Figure 1: In the center, maps with sites visited to record song and collect the specimen series. On the sides, various pictures taken in the field. Our field sites (located from 2000 to 3800 masl) included pristine cloud forests but also highly intervened agricultural landscapes where we found Myioborus in small patches of scrub. Photos by: Paulo Pulgarín, Andrés Cuervo, Daniel Céspedes, David Ocampo and Laura Céspedes. Illustrations by: Maria P. Bustamante.

After fieldwork, the project continued in the lab and the museum. Using collected tissues and others available from museum collections, we extracted DNA and obtained sequences for one mitochondrial gene. By collaborating with researchers at Universidad San Francisco de Quito (Elisa Bonaccorso), Universidad Central de Venezuela (Maria Alejandra Castro and Jorge Pérez-Emán) and University of New Mexico (Christopher Witt) we were also able to include samples to cover all the main areas where the Spectacled Redstart and Golden-Fronted Redstart occur along the Andes, from Venezuela to Bolivia.

Using a complete geographic sampling we found that there is an overall low genetic structure within the Spectacled/Golden-Fronted group. However, we identified a differentiated genetic group which includes exclusively samples collected south of the Marañón River Valley in Peru, which is a dry hot area that breaks the continuity of the cloud forest. Interestingly, Spectacled Redstarts that inhabit south of the Marañón all lack of the rufous crown lack present in Ecuadorian populations (see photo here). Despite of marked differences in head coloration between the hybridizing subspecies (Illustrated in Figure 1) there is no clear differentiation between them in the mitochondrial gene that we sequenced. The genetic group including the hybridizing subspecies also included the subspecies from the eastern cordillera of the Golden-fronted Redstart that is characterized by a white rather than yellow face (see photo here). Overall, these results are consistent with a history of rapid diversification of head color patterns in these warblers.

Head coloration is the most striking difference between the Spectacled Redstart and the Golden-Fronted Redstart and therefore we focused on describing variation these plumage traits across our sampling transect. One interesting finding is that there are many different intermediate phenotypes, with multiple combinations of both species facial traits (Figure 2). Also, according to variation in head color pattern this hybrid zone expands for ~200 km (relatively wide compared to other warbler hybrid zones) were the intermediates are very common. All of these suggest that hybridization between these species is quite extensive, and the high diversity of intermediate plumage phenotypes (Figure 2) could suggest that many individuals might represent advanced generation hybrids (individuals that are product of several generations of hybrids crossing) and backcrosses (hybrid x ‘pure’). We also evaluated how other variable plumage traits varied across this hybrid zone: tail color pattern and underpart coloration. In spite of previous descriptions and significant variation among individuals, neither of these plumage traits show an association with geography. Therefore, the head coloration pattern appears to be the only plumage trait that shows variation across geography in our sampling transect.

 

Figure 2. Different plumage phenotypes found across the hybrid zone. On the left, typical phenotype of Myioborus melanocephalus ruficoronatus (Spectacled Redstart) and in the right, typical plumage of Myioborus ornatus chrysops (Golden-Fronted Redstart). In the middle a sample of individuals with intermediate phenotype. Note, for example, that some have a full rufous crown and yellow face while others also have hints of black on the front and/or sides. Photos by: Laura Céspedes, Andrés Cuervo, Paulo Pulgarín, Alejandro Mendoza and David Ocampo.

Using 134 song recordings recorded across the hybrid zone (by us and few other recordists that archived them in xenocanto and Macaulay Library) we found that songs are longer towards the south, where ‘pure’ Spectacled Redstarts occur. Other song parameters, related to frequency (how high or low-pitched is a sound), did not vary much between distant sampling localities. Myioborus songs are very complex, often composed by over 70 notes (especially in the south) and our data suggest that there is high within-site variation. While in the field we observed that these warblers often participate in duets, especially in response to playback and other Myioborus in the area (link here). Giving the vocal complexity that we found, it is worth studying more thoroughly vocal behavior (repertoires, duets, female songs) of these redstarts in the future to be able to better describe the acoustic variation across this hybrid zone.

Last December I successfully defended my master thesis. However, the project is still in progress! Obtaining genomic data is in the near future plans and this information will be key to understand the evolutionary processes at play in this hybrid zone. Thank you all again for contributing to this research project!

Laura Céspedes Arias