COLOR VARIATION AMONG NESTLING BROWN-HEADED COWBIRDS (MOLOTHRUS ATER) DOES NOT REFLECT DIFFERENTIAL SUCCESS WITH HOSTS IN TEXAS

ABSTRACT.-

As brood parasites, nestling Brown-headed Cowbirds (Molothrus ater) may exhibit characters that improve their fledging success when reared alongside host young. The coloration of mouthparts of nestlings can influence adult care and, thus, the polymorphism of yellow or white rictal flanges among nestling Brown-headed Cowbirds may reflect differential success with different hosts on the basis of flange color. Moreover, because Brown-headed Cowbirds in the southern United States co-occur with Bronzed Cowbirds (M. aeneus), whose young have white flanges, cowbird nestlings' flange colors may reflect a means for reducing interspecific competition through partitioning of host species on the basis of nestling flange color. To determine whether flange color influences cowbird fledging success with hosts of either color, we recorded the flange colors of cowbirds and their hosts at a site in Texas. We also tested whether flange color was influenced by nestling sex. Most hosts of Brown-headed Cowbirds had young with yellow flanges (81%, n = 16 spp.), yet Brown-headed Cowbirds with white flanges were more common (61%, n = 107). Bronzed Cowbirds parasitized primarily species whose young had white flanges (86%, n = 348 eggs). Despite the differential use of hosts with regard to flange color, the frequencies of each were similar among nestling and juvenile Brown-headed Cowbirds. Likewise, the frequencies of each color did not differ significantly between the sexes. Therefore, we suggest that a flange color matching that of nestmates is not strongly selected for by hosts.Received 3 October 2003, accepted 25 April 2006.

Key words: bill color, Bronzed Cowbird, Brown-headed Cowbird, Molothrus aeneus, M. ater, nestling discrimination, polymorphism.

La Variación del Color entre Polluelos de Molothrus ater no Refleja Éxito Diferencial al Ser Criados por Especies Hospederas en Texas

RESUMEN.-Como un ave parásita de nido, Molothrus ater puede exhibir caracteres que aumentan su éxito de emplumamiento cuando sus polluelos son criados junto a los de su hospedero. La coloración de la boca de los polluelos puede influenciar el cuidado por parte de los padres, por lo que el polimorfismo de la coloración blanca o amarilla de la parte lateral de la boca de los polluelos de M. ater podría conducir a diferencias en el éxito al ser criados por especies hospederas. Además, debido a que M. ater coexiste con M. aeneus, cuyos polluelos tienen la parte lateral de la boca de color blanco, la coloración de esta parte en los polluelos de Malotrus podria reflejar un medio por el cual se reduce la competencia interespecifica a través de la partición de las especies hospederas. Para determinar si la coloracion de la parte lateral de la boca influencia el éxito de emplumamiento de los polluelos con hospederos de distintos colores, registramos esta coloración de los polluelos de Molothrus y de sus hospederos en una localidad en Texas. También probamos si esta coloración estaba influenciada por el sexo de los polluelos. La mayoría de las especies hospederas de M. citer tuvieron polluelos con la parte latéral de la boca de color amarillo (81%, n = 16 spp.), pero los polluelos de M. ater tuvieron una mayor frecuencia de coloración blanca (61%, n = 107). Molothrus aeneus parasitó principalmente a especies cuyos polluelos tenian la parte lateral de la boca de color blanco (86%, n = 348 huevos). A pesar del uso diferencial de hospederos con respecto a la coloración de la boca, las frecuencias de cada color fueron similares entre polluelos y juveniles de M. ater. De modo similar, las frecuencias de cada color no fueron diferentes entre sexos. Sugerimos que un color de la boca igual al de los polluelos compañeros en el nido no es un rasgo que se encuentra bajo una presión selectiva fuerte por parte de los hospederos.

AFTER HATCHING, FEATURES of the gape and associated movements are some of the earliest forms of communication between young and adult songbirds. Internal (gape lining, mouth markings or nodules or both) and external (tomium proper, rictal flange; see Fig. 1) mouthparts of nestlings are important features of nestling communication. For some species, gape coloration is an important indicator of nestling physiological status (Kilner 1998, Clotfelter et al. 2003, Saino et al. 2003). In species exposed to brood parasitism, coloration of both internal and external mouthparts may be associated with discrimination of parasitic young (Nicolai 1974, Payne et al. 2001). Brood parasites, therefore, provide interesting cases for the study of communication between adults and their young, because dissimilar parasitic young that are often reared at a reproductive cost to the hosts (Dearborn et al. 1998, Lorenzana and Sealy 1999) may reveal the primary mechanisms by which nestlings convey their needs to caregivers (Dearborn and Lichtenstein 2002). The external mouthparts of young of most songbirds are usually white or yellow, and the color does not vary within each species (Ficken 1965, Rothstein 1978, Skutch 1976). Therefore, it is noteworthy that the nestlings of several brood parasites are polymorphic in the color of their mouthparts (Cuculiis: Gill 1998; Vidua: Nicolai 1974, Payne et al. 2001; Molothnts: Fraga 1978, Rothstein 1978).