Searching for a hybrid zone between two Andean warblers

Extensive phenotypic variation across a transect between Ecuador and Colombia.

From the 1930s into 1950s, John Zimmer published numerous extensive monographs about “Studies on Peruvian Birds”. In number 54 of the series, he focused on the bird families Catamblyrhynchidae (now part of the Thraupidae) and Parulidae. While describing specimens of the genus Myioborus, he commented on some “puzzling specimens” that showed characteristics of two subspecies of the Golden-fronted Redstart (M. ornatus chrysops) and the Spectacled Redstart (M. melanocephalus ruficoronatus).

The extreme characters of ruficoronatus strongly suggest those of the ornatus group, and an occasional specimen of o. chrysops shows a noticeable patch of rufous in the center of the crown, strongly suggesting ruficoronatus. It is not impossible, therefore, that the puzzling specimens of one sort and another may represent intergrades or even hybrids between the two groups, but much more material will be necessary before an adequate solution is reached.

Several researchers followed the advice of Zimmer and collected more material on these birds. A recent study in the journal Ornithology reported on their findings. Is there a hybrid zone or not?

Two Subspecies

Before we delve into the possibility of a hybrid zone, we need to clarify the distribution of the M. ornatusM. melanocephalus species complex. First, the Spectacled Redstart (M. melanocephalus) is divided into five subspecies that replace each other when you travel along the Andes from Bolivia to Ecuador (see dots in the map below). The southern subspecies (malaris, melanocephalus and bolivianus) lack a rufous crown, which is present in the two northern subspecies (griseonuchus and ruficoronatus). Second, the Golden-fronted Redstart (M. ornatus) comprises two subspecies (ornatus and chrysops) that occur in different cordilleras in Colombia and Venezuela (see triangles in the map below). The putative hybrid zone concerns interactions between the most northern subspecies of the Spectacled Redstart (ruficoronatus) and the western subspecies of the Golden-fronted Redstart (chrysops). Laura Céspedes-Arias and her colleagues collected samples of these subspecies across a transect running from Ecuador into Colombia.

(A) Map showing the distribution of the subspecies in the M. ornatus-M. melanocephalus species complex. (B) Overview of the sampling transect in the study. From: Céspedes-Arias et al. (2021) Ornithology.

Clinal Patterns

Plumage analyses of over 300 specimens revealed a wide variety of phenotypes, representing different trait combination of both subspecies. It quickly became clear that individuals with intermediate phenotypes were most common along the transect. Some traits, such as head and chest coloration, showed a clear clinal transition from one subspecies into the other (see this blog post for more information on cline theory). All in all, these morphological patterns pointed to a roughly 200 kilometer wide hybrid zone, confirming the suspicion of John Zimmer.

Next, the researchers turned to genetic data by sequencing the mitochondrial gene ND2. In contrast to the plumage traits, this gene did not show a smooth transition. Instead the researchers found extensive haplotype sharing between the subspecies. This pattern can be explained by the recent origin of these subspecies (i.e. incomplete lineage sorting) or by extensive introgression due to hybridization. Genomic analyses will be needed to discriminate between these possibilities. Nonetheless, the most likely scenario seems to entail allopatric divergence leading to differences in plumage traits, followed by secondary contact and extensive hybridization. Another exciting avian hybrid zone to study in more detail.

A sample of the diversity of plumage phenotypes across the hybrid zone, from a typical chrysops (top) over intermediates to a typical ruficoronatus (bottom). From: Céspedes-Arias et al. (2021) Ornithology.

References

Céspedes-Arias, L. N., Cuervo, A. M., Bonaccorso, E., Castro-Farias, M., Mendoza-Santacruz, A., Pérez-Emán, J. L., Witt, C. C. & Cadena, C. D. (2021). Extensive hybridization between two Andean warbler species with shallow divergence in mtDNA. Ornithology138(1), ukaa065.

Featured image: Spectacled Redstart (Myioborus melanocephalus) © Francesco Veronesi | Wikimedia Commons

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