The Ecological Succession Story of Hybridizing Bluebirds

Avian hybrid zones have been studied extensively for decades. You would think that ornithologists looked at these zones from all possible angles. Nonetheless, Renée A. Duckworth and Georgy A. Semenov provide a fresh perspective on a particular hybrid zone between two Sialia species.

Ecological succession refers to the situation where the species composition of a community changes over time. Mostly, succession occurs after an environmental disturbance, such as a forest fire. The resulting habitat is first colonized by the most dispersive species. Later on, other species – often better competitors – invade and start to replace the early arrivers.

This is what happened in the northwestern United States.  The open meadows of Montana are not the ideal habitat for bluebirds (genus Sialia), but the placement of artificial nest boxes provided the ideal conditions for these cavity breeders. So, the introduction of nestboxes can be considered the ‘environmental disturbance’. The first species to take advantage of these new nesting places was the  Mountain Bluebird (S. currucoides). The more aggressive Western Bluebirds (S. mexicana) were delayed in their arrival, but once they showed up, their numbers increased rapidly.

Using data from 10 populations from 2001 to 2014, Duckworth and Semenov investigated the occurrence of matings between these two Bluebird species during different stages of the ecological succession. In addition, to check whether these heterospecific pairs actually produced hybrids (you never know if there are extra-pair copulations!), they also looked into the genetics of these birds using microsatellites.



The early arriving Mountain Bluebird (left) and the delayed, more aggressive Western Bluebird


The results show that heterospecific matings only occurred during the early stages of succession. This is in accordance with Hubb’s principle, which states that hybridization is more likely when one species is rare. In this case, the first wave of invading Western Bluebirds were outnumbered by Mountain Bluebirds. Unable to find another Western Bluebird to mate with, they ‘settled’ for a Mountain Bluebird. Later in the succession, the number of Western Bluebirds increased and the chances of hybridization diminished.

Although the frequency of hybridization seems to decline during the ecological succession, the genetic effects of interbreeding are still measurable in later generations. This could have important evolutionary consequences for these species.


Duckworth, R. A. & Semenov, G. A. (2017). Hybridization Associated with Cycles of Ecological Succession in a Passerine Bird. The American Naturalist 190.


Thanks to Renée A. Duckworth for sending me a copy of this paper. The information has been added to the Turdidae page.


Hybrid Geese: A Trilogy of Papers

With the publication of ‘A History of Hybrids? Genomic Patterns of Introgression in the True Geese’ in BMC Evolutionary Biology, the three goose papers from my PhD thesis have made it into scientific journals. The trilogy is complete, but the story continues…

During my PhD, I studied the evolutionary history of the True Geese. This bird group contains about 17 species (depending on which authority you follow) and is traditionally divided into two genera: Anser and Branta. At the start of my PhD, I was surprised to find out that the phylogeny (i.e. evolutionary tree) of the geese was still unresolved. The failure to resolve the relationships between these bird species is probably due to high levels of hybridization. My goal was to solve this phylogenetic conundrum and further explore the influence of hybridization during the evolutionary history of the True Geese.


Part 1: Goose Hybrids

Although the main focus of my research was to quantify the effects of hybridization on an evolutionary timescale, I wanted to know the current state of events. How often do birders see hybrid geese? Which species are interbreeding? Are these hybrids fertile? And why does a goose choose a partner of another species? These questions formed the basis for part one of the goose trilogy. This first story was published in Frontiers in Zoology, entitled ‘Hybridization in Geese: A Review’.

It turns out that the majority of goose species have interbred at some point (in captivity or in the wild). Hybrids are thus common on a species-level, but rare on a per-individual level. The origin of particular goose hybrids is difficult to deduce but several mechanisms, such as interspecific nest parasitism and extra-pair copulations, are possible. The different mechanisms are not mutually exclusive and it is currently not possible to discriminate between these mechanisms without quantitative data.

Most hybrid geese are fertile; only in crosses between distantly related species do female hybrids become sterile. This fertility pattern, which is in line with Haldane’s Rule, may facilitate interspecific gene flow between closely related species. This finding is important for the other stories in the goose trilogy.

nest parasitism


Part 2: A Tree of Geese

Before I could investigate the role of hybridization in goose evolution, I needed a proper phylogenetic framework. The construction of this framework was the focus of my second story, which was published in Molecular Phylogenetics and Evolution under the title ‘A Tree of Geese: A Phylogenomic Perspective on the Evolutionary History of True Geese‘.

For this study, I collected blood samples from all goose species. Sequencing the whole genome of these species provided me with a huge amount of data to resolve the phylogenetic tree of this bird group. I won’t bother you with the technical details (e.g., we opted for an exon-based approach with both concatenation and consensus analyses). Let’s jump straight to the main results!

The split between Anser and Branta was already well-established, but the relationships within these genera were contentious. Using whole genome data, I was able to resolve the phylogenetic relationships between the different goose species.

Within the genus Branta (commonly referred to as the Black Geese) there is a group of White-cheeked Geese – Canada Goose (B. canadensis), Cackling Goose (B. hutchinsii), Barnacle Goose (B. leucopsis) and Hawaiian Goose (B. sandvicensis) – and two basal splits – leading to Brent Goose (B. bernicla) and Red-breasted Goose (B. ruficollis).

In the genus Anser, the most basal split leads to the morphologically divergent Bar-headed Goose (A. indicus). Next, two main groups can be recognised: the White Geese – Snow Goose (A. caerulescens), Ross’ Goose (A. rossii) and Emperor Goose (A. canagicus) – and the Grey Geese – Greylag Goose (A. anser), Swan Goose (A. cygnoides), the White-fronted Geese (A. albifrons and A. erythropus) and the Bean Goose complex (A. fabalis, A. serrirostris and A. brachyrhynchus).

A molecular clock analysis indicated that the majority of speciation events took place at the end of the Pliocene. The approximate date of diversification coincides with the beginning of a period of climatic oscillations between 3.2 and 1.9 million years ago. This period was part of a fast global cooling trend, following the closure of the Panama Seaway and the uplifting of the Tibetan Plateau around four million years ago. This resulted in the formation of permanent Northern Hemisphere ice sheets, the establishment of a circumpolar tundra belt and the emergence of temperate grasslands, which opened up new ecological niches in which new groups of animals and plants were able to spread. The tundra habitat serves as breeding ground for geese, while the temperate grasslands act as wintering grounds where mate choice takes place. Moreover, these tundra and grassland habitats provided ample opportunity for geese to explore new ecological niches and diversify in beak morphology.

A Tree of Geese

More importantly, the comparison of different gene trees revealed that different genes tell different stories. This observation, called gene tree discordance, can be caused by rapid speciation (leading to a phenomenon known as incomplete lineage sorting or ILS) and hybridization. Disentangling the contributions of ILS and hybridization is the focus of the third story.


Part 3: A History of Hybrids

And so we arrive at the final story in this trilogy where I explored the role of hybridization during the evolutionary history of the True Geese. As mentioned in the introduction, this story was published in BMC Evolutionary Biology, entitled ‘A History of Hybrids? Genomic Patterns of Introgression in the True Geese‘.

I found indications for ancient gene flow during the diversification of the True Geese and I was able to pinpoint several putative hybridization events. Specifically, in the genus Branta, both the ancestor of the White-cheeked Geese (Hawaiian Goose, Canada Goose, Cackling Goose and Barnacle Goose) and the ancestor of the Brent Goose hybridized with Red-breasted Goose.

The reconstruction of historical effective population sizes shows that most species experienced a steady increase during the Pliocene and Pleistocene (in agreement with the conclusions from story 2). These large effective population sizes might have facilitated contact between diverging goose species, resulting in the establishment of hybrid zones and consequent gene flow.

I can definitely conclude that the evolution of goose species follows a complex speciation model high levels of gene flow during species diversification. Unfortunately, I did not have the data to determine whether this gene flow is the outcome of (repeated) secondary contact or divergence-with-gene-flow. This warrants a population genomic approach whereby multiple individuals of one population are sequenced. In fact, this is exactly what I plan to do during my postdoc with Hans Ellegren at Uppsala University.

To be continued…

Goose Demography.jpg



Ottenburghs, J., van Hooft, P., van Wieren, S.E., Ydenberg, R.C. & Prins, H.H.T. (2016). Hybridization in Geese: A Review. Frontiers in Zoology. 13:20 DOI: 10.1186/s12983-016-0153-1

Ottenburghs, J., Megens, H.-J., Kraus, R.H.S., Madsen, O., van Hooft, P., van Wieren, S.E., Crooijmans, R.P.M.A., Ydenberg, R.C., Groenen, M.A.M. & Prins, H.H.T. (2016). A Tree of Geese: A Phylogenomic Perspective on the Evolutionary History of True Geese. Molecular Phylogenetics and Evolution. 101:303-313  doi:10.1016/j.ympev.2016.05.021

Ottenburghs, J., Megens, H.-J., Kraus, R.H.S., van Hooft, P., van Wieren, S.E., Crooijmans, R.P.M.A., Ydenberg, R.C., Groenen, M.A.M. & Prins, H.H.T. (2017). A History of Hybrids? Genomic Patterns of Introgression in the True Geese. BMC Evolutionary Biology. 17:201



Throwback Thursday: Observations of Red-shafted and Yellow-shafted Flickers in a 1893 letter to Science

It’s Thursday, so time for an old avian hybrids paper! This week: a short letter to Science on hybrid woodpeckers.

Being a scientist in the 1800s must have been great. Nowadays, researchers struggle to get something published in high ranking journals, such as Science or Nature. Back in the days, you could just send a letter to the editor of those journals and you would have a reasonable chance of getting published.

In 1893, Alvah Eaton reacted to a summary of hybridization in the genus Colaptes in a previous edition of Science. Here is the entire letter:


A letter in Science based on a few presumed hybrids between Red-shafted Flicker (C. cafer) and Yellow-shafted Flicker (C. auratus). This would be unthinkable nowadays. However, these two woodpecker species have become one of the model systems for avian hybridization in North America. You can read more about them here.


Yellow-shafted and Red-shafted Flicker

On a side note, the letter following the piece by Eaton shows how scientists communicated before the arrival of the internet. Want to know how to study ants? Just send a letter to Science!



Eaton, A. A. 1893. Hybridism in Genus Colaptes. Science (New York, N.Y.), 21: 25-25.