Resistance of flight feathers to mechanical fatigue covaries with moult strategy in two warbler species

Flight feather moult in small passerines is realized in several ways. Some species moult once after breeding or once on their wintering grounds; others even moult twice. The adaptive significance of this diversity is still largely unknown. We compared the resistance to mechanical fatigue of flight feathers from the chiffchaff Phylloscopus collybita, a migratory species moulting once on its breeding grounds, with feathers from the willow warbler Phylloscopus trochilus, a migratory species moulting in both its breeding and wintering grounds. We found that flight feathers of willow warblers, which have a shaft with a comparatively large diameter, become fatigued much faster than feathers of chiffchaffs under an artificial cyclic bending regime. We propose that willow warblers may strengthen their flight feathers by increasing the diameter of the shaft, which may lead to a more rapid accumulation of damage in willow warblers than in chiffchaffs

Bacteria and the evolution of honest signals. The case of ornamental throat feathers in spotless starlings

1. Mechanisms guaranteeing reliability of messages are essential in understanding the underlying information and evolution of signals. Micro-organisms may degrade signalling traits and therefore influence the transmitted information and evolution of these characters. The role of micro-organisms in animal signalling has, however, rarely been investigated. 2. Here, we explore a possible role for feather-degrading bacteria driving the design of ornamental throat feathers in male spotless starlings (Sturnus unicolor). We estimated length, bacterial load, degradation status and susceptibility to degradation by keratinolytic bacteria in those feathers, compared with non-ornamental adjacent feathers in males, as well as to throat feathers in females. In addition, the volume of the uropygial gland and its secretion was measured and the secretion extracted. We also experimentally evaluated the capacity of each secretion to inhibit growth of a keratinolytic bacterium. 3. The apical part of male ornamental throat feathers harboured more bacteria and degraded more quickly than the basal part; these patterns were not detected in female throat feathers or in non-ornamental male feathers. Moreover, degradation status of male and female throat feathers did not differ, but was positively associated with feather bacterial density. Finally, the size of the uropygial gland in both males and females predicted volume and the inhibitory capacity of secretion against feather-degrading bacteria. Only in males was uropygial gland size negatively associated with the level of feather degradation. 4. All results indicate differential susceptibility of different parts of throat feathers to keratinolytic bacterial attack, which supports the possibility that throat feathers in starlings reflect individual ability to combat feather-degrading bacteria honestly. This is further supported by the relationship detected between antimicrobial properties of uropygial secretion and the level of feather degradation. 5. Our results suggest that selection pressures exerted by feather-degrading bacteria on hosts may promote evolution of particular morphologies of secondary sexual traits with different susceptibility to bacterial degradation that reliably inform of their bacterial load. Those results will help to understand the evolution of ornamental signals.This work was financed by Spanish Ministerio de Ciencia e Innovaci on, European funds (FEDER) (CGL2010-19233-C03-01, CGL2013-48193-C3-1-P). MRR and DMG received a postdoc from the program “JAE-Doc”, GT from the “Juan de la Cierva”, and CRC had a predoctoral fellowship, all from the Spanish Government.Peer reviewe

From Abbott Thayer to the present day: what have we learned about the function of countershading?

Of the many visual characteristics of animals, countershading (darker pigmentation on those surfaces exposed to the most lighting) is one of the most common, and paradoxically one of the least well understood. Countershading has been hypothesized to reduce the detectability of prey to visually hunting predators, and while the function of a countershaded colour pattern was proposed over 100 years ago, the field has progressed slowly; convincing evidence for the protective effects of countershading has only recently emerged. Several mechanisms have been invoked for the concealing function of countershading and are discussed in this review, but the actual mechanisms by which countershading functions to reduce attacks by predators lack firm empirical testing. While there is some subjective evidence that countershaded animals match the background on which they rest, no quantitative measure of background matching has been published for countershaded animals; I now present the first such results. Most studies also fail to consider plausible alternative explanations for the colour pattern, such as protection from UV or abrasion, and thermoregulation. This paper examines the evidence to support each of these possible explanations for countershading and discusses the need for future empirical work

Environmental-induced acquisition of nuptial plumage expression: a role of denaturation of feather carotenoproteins?

Several avian species show a bright carotenoid-based coloration during spring and following a period of duller coloration during the previous winter, despite carotenoids presumably being fully deposited in feathers during the autumn moult. Carotenoid-based breast feathers of male linnets (Carduelis cannabina) increased in hue (redness), saturation and brightness after exposing them to outdoor conditions from winter to spring. This represents the first experimental evidence showing that carotenoid-based plumage coloration may increase towards a colourful expression due to biotic or abiotic environmental factors acting directly on full-grown feathers when carotenoids may be fully functional. Sunlight ultraviolet (UV) irradiation was hypothesized to denature keratin and other proteins that might protect pigments from degradation by this and other environmental factors, suggesting that sunlight UV irradiation is a major factor in the colour increase from winter to spring. Feather proteins and other binding molecules, if existing in the follicles, may be linked to carotenoids since their deposition into feathers to protect colourful features of associated carotenoids during the non-breeding season when its main signalling function may be relaxed. Progress towards uncovering the significance of concealment and subsequent display of colour expression should consider the potential binding and protecting nature of feather proteins associated with carotenoids

Melanin pigmentation negatively correlates with plumage preening effort in barn owls

1. According to the handicap principle of sexual selection, colourful ornaments honestly signal absolute quality only if they entail fitness costs. The degree of melanism often covaries positively with aspects of individual quality, and hence melanin-based coloration should be costly to produce or to maintain in a good shape. This is, however, unlikely because melanin-based coloration is often strongly heritable and in birds the rate of feather wear decreases with the amount of melanin pigments packed in feathers. 2. The hypothesis that melanin pigments reduce the cost of maintaining colourful ornaments in a good shape predicts a negative correlation between the degree of melanism and both the size of the uropygial gland that produces preening secretions and the intensity of preening behaviour. 3. Using a correlative approach, I evaluated these two predictions in the barn owl Tyto alba in which the body underside varies from immaculate to heavily marked with black spots, a eumelanin-based trait, and from white to reddish-brown, a phaeomelanin-based trait. I correlated plumage traits with preening behaviour in nestlings and with the size and mass of the uropygial gland in dead adults. I also weighed nonornamental wing and tail feathers to assess whether the quality of nonornamental feathers is positively correlated with the degree of melanism of an ornamental plumage trait. 4. The degree of phaeomelanism was neither associated with preening behaviour nor with the size and mass of the uropygial gland. In line with the two predictions, individuals with more and larger black spots had a lighter uropygial gland and preened less frequently. Because nonornamental wing and tail feathers of spottier individuals were heavier per unit of surface area, the entire plumage of eumelanic individuals may be more robust and in turn require less care than the plumage of nonmelanic conspecifics. 5. In conclusion, the degree of eumelanism can be associated with aspects of individual quality even if eumelanic ornaments are neither costly to produce nor to maintain in a good shape. Document Type: Articl