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Abstract

Hybridization is predicted to occur most frequently in closely related species when one species is rare (scarcity of mates hypothesis) and when the rare species of a heterospecific pair is more likely to be female (sexual selection hypothesis). We studied hybridization in mixed breeding colonies of the Western Grebe (Aechmophorus occidentalis) and Clark’s Grebe (A. clarkii) at Clear Lake, California, during 2011–2019. Of 203 mated pairs with at least one Clark’s Grebe (the rarer species, attending 15% of nests), 77.3% were conspecific pairs, 20.7% were heterospecific pairs, and 2.0% comprised a Clark’s Grebe paired with a presumed hybrid. The proportion of heterospecific pairs was no greater in smaller colonies or later during the breeding season, contradicting the scarcity of mates hypothesis. Of 42 heterospecific pairs, the Clark’s Grebe was just as likely to be male (45.2%) as female (54.8%), contradicting the sexual selection hypothesis. Hybridization between these species more likely results from an error in sexual imprinting due to interspecific brood parasitism or from extra-pair copulations.


Se predice que la hibridacion ocurrira con mayor frecuencia en especies estrechamente relacionadas cuando una es rara (hipotesis de escasez de parejas) y es mas probable que la especie rara de una pareja heteroespecifica sea hembra (hipotesis de seleccion sexual). Estudiamos la hibridacion en colonias reproductoras mixtas del achichilique occidental (Aechmophorus occidentalis) y achichilique pico naranja (A. clarkii) en Clear Lake, California, durante 2011–2019. De 203 parejas apareadas con al menos un achichilique pico naranja (la especie mas rara, que atiende al 15% de los nidos), el 77.3% eran parejas conespecificas, el 20.7% eran parejas heteroespecificas, y el 2.0% comprendia un achichilique pico naranja emparejado con un presunto hibrido. La proporcion de parejas heteroespecificas no fue mayor en colonias mas pequenas o mas tarde durante la temporada reproductiva, contradiciendo la hipotesis de la escasez de parejas. De 42 parejas heteroespecificas, el achichilique pico naranja tenia la misma probabilidad de ser macho (45.2%) que hembra (54.8%), contradiciendo la hipotesis de la seleccion sexual. Es mas probable que la hibridacion entre estas especies se deba a un error en la impronta sexual debido al parasitismo de cria interespecifico o a copulas extraparejas.

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