Plumage convergence resulting from social mimicry in birds? A tetrachromatic view

ABSTRACTSocial mimicry may lead to convergent evolution when interactions with conspecific and heterospecific individuals drive evolution towards similar phenotypes in different species. Several hypotheses accounting for convergence in communication signals based on mechanisms of social mimicry exist, but evaluations of how similar species are given the visual system of receptors of such signals have been ostensibly missing from tests of such hypotheses. We used plumage reflectance measurements and models of avian color discrimination to evaluate the efficacy of visual deception and therefore the plausibility of mimicry hypotheses accounting for plumage convergence among six species of passerine birds in the flycatcher family (Tyrannidae) with strikingly similar plumage. We rejected interspecific social mimicry hypotheses as an explanation for the similarity between one putative model species and putative mimics because deception seems unlikely given the visual system of passerines. However, plumage similarity was consistent with a role for selective pressures exerted by predators because dorsal coloration of putative model and mimic species was indistinguishable by visually oriented raptors. Experiments and behavioral observations are necessary to better characterize social interactions and to test predictions of alternative mimicry hypotheses proposed to account for convergence.

The evolution of mimicry of friarbirds by orioles (Aves: Passeriformes) in Australo-Pacific archipelagos

Observations by Alfred Wallace and Jared Diamond of plumage similarities between co-occurring orioles ( Oriolus ) and friarbirds ( Philemon ) in the Malay archipelago led them to conclude that the former represent visual mimics of the latter. Here, we use molecular phylogenies and plumage reflectance measurements to test several key predictions of the mimicry hypothesis. We show that friarbirds originated before brown orioles, that the two groups did not co-speciate, although there is one plausible instance of co-speciation among species on the neighbouring Moluccan islands of Buru and Seram. Furthermore, we show that greater size disparity between model and mimic and a longer history of co-occurrence have resulted in a stronger plumage similarity (mimicry). This suggests that resemblance between orioles and friarbirds represents mimicry and that colonization of islands by brown orioles has been facilitated by their ability to mimic the aggressive friarbirds.

Fat quill secretion in pigeons: could it function as a cosmetic?

In order to maintain functionality of the plumage, birds apply a variety of fatty substances to the feathers, usually derived from the preen gland. Such secretions could also have cosmetic functions, altering plumage reflectance, and there is indeed some evidence for such cosmetic functions of preen fats. Some birds, noticeably pigeons, use fats derived from modified feathers, so called fat quills, in a presumably similar fashion, but almost nothing is known about their putative functions. Here we present a first test of the possibility that fat quill secretion could function as a cosmetic. As models we used domestic pigeon races that produce fat quills, we first confirm their extensive use in plumage maintenance behaviour. We then assessed the effect of experimental addition and removal fat quill secretion on reflectance of white feathers using reflectance spectrometry and physiological models of avian colour vision. Addition of secretion to cleaned white feathers resulted in a significant, discriminable change in the feather reflectance spectrum, which was strikingly similar to the absorbance spectrum of passerine preen gland secretion. However, like previous studies on preen gland secretion, removal of fats from intact feathers did not significantly alter feather reflectance, indicating that fat quill secretion when present in quantities as applied by the birds does not noticeably affect plumage reflectance. Therefore we conclude that the potential for the fat quill secretion to modify plumage colour appears limited at best, and other functions, such as antibiotic defense against feather-degrading bacteria, should be considered.

Cryptic Dichromatism and Seasonal Color Variation in the Diademed Tanager

We studied the patterns of sexual dichromatism and seasonal variation in plumage color in the Diademed Tanager (Stephanophorus diadematus), a species previously considered devoid of variation in adult plumage. The general coloration of this species is dark blue-violet, with a white-blue and red crown. Plumage reflectance of seven body regions from 33 study skins belonging to adults of both sexes was measured. Reflectance values were used in a principal components analysis (PCA) and hue, short-wave chroma, and UV chroma were also measured directly on the spectra. Both PCA factor scores and these latter variables were subjected to two-way ANCOVAs with sex and season as main factors and the year of capture as a covariate. We found that crowns of males were significantly brighter than those of females. In addition, the nape, chest, and belly showed significant differences in spectral shape, with relatively greater short-wave reflectance and less long-wave reflectance in males than in females. Although sexes were alike in hue, they differed in chroma in almost all body regions. Brightness also differed between seasons, and contrary to our expectation nonbreeding birds were brighter than breeding ones. This result may be a consequence of the particular molt program of tanagers that includes only a complete post-reproductive molt. Despite finding seasonal differences in spectral shape in various body regions, no significant changes in hue, short-wave chroma, or UV chroma were evident. To our knowledge, this is the first report of variation in adult plumage color for the Diademed Tanager, and we suggest that dichromatism in tanagers may be even more pervasive than is currently recognized. Dicromatismo Críptico y Variación Estacional de Color en Stephanophorus diadematus Resumen. Estudiamos los patrones de dicromatismo sexual y variación estacional en la coloración del plumaje de Stephanophorus diadematus, una especie previamente considerada carente de variación en la coloración del plumaje adulto. La coloración general de esta especie es azul violáceo oscuro, con una corona blanca azulada y roja. Se midió la reflectancia de siete regiones corporales en 33 pieles de estudio pertenecientes a adultos de ambos sexos. Los valores de reflectancia se utilizaron en un análisis de componentes principales, y además se midieron el tono (hue), la intensidad del color de onda corta y la intensidad del color de UV directamente sobre los espectros. Tanto los factores del análisis de componentes principales como las variables mencionadas fueron sujetos a ANCOVAs de dos factores, considerando el sexo y la estación como factores principales, y el año de captura como covariable. Estos análisis mostraron que la corona de los machos es significativamente más brillante que la de las hembras. Además, la nuca, el pecho y el vientre mostraron diferencias significativas en la forma espectral, presentando los machos mayor reflectancia en la zona de onda corta y menor en la zona de onda larga que las hembras. Si bien el tono no difirió entre sexos, la intensidad del color difirió en la mayoría de las regiones corporales entre machos y hembras. El brillo también difirió entre temporadas y, contrariamente a nuestra expectativa, los individuos capturados en la temporada no reproductiva fueron más brillantes que aquellos capturados en la temporada reproductiva. Este resultado podría deberse al programa de muda particular presente en Thraupidae, que incluye una única muda post-reproductiva completa. Si bien encontramos diferencias entre estaciones en la forma espectral en varias regiones corporales, no se detectaron diferencias en el tono, la intensidad del color de onda corta ni la intensidad del color de UV. Este es, de acuerdo a nuestro conocimiento, el primer estudio que muestra variación en la coloración del plumaje adulto de S. diadematus. Sugerimos que el dicromatismo en Thraupidae podría ser más común de lo que actualmente se piensa.