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Causes for genomic and morphological similarities among recently radiated species are often multifaceted and are further convoluted among species that readily interbreed. Here, we couple genomic and morphological trait comparisons to test the extent that ancestry and gene flow explain the retention of mallard-like traits within a sister species, the Mexican duck. First, we confirm that these taxa remain genetically structured, and that Mexican ducks exhibit an isolation-by-distance pattern. Despite the assumption of wide-spread hybridization, we found only a few late-stage hybrids, all from the southwestern USA. Next, assessing 23 morphological traits, we developed a genetically-vetted morphological key that is > 97% accurate in distinguishing across sex-age cohorts of Mexican ducks, mallards, and hybrids. During key development, we determined that 25% of genetically pure, immature male Mexican ducks of the northern population naturally displayed mallard-like traits in their formative plumage. In fact, applying this key to 55 museum specimens, we identified that only four of the 14 specimens originally classified as phenotypic hybrids were truly hybrids. We discuss how genomic and morphological comparisons shed light into the mechanism(s) underlying the evolution of complex phenotypic traits in recent radiations, and how misunderstanding the true morphological diversity within Mexican ducks resulted in taxonomic revisions that hindered conservation efforts.

Abstract Causes for genomic and morphological similarities among recently radiated species are often multifaceted and are further convoluted among species that readily interbreed. Here, we couple genomic and morphological trait comparisons to test the extent that ancestry and gene flow explain the retention of mallard-like traits within a sister species, the Mexican duck. First, we confirm that these taxa remain genetically structured, and that Mexican ducks exhibit an isolation-by-distance pattern. Despite the assumption of wide-spread hybridization, we found only a few late-stage hybrids, all from the southwestern USA. Next, assessing 23 morphological traits, we developed a genetically-vetted morphological key that is > 97% accurate in distinguishing across sex-age cohorts of Mexican ducks, mallards, and hybrids. During key development, we determined that 25% of genetically pure, immature male Mexican ducks of the northern population naturally displayed mallard-like traits in their formative plumage. In fact, applying this key to 55 museum specimens, we identified that only four of the 14 specimens originally classified as phenotypic hybrids were truly hybrids. We discuss how genomic and morphological comparisons shed light into the mechanism(s) underlying the evolution of complex phenotypic traits in recent radiations, and how misunderstanding the true morphological diversity within Mexican ducks resulted in taxonomic revisions that hindered conservation efforts.

The Brown-headed Cowbird (Molothrus ater) is a well-known obligate brood parasite that spread northward into California's Sacramento Valley in the 1930s, supported by anthropogenic changes to the landscape. Many species of western birds now serve as hosts for Brown-headed Cowbirds and brood parasitism has contributed to the decline of several passerines in California. Parasitism of Eastern Phoebe (Sayornis phoebe) and Say's Phoebe (S. saya) is well documented, but it is unreported in Black Phoebe (S. nigricans). Here we report the first confirmed nest parasitism of Black Phoebe by Brown-headed Cowbird. While monitoring Black Phoebe nests in Glenn County, California, we documented 4 instances of Brown-headed Cowbirds parasitizing active Black Phoebe nests. Brown-headed Cowbird nestlings likely fledged in 2 of the instances, 1 nest fell to the ground, and the fourth was likely depredated. In 1 successful nest, the Black Phoebe parents fledged 1 of their own young, along with the parasitic nestling. Based on our study, and the lack of detection in earlier studies, the current rate of Brown-headed Cowbird parasitism of Black Phoebes remains low, and we perceive no immediate conservation concern. However, further research and monitoring is needed to determine if parasitism is an increasing trend that could impact Black Phoebe productivity. El tordo Molothrus ater es un parásito obligado de nidada conocido que se dispersó hacia el norte en el Valle de Sacramento, California, en los 1930's, apoyado en los cambios antropogénicos del paisaje. Muchas especies de aves occidentales sirven ahora de hospedero para los tordos Molothrus ater y el parasitismo de nidada ha contribuido al declive de varios paserinos en California. El parasitismo de los mosqueros Sayornis phoebe y S. saya está bien documentado, pero no está reportado en el mosquero S. nigricans. Aquí reportamos el primer caso confirmado de parasitismo de nido de mosquero S. nigricans por tordo Molothrus ater. Mientras monitoreabamos nidos de mosquero S. nigricans en Glenn County, California, documentamos 4 ocurrencias de tordo Molothrus ater parasitando activamente nidos de mosquero S. nigricans. Las camadas de tordo Molothrus ater posiblemente se emanciparon en 2 de estos casos, 1 nido cayó al suelo, y el cuarto fue depredado. En uno de los nidos exitosos, los padres mosquero S. nigricans empollaron a una cría propia junto con la nidada del parásito. Basado en nuestro estudio y en la falta de detección de estudios previos, la tasa de parasitismo de tordo Molothrus ater en mosquero S. nigricans sigue baja y no percibimos amenaza a la conservación inmediata. Sin embargo, se necesita más investigación y monitoreo para determinar si este parasitismo tiene tendencia creciente que podría impactar la productividad de mosquero S. nigricans. Palabras clave: Glenn County, monitoreo de nidos, nido en copa, parásito. de nidada, tiránido, volantones. [ABSTRACT FROM AUTHOR]