Some peculiar duck hybrids provide insights into the evolution of sexual dichromatism

A closer look at captive hybrids between the Chiloé Wigeon and the Philippine Duck.

The Avian Hybrids Project officially started in 2015 with the publication of a short paper in Ibis. My main goal was to gather the scientific literature on avian hybridization in one place. Later on, I started publishing blog posts on a wide range of bird-related topics, summarizing many scientific publications. The blog post you are currently reading is number 400. An important milestone for the Avian Hybrids Project. To celebrate this achievement, I decided to cover one of my own recent papers that was recently published in the journal Ecology and Evolution. In this paper, Jan Harteman and I describe some interesting hybrids between the Chiloé Wigeon (Mareca sibilatrix) and the Philippine Duck (Anas luzonica).

Plumage Patterns

Let’s start from the beginning. In the spring of 2020 Jan contacted me with an intriguing story. A female Chiloé Wigeon had mated with a male Philippine Duck, producing four hybrids. He asked me if these hybrids would be interesting to study in more detail. We decided to let them grow up to see how their plumage patterns would develop. And we were in for a surprise! Chiloé Wigeon and Philippine Duck are both sexually monochromatic (i.e., both sexes look alike). But the hybrids showed clear sexual dichromatism. The males exhibited the iridescent green head pattern of the Chiloé Wigeon, whereas the females developed the dark crown and eye stripe of the Philippine Duck.

This observation triggered me to dive into the literature and learn more about the genetic and developmental mechanisms of sexual mono- and dichromatism in ducks. It turns out that the showy male plumage is the default state in both sexes, while the production of estrogen culminates in the development of cryptic female-type plumage. The plumage patterns in the hybrids were probably the outcome of different levels of estrogen production. Chiloé Wigeon and Philippine Duck belong to different branches on the evolutionary tree, diverging about 13 million years ago. It is thus likely that sexual monochromatism arose independently in these species, with different modifier genes controlling the production of estrogen. More detailed genetic analyses will be needed to identify these genes.

In addition, the hybrids formed two pairs, of which one pair produced a clutch of six unfertilized eggs. It is difficult to pinpoint the exact reason for infertility of these eggs. One of the sexes (or both) might be sterile, or fertilization was unsuccessful due to genetic incompatibilities between sperm and egg cells. Another interesting question to explore further.

Pictures of the parental species – (a) Chiloé Wigeon and (b) Philippine Duck – and their hybrids (c–d). © Jan Harteman.

Documenting Hybrids

This study takes me back to the basis of the Avian Hybrids Project: documenting hybridization in birds. In the current scientific climate of big grants and fancy genomic tools, it is easy to forget about the simple description of interesting observations. Other notable examples include the pairing between between a Cerulean Warbler and a Black-throated Blue Warbler in Indiana (see this blog post) or the reports of several fairywren hybrids (see this blog post). The disdain for such descriptive studies by some researchers was nicely illustrated by one of the reviewers who wrote that “there is not really a whole lot here.” Luckily, the other reviewer and the editor were enthusiastic about our work, resulting in a swift acceptance.

In addition, this paper highlights the importance of reporting avian hybrids in the scientific literature. If Jan had not contacted me, these hybrids might have gone unnoticed. Getting an overview of all known avian hybrids is a daunting task. Eugene McCarthy has produced a nice overview of hybridization in birds with his “Handbook of Avian Hybrids of the World”. However, several hybrid records in this book are unreliable and require further investigation (a quest that I have recently started with by checking the reliability of tinamou hybrids). The description of hybrids in peer-reviewed papers is sorely needed to obtain a better overview of the incidence of hybridization in birds. And who knows? Maybe some hybrids might even provide some crucial insights into fundamental scientific questions, such as the evolution of sexual mono- and dichromatism in ducks.

References

Ottenburghs, J. & Harteman, J. (2021) Sexually dichromatic hybrids between two monochromatic duck species, the Chiloé wigeon and the Philippine duck. Ecology and Evolution.

Featured image: Hybrids between the Chiloé wigeon (Mareca sibilatrix) and the Philippine duck (Anas luzonica) © Jan Harteman.