have been many discussions of the selective forces acting on the coloration of birds and we review the major proposals in some detail. The earliest suggestion (Darwin 1871) was that the bright coloration of many male birds originated through sexual selection by female choice of the most exotic variants in male plumage. A conflicting view (Hingston 1933) is that brightness has an intimidatory effect on opponents, and that bright male coloration arose through sexual selection but largely due to its advantages against other males in disputes concerning mating access to females. Bird coloration has also been considered in terms of predation (see, for example, Cott 1964 a ). Some birds with bright plumage patterns are known to be unpalatable compared to cryptic species, and certain other patterns have been interpreted as adaptations to confuse predators. Bright colours may commonly be favoured when an individual is anyhow obvious (e.g. through activity) and where it represents an 'unprofitable’ prey for a predator. This interpretation may be particularly relevant to lekking among polygamous males. A special case of unprofitable prey may be ‘perception advertisement’, where an animal signals (by flash patterns or alarm calls) that it has seen a predator (or opponent). It is also possible that bright coloration may serve to deflect predators away from the nest site; this requires in many ways conditions opposite to those for the unprofitable prey solution. Finally, bird colorations can act as a variety of social signals other than threat. Analysis . The coloration of the 516 species of birds that breed and/ or winter in the Western Palaearctic was analysed by multiple regression. Seven regions of a bird’s body plus two areas of flash coloration were recognized and scored for colour on a cryptic/conspicuous scale. Five different age/sex/season categories were recognized and scored separately for each species. These dependent variables were each analysed with respect to 17 independent variables that reflect different aspects of the reproductive biology and ecology of the birds. The advantages and disadvantages of multiple regression as an analytical technique are discussed. Results . The analysis identifies associations between the reproductive biology and ecology of the birds and the coloration of the different regions of the body of the different age/sex/season classes. Apparent exceptions to these associations are also identified and discussed. A relatively large proportion of the associations made sense in terms of the theories presented and usually there was a strong implication that for any specific association one theory was more relevant than any of the others. The results indicate that by far the most potent selective pressures to have shaped bird coloration are those related to predation risk. A number of the theories make use of predation risk, each in a different way, and for most of them some support can be gained for their involvement in the evolution of bird coloration. Of all the theories, however, it is the unprofitable prey model that seems to account for the major part of the variation in bird coloration. By contrast, no clear evidence for the involvement of sexual selection in the evolution of bird coloration could be found. Indeed, many associations, such as that between sexual dimorphism and polygamy, were more readily explicable in terms of selection pressures due to predation risk than of sexual pressures. The suggestion that bird coloration is shaped by predation rather than by sexual selection in no way prevents the coloration, as it evolves, being incorporated within the species and sex recognition system. Conclusions . It is concluded that bird coloration has evolved almost entirely in response to predation-based selective pressures. Although plumage and coloration are involved in species and sex recognition systems, they have not evolved in response to sexual selection pressures. In species that are sexually dimorphic, the male is not brightly coloured as a result of female choice or male: male competition but because he represents a less profitable prey to a predator than the females and juveniles. We predict that brightly coloured birds will most often be found to suffer less from predation than will comparable more cryptic birds (though one of the predation-risk theories does allow the converse to be true).
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Introduction The nature of the following work will be best understood by a brief account of how it came to be written. During many years I collected notes on the origin or descent of man, without any intention of publishing on the subject, but...
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In studies of animal colouration it is no longer necessary to rely on subjective assessments of colour and conspicuousness, nor on methods which rely upon human vision. This is important because animals vary greatly in colour vision and colour is context-dependent. New methods make it practical to measure the colour spectrum of pattern elements (patches) of animals and their visual backgrounds for the conditions under which patch spectra reach the conspecific's, predator's or prey's eyes. These methods can be used in both terrestrial and aquatic habitats. A patch's colour is dependent not only upon its reflectance spectrum, but also upon the ambient light spectrum, the transmission properties of air or water, and the veiling light spectrum. These factors change with time of day, weather, season and microhabitat, so colours must be measured under the conditions prevalent when colour patterns are normally used. Methods of measuring, classifying and comparing colours are presented, as well as techniques for assessing the conspicuousness of colour patterns as a whole. Some implications of the effect of environmental light and vision are also discussed.
Colour Vision
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