How many hybrid bird species are there?

A review paper on hybrid speciation in birds proposes new way to classify hybrid species.

This week, I published my first single-author paper in a scientific journal: a review on hybrid speciation in birds. It feels strange to write a blog post about your own work, but I will do it anyway. First, I will briefly summarize the main points of the paper. Then I will provide some insights into the origin of this review.


Putative Hybrid Species

The last couple of years, ornithologists have speculated that several bird species have a hybrid origin, namely:

  • Italian sparrow (Passer italiae)
  • Audubon’s warbler (Setophaga auduboni)
  • Genovesa mockingbird (Mimus parvulus bauri)
  • Hawaiian duck (Anas wyvilliana)
  • red‐breasted goose (Branta ruficollis)
  • golden‐crowned manakin (Lepidothrix vilasboasi)
  • “Big Bird” (Geospiza spp.)

The evidence supporting these claims varies for each species. Some cases (e.g., the Italian sparrow and “Big Bird”) are quite solid, while other putative hybrid species (e.g., red-breasted goose and Genovesa mockingbird) need more research. In each case, there is convincing evidence for hybridization, but not all studies could confidently discriminate between hybrid speciation and (recurrent) introgressive hybridization.

I have written about several of these hybrid species before (see here for “Big Bird” and golden-crowned manakin, here for the Italian sparrow and here for red-breasted goose).


The red-breasted goose, a hybrid species or not? (from:


Two Types?

When do you consider a species to have a hybrid origin? In 2014, Molly Schumer and her colleagues provided three criteria that should be satisfied: (1) genetic or morphological evidence for hybridization, (2) reproductive isolation of the hybrid lineage from its parental species, and (3) evidence that reproductive isolation is a direct consequence of past hybridization. Some authors argued that the third criterion is too strict and “focusing exclusively on [reproductive isolation] may shift the interest away from other crucial elements in HHS, that is, the ecological dimensions of the process and the production of novel diversity.”

As a solution to this debate, I propose to discriminate between two types of hybrid species: type I where reproductive isolation is a direct consequence of hybridization and type II where reproductive isolation is the by‐product of other processes, such as geographical isolation. I applied this classification scheme to the proposed hybrid bird species. “Big Bird” can be considered a type I hybrid species, while the Italian sparrow and the golden‐crowned manakin are type II hybrid species. For the other species, the evidence is still inconclusive.

italian sparrow

The Italian sparrow: a type II hybrid species (from:


Exploring the speciation continuum

The overview of hybrid bird species revealed hybrid lineages of different ages, ranging from a few generations (“Bird Bird”) over thousands of years (e.g., Italian sparrow and golden‐crowned manakin) to millions of years old (red‐breasted goose). This spectrum of divergence times allows for the comparison of hybrid genome stabilization and adaptation over time, while taking into account species‐specific processes. The exploration of this hybrid speciation continuum will definitely lead to some important insights.


The hybrid speciation continuum in birds (from Ottenburghs 2018, Ecology and Evolution)


On the origin of this paper

The story behind this review is quite intriguing. Let’s start from the beginning: a few years ago I wanted to publish one of PhD chapters as an extensive review. This chapter (which you can read here) provided an overview of avian hybrid zones and patterns of introgression. Unfortunately, the manuscript was rejected at Biological Reviews. One of the reviewers commented on my section about hybrid speciation in birds. He or she stated that this process was so rare in nature that it did not deserve a separate section in the paper. Being a stubborn PhD-student, I decided to turn the section into a separate paper.

While writing this hybrid speciation paper, I came across a discussion in Heredity on the classification of hybrid species (you can find the papers here and here). I wanted to provide my small contribution to this debate and proposed the classification described above.

The manuscript was send to Journal of Evolutionary Biology, where – despite largely positive reviews – it was rejected. The main reason for the rejection was the focus on birds. One reviewer wanted me to apply the classification scheme more broadly. Luckily, it was cascaded down to Ecology and Evolution where is was accepted with minor revisions.


A final note of frustration

I do not like to complain, but I want to share the final part of  the publication history. If you check the publication, you will see that it was accepted on 29 August 2018. It appeared online on 05 December 2018. Why did it take so long? The reason is the payment of the Open Access costs. My previous research group at Wageningen University agreed to pay the costs because the paper is part of my PhD project. So, I send the invoice to the financial department of Wageningen University where it got caught up in an administrative hassle. Apparently, Wageningen University needs a very specific format of invoice before they can process it. After several months, I got so fed up with this incompetent indecisiveness and decided to pay the costs myself. It literally took me less than 5 minutes. How difficult can it be to pay a bill?! Anyway, the paper is published. Let’s focus on the positive side.




Ottenburghs (2018) Exploring the hybrid speciation continuum in birds. Ecology and Evolution. Early Online

8 thoughts on “How many hybrid bird species are there?

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