MOLT, PLUMAGE ABRASION, AND COLOR CHANGE IN LAWRENCE'S GOLDFINCH

A Breeding Plumage in the Making: The Unique Process of Plumage Coloration in the Crested Ibis in Terms of Chemical Composition and Sex Hormones

The Crested Ibis (Nipponia nippon) has long fascinated ornithologists with its enigmatic plumage color change. After more than a century of curiosity, the mystery was finally unraveled in the 1970s, unveiling the mechanism behind this remarkable transformation. Unlike other bird species, the Crested Ibis achieves its nuptial plumage coloration through a unique daubing behavior. After a water-bathing, it applies a sticky black substance secreted by a patch of skin in the neck and throat region. However, the chemical components of this black substance have not been studied in detail until now. To address this issue, we conducted a study to detect the components of the black substance and explore the relationship between sex hormone levels and the secretion of the black substance. We used enzyme-linked immunosorbent assay (ELISA) to measure the monthly changes in steroid hormone levels (estradiol E2, testosterone T, and progesterone PROG) levels in feces. We also analyzed the correlation between sex hormone levels and daubing behavior. The results showed that the sex hormone levels are closely related to the secretion and application of the black substance. In addition, we qualitatively analyzed the chemical components of the black substance using gas chromatography-mass spectrometry (GC-MS), uncovering the presence of 117 distinct chemical components. We assume that the black coloration results from the polymerization of selected chemical constituents among these components. These findings provide a groundwork for further exploration into the biological significance of the black substance. Overall, our study detected components in the black substance and studied how sex hormone levels relate to its secretion. Understanding the hormone effects on coloration helps in precise habitat management, like wetland preservation, crucial for Crested Ibis survival. Implementing hormone-boosting measures during breeding seasons enhances reproduction and health, vital for their conservation.

The color of greater flamingo feathers fades when no cosmetics are applied

Greater flamingos use cosmetic coloration by spreading uropygial secretions pigmented with carotenoids over their feathers, which makes the plumage redder. Because flamingos inhabit open environments that receive direct solar radiation during daytime, and carotenoids bleach when exposed to solar radiation, we expected that the plumage color would fade if there is no maintenance for cosmetic purposes. Here, we show that the concentrations of pigments inside feathers and on the surface of feathers were correlated, as well as that there was a correlation between the concentrations of pigments in the uropygial secretions and on the surface of feathers. There was fading in color (becoming less red) in feathers that received direct solar radiation when there was no plumage maintenance, but not so in others maintained in darkness. When we controlled for the initial color of feathers, the feathers of those individuals with higher concentration of pigments on the feather surfaces were those that lost less coloration after experimental exposure of feathers to sunny conditions. These results indicate that exposure to sunlight is correlated with the fading of feather color, which suggests that individuals need to regularly apply makeup to be more colorful. These results also reinforce the view that these birds use cosmetic coloration as a signal amplifier of plumage color. This may be important in species using highly variable habitats, such as wetlands, since the conditions experienced when molting may differ from those when the signal should be functional, usually months after molting.

Evolution of long-term coloration trends with biochemically unstable ingredients

The evolutionarily persistent and widespread use of carotenoid pigments in animal coloration contrasts with their biochemical instability. Consequently, evolution of carotenoid-based displays should include mechanisms to accommodate or limit pigment degradation. In birds, this could involve two strategies: (i) evolution of a moult immediately prior to the mating season, enabling the use of particularly fast-degrading carotenoids and (ii) evolution of the ability to stabilize dietary carotenoids through metabolic modification or association with feather keratins. Here, we examine evolutionary lability and transitions between the two strategies across 126 species of birds. We report that species that express mostly unmodified, fast-degrading, carotenoids have pre-breeding moults, and a particularly short time between carotenoid deposition and the subsequent breeding season. Species that expressed mostly slow-degrading carotenoids in their plumage accomplished this through increased metabolic modification of dietary carotenoids, and the selective expression of these slow-degrading compounds. In these species, the timing of moult was not associated with carotenoid composition of plumage displays. Using repeated samples from individuals of one species, we found that metabolic modification of dietary carotenoids significantly slowed their degradation between moult and breeding season. Thus, the most complex and colourful ornamentation is likely the most biochemically stable in birds, and depends less on ecological factors, such as moult timing and migration tendency. We suggest that coevolution of metabolic modification, selective expression and biochemical stability of plumage carotenoids enables the use of unstable pigments in long-term evolutionary trends in plumage coloration.

Feasibility study: fluorescence lidar for remote bird classification

We present a method for remote classification of birds based on eye-safe fluorescence lidar techniques. Mechanisms of quenching are discussed. Plumage reflectance is related to plumage fluorescence. Laboratory measurements on reflectance and fluorescence are presented, as well as test-range measurements. Also we present examples of birds' in-flight lidar returns. The methods are suitable for studies of night migrating species and high-altitude classification with implications for the detailed understanding of bird migration and global virus spread.

Seasonal changes in colour: a comparison of structural, melanin- and carotenoid-based plumage colours

Background

Plumage coloration is important for bird communication, most notably in sexual signalling. Colour is often considered a good quality indicator, and the expression of exaggerated colours may depend on individual condition during moult. After moult, plumage coloration has been deemed fixed due to the fact that feathers are dead structures. Still, many plumage colours change after moult, although whether this affects signalling has not been sufficiently assessed.

Methodology/Principal Findings

We studied changes in coloration after moult in four passerine birds (robin, Erithacus rubecula; blackbird, Turdus merula; blue tit, Cyanistes caeruleus; and great tit, Parus major) displaying various coloration types (melanin-, carotenoid-based and structural). Birds were caught regularly during three years to measure plumage reflectance. We used models of avian colour vision to derive two variables, one describing chromatic and the other achromatic variation over the year that can be compared in magnitude among different colour types. All studied plumage patches but one (yellow breast of the blue tit) showed significant chromatic changes over the year, although these were smaller than for a typical dynamic trait (bill colour). Overall, structural colours showed a reduction in relative reflectance at shorter wavelengths, carotenoid-based colours the opposite pattern, while no general pattern was found for melanin-based colours. Achromatic changes were also common, but there were no consistent patterns of change for the different types of colours.

Conclusions/Significance

Changes of plumage coloration independent of moult are probably widespread; they should be perceivable by birds and have the potential to affect colour signalling.

Soil and preen waxes influence the expression of carotenoid-based plumage coloration

The signaling function of carotenoid-based plumage is mainly determined by the concentration of pigments in feathers. For this reason, most studies of the proximate control of coloration focus on processes during and preceding moult. In great tits Parus major, past research demonstrates that carotenoid-based plumage coloration honestly indicates male quality and, thus, may be a sexually selected signal. In this study, we investigate how dirt and preen oil influence the coloration of carotenoid-based feathers in the great tit. We collected six feathers from each individual bird; three feathers served as controls while the remaining three feathers were washed with a chloroform/methanol mixture to remove soil and preen waxes. We assessed plumage coloration using digital photography. This cleaning procedure slightly enhanced ornamentation; the experimentally cleaned feathers expressed hues shifted towards shorter wavelengths and expressed brighter overall coloration than control feathers. This is the first experimental study conducted on wild birds demonstrating that, in addition to pigment concentration, the presence of preen waxes and soils on feathers may contribute to variation in coloration.

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.

Plumage color as a dynamic trait: carotenoid pigmentation of male house finches (Carpodacus mexicanus) fades during the breeding season

Evolutionary biologists studying sexually selected bird plumage generally consider this trait to be static throughout a breeding season and assign trait values to individuals on the basis of single measurements. We investigated the propensity for carotenoid-based color of feather patches in male house finches (Carpodacus mexicanus ( Muller, 1776)) to change during the breeding period. We recaptured and rescored 63 males and found that the hue of feathers faded significantly over the season. The degree of hue change was a direct function of the amount of time between plumage scores; feathers faded more as the interval between measurements increased. The magnitude of hue change was not, however, related to an individual's age or initial plumage redness, which suggests that certain birds are not more or less prone to fading. Collectively, these findings imply that researchers should more carefully track plumage color expression during the course of a year, as seasonal color shifts may have important consequences for late-season male–male competitive interactions and flexible female mating tactics (e.g., social mate switching, choice of extra-pair partners). Potential mechanisms for this seasonal plumage color shift are discussed.