The outcome of social and sexual competition in animals is typically mediated through the expression of body traits. Conspicuous characters such as yellow, orange, and red colorations in skin, scales, and feathers are often posited as quality-dependent signals, because such colors are made of dietary carotenoids and their use for signaling conflicts with health functions. Raptors often lack brightly colored feathers but most diurnal species display intense orange and yellow hues in the cere and legs. Here we test the hypothesis that integument coloration functions as a signal of status in wild raptors, revealing availability of carotenoid pigments. As study model we used the Black Kite (Milvus migrans), a highly social, long-lived, and sexually monogamous Accipitriform. Regular trapping of adults throughout the breeding season revealed that circulating carotenoid levels were highest in breeding males, whereas breeding females and floaters showed moderate and statistically similar carotenoid titers. Plasma levels of carotenoids showed a slight seasonal decline, especially marked in post-laying females. Leg and cere colorations were visually assessed through comparison to a color chart, yielding a very high interobserver reliability and consistency with simultaneous spectrophotometric measures. Integument color was similar between the sexes, brighter in breeders compared to floaters, and positively related to circulating carotenoids, but only in the floaters. These results suggest that the physiological regulation of signal expression is different in breeders and floaters, possibly involving a costly social or physiological mechanism that ensures the honesty of coloration as a quality-dependent signal. In addition, breeding males and females displayed similar color scores despite strong differences in plasma carotenoids, indicating sex-related differences in physiological regulation that were not apparent in floaters. Our results are consistent with a role of integument coloration in status signaling in wider competitive contexts than those enforced by sexual selection. The reported patterns of coloration can be ultimately explained by two alternative, non-mutually exclusive hypotheses: (1) the lower coloration displayed by floaters reflects underlying physiological limitations and mediates the access to breeding resources through social competition (constraint hypothesis), and (2) young floaters have evolved mechanisms to restrain color expression and thus signal their competitive inferiority, avoiding physiological and social costs (restraint hypothesis).
El resultado de la competencia social y sexual en animales es a menudo facilitado por la expresión de caracteres corporales. Las tonalidades amarillas, anaranjadas y rojas que colorean la piel, escamas o plumas de los vertebrados suelen funcionar como señales indicadoras de la calidad del individuo, pues dependen de la ingesta de pigmentos carotenoides y su utilización ornamental impide otros usos relacionados con la salud. Las aves rapaces suelen carecer de plumajes vivamente coloreados, pero la mayoría de las especies diurnas muestran intensas tonalidades amarillas y anaranjadas en la piel de la cera y las patas. En el presente estudio comprobamos la hipótesis de que la coloración del tegumento funciona como una señal de estatus social en rapaces silvestres, revelando la disponibilidad de pigmentos carotenoides. Como modelo de estudio utilizamos a Milvus migrans, un Accipitriforme altamente social, longevo y monógamo. La captura y muestreo regular de adultos a lo largo de la estación de cría mostró niveles máximos de carotenoides circulantes en los machos reproductores, mientras que las hembras y los individuos flotantes tuvieron niveles moderados y estadísticamente similares. Las concentraciones plasmáticas de carotenos sufrieron una ligera disminución estacio
