Reader’s Hybrids: Ducks and Geese

Some pictures of the masters of hybridization!

Last year, reader Nico Rosseel send me a nice collection of hybrid parrots (see here). Recently, he surprised me with some hybrid pictures of my favorite bird order: the Anseriformes, better known as ducks, geese and swans. These birds are infamous for interbreeding, resulting in some unexpected combinations. You can check out the Anseriformes-page for the latest scientific findings in this group.

But why do they hybridize so often? In my review on goose hybridization, I considered three mechanisms: forced extra-pair copulations, interspecific nest parasitism and scarcity of conspecifics. The latter situation usually occurs in captivity where different species are kept together. When the breeding season starts and an individual cannot find a mate, it might settle for a partner from another species. Given that waterbirds are very popular among bird-keepers, it is no surprise that hybrids are some common among ducks and geese. Indeed, all the pictures are from captive birds. Let’s have a look!


Goose Hybrids

The first hybrid is a tricky one. It probably concerns a Barnacle Goose (Branta leucopsis) x Snow Goose (Anser caerulescens) because both parental species were present in the park. However, Nico noted that he sees some traces of Emperor Goose (Anser canagicus).


A probable Barnacle Goose x Snow Goose.


The second goose hybrid also involves Snow Goose, but this time mixed with a Bar-headed Goose (Anser indicus). Although it might be just a Bar-headed Goose without a head pattern, other characteristics (including beak color and markings on the belly) suggest Snow Goose influences. There is very interesting paper in Ornis Svecica on the traits of this particular hybrid combination.


A hybrid between Snow Goose and Bar-headed Goose.


Duck Hybrids

And now for some ducks! First, a hybrid from Tierpark Friedrichsfelde (Germany): Ruddy Shelduck (Tadorna ferruginea) x Common Shelduck (Tadorna tadorna). Most species in the genus Tadorna hybridize, but surprisingly this group of ducks has not been studied in great detail yet (in terms of hybridization).


A Ruddy Shelduck x Common Shelduck


Then we move to the king of duck hybrids: the Mallard (Anas platyrhynchos). Recently, I published a paper on multispecies hybridization in birds and the Mallard came out on top (pun not intended): it hybridized with 39 different species! Here are two examples of Mallard hybrids with Rosy-billed Pochard (Netta peposaca) and Red-billed Teal (Anas erythrorhyncha).


Two Mallard hybrids with Rosy-billed Pochard (left) and Red-billed Teal (right).



Lehmhus, J. & Gustavsson, C. G. (2014). Hybrids between Bar-headed Goose Anser indicus and Snow Goose Anser caerulescensOrnis Scvecica, 147-163.

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

Ottenburghs, J. (2019) Multispecies Hybridization in Birds. Avian Research, 10:20.


Genomics on the Galapagos: Exploring the genetic diversity of Darwin’s Finches and their relatives

Can we use the knowledge on genomic diversity to inform conservation?

The physicist Richard Feynman said: “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.” This simple statement captures the essence of science: you formulate a hypothesis and you test it with observations or experiments. And if your hypothesis is not explained by the data, it is wrong. Time to move on to the next guess (Of course, things are more complicated in practise, but you get the main idea). Let’s try this approach with some population genomic data:

  1. Genomic diversity will increase with population size.
  2. Genomic diversity will decrease with body size.

The first hypothesis is intuitive: a bigger population has more individuals which probably differ genetically, leading to more genomic diversity. The second hypothesis requires a bit more explanation. Big animals tend to have longer generation times, less offspring and smaller population sizes compared to small animals. Together these factors culminate in lower genomic diversity. A recent study in the journal Molecular Ecology tested these hypotheses using island populations of several bird species. What did they find?


The Lesser Antillean Bullfinch © Dick Daniels | Carolina Birds


17 species

To test these hypotheses, Anna Brüniche‐Olsen and her colleagues used published data from all 15 species of Darwin’s Finches that reside on the Galapagos Islands (and nearby Coco Island) and two Tanager species from Barbados (the Black‐faced Grassquit Tiaris bicolor and the Lesser Antillean Bullfinch Loxigilla noctis). For each island population, they estimated genetic diversity based on heterozygosity. Next, they correlated this estimate with island area and body size.

The results were in line with the hypotheses mentioned above. The researchers write that “We find [a] significant positive correlation between island size and genomic diversity, [and] a significant negative correlation between body size and genomic diversity.”


Green Warbler Finch © Paul McFarling | Darwin Foundation


Red List Status

These findings suggest that we can use genomic diversity to assess the conservation status of small populations. So, the researchers compared the estimates of heterozygosity with conservation status on the IUCN Red List (threatened vs. unthreatened). Surprisingly, there was no effect of Red List status on heterozygosity. What an anticlimax!

But wait, there is more. They also tested another measure of genetic diversity: Watterson’s θ. In this case, there was a significant effect: threatened species had significantly lower estimates of this measure compared to unthreatened species. This result can probably be explained by the way Watterson’s θ is calculated. It is the product of the effective population size and the neutral mutation rate. Taking into account effective population size provides a rough (but reliable) estimate of actual population size. So, this statistic might be a useful indicator for species at significant risk of decline. 


A Vegetarian Finch looking away from the significant difference in Watterson’s θ between non-threatened and threatened species. © Mike’s Birds | Flickr



Brüniche‐Olsen, A., Kellner, K. F., & DeWoody, J. A. (2019). Island area, body size and demographic history shape genomic diversity in Darwin’s finches and related tanagers. Molecular Ecology, 28(22), 4914-4925.

A genetic model for puntuated equilibria

Using a combination of developmental genes and transposable elements to explain patterns in the fossil record.

The evolutionary biologist Stephen Jay Gould is one of my personal heroes. He has written wonderful essays about evolution and the history of science. And he was not afraid to challenge the scientific status-quo with radical new ideas. I think every evolutionary biologist should read “The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme” (with Richard Lewontin) and “Punctuated equilibria: an alternative to phyletic gradualism” (with Niles Eldredge).

In this blog post, I will focus on the theory of punctuated equilibria, which tries to explain the patterns we see in the fossil record. Darwin’s theory of evolution focused on gradualism, the slow change of species over time. But this gradual change was not apparent in the fossil record, instead new species seem to suddenly pop into existence (from a paleontological perspective). This mismatch between Darwin’s gradualism and the sudden appearance of new fossil species was attributed to the incompleteness of the fossil record. Gould and Eldredge challenged this explanation and argued that the pattern in the fossils reflects reality. For most of evolutionary time, species do not change much (so-called stasis), but during speciation events organisms change quickly and drastically. Hence, the stable stasis (equilibrium) is occasionally punctuated by rapid speciation events and big morphological changes.


The model of punctuated equilibria versus gradualism. From: Wikipedia


Developmental Regulatory Genes

Since the publication of this model in 1972, numerous studies have confirmed patterns of punctuated equilibria (although some fossil series are still better explained by gradualism). However, one main weakness of punctuated equilibria is the absence of a genetic mechanism. Recently, Emily Casanova and Miriam Konkel proposed a genetic model in the journal BioEssays.

Their idea – dubbed the Developmental Gene Hypothesis – focuses on conserved non-coding elements (CNEs) in the genomes of organisms. These DNA-sequences are shared between distantly related species and often play a role in gene regulation. Specifically, the CNEs that contribute to embryonic development, the developmental regulatory genes (DevReg), might be key to explaining punctuated equilibria. These genes are under strong stabilizing selection (a wrong mutation mostly leads to an unviable embryo) and they can thus account for the morphological stasis we observe in the fossil record.


Transposable Elements

What about the punctuations? Here, transposable elements (TEs) come into the picture. These genetic parasites can jump around the genome through a cut-and-paste or copy-and-paste mechanism. Mostly they wreak havoc in the genome, but occasionally they end up in the right location and invoke a significant change in the organism. If they jump into or close to a developmental regulatory gene, TEs can lead to important morphological changes by altering developmental pathways. The result is rapid evolutionary change: a punctuation.

In their paper, Casanova and Konkel give the example of live birth (vivipary). TEs have played an important role in the evolution of mammalian pregnancy. The gene syncytin – which contributes to the formation of the placenta – can be traced back to a retrovirus. Interestingly, the Mabuya lizard, which is also viviparous, has a mammal-like placenta that expresses a retrovirus-derived gene similar to syncytin. And the fossil record harbors evidence of marine reptiles (the ichthyosaurs) giving birth to live young. Could this also be mediated by TEs?


A fossil of an ichthyosaur giving birth. From: Casanova & Konkel (2019) BioEssays



What does this model have to do with hybridization (the main topic of this website). At first sight, not much. I just thought this paper was really cool and decided to cover it. However, there is a link with hybrdization (which the authors don’t mention). It has been shown that hybridization leads to genome instability and the activation of silenced TEs. Hence, hybridization is another mechanism that can lead to rapid evolutionary change. Perhaps some of the punctuated patterns in the fossil record can be explained by massive hybridization events?



Casanova, E. L., & Konkel, M. K. (2019). The Developmental Gene Hypothesis for Punctuated Equilibrium: Combined Roles of Developmental Regulatory Genes and Transposable Elements. BioEssays, 1900173.

Drawing lines between larks: How many species of Horned Lark are there?

A genetic study reveals four primary lineages. But are they also distinct species?

During my student-days in Belgium, I was asked to prepare the sports questions for a pub quiz. Being a proper ornithologist, I managed to sneak in a bird-related question: “Which bird species is mentioned in the Liverpool-anthem You’ll Never Walk Alone?” The answer is obvious:

When you walk through a storm
Hold your head up high
And don’t be afraid of the dark
At the end of a storm is a golden sky
And the sweet silver song of a lark

But which lark are the Liverpool-fans referring to? The lark-family Alaudidae contains numerous species. One specific genus of larks has a particularly complex taxonomic history. This genus – Eremophila – currently contains two species, but that might change…


A Horned Lark in the snow © Tom Koerner | Wikimedia Commons


It’s complicated

In 1904, Bianchi reviewed the genus Eremophila (then known as Otocorys) and divided it into eight distinct species. Over the years, the number of Eremophilaspecies has declined and we are now left with two species: the Horned Lark (E. alpestris) and the Temmick’s Lark (E. bilopha). Both species are widely distributed: the Temmick’s Lark occurs in North Africa and the Middle East while the Horned Lark is found across the Holarctic. The widespread occurrence of the Horned Lark (it breeds on five continents!) and the variation in head patterns has led to a proliferation of subspecies: up to 42 have been proposed over the years.

The wide distribution and the considerable morphological variation suggest that there might be more than one species of Horned Lark. And indeed, based on several genetic markers, Sergei Drovetski and his colleagues proposed to divide the Horned Lark into six distinct species. Now, another study in the Journal of Ornithology – using new samples – took a closer look at the genus Eremophila. How many species did they recognize?


A Temminck’s Lark in Morocco © Francesco Veronesi | Wikimedia Commons


Four Lineages

Fatemeh Ghorbani and colleagues looked at two mitochondrial genes (cytb and ND2) for 46 samples. Phylogenetic analyses of these genes uncovered four primary lineages that originated about 3 million years ago. The exact relationships between these lineages are uncertain (they can probably be resolved with genomic data), making it difficult to pinpoint the origin of this genus. Nonetheless, the researchers propose to treat the four lineages as distinct species:

  1. Himalayan Horned Lark E. longirostris (comprising E. l. longirostrisE. l. deosaiensisE. l. elwesiE. l. khamensisE. l. przewalskiiE. l. argaleaE. l. teleschowi, and E. l. nigrifrons) from the Himalayas and Qinghai–Tibetan plateau
  2. Temminck’s Lark E. bilopha (monotypic), from North Africa to the Middle East
  3. Mountain Horned Lark E. penicillata (E. p. penicillataE. p. atlasE. p. albigulaE. p. balcanica, and E. p. bicornis) from northwest Africa and southeast Europe/southwest Asia
  4. Common Horned Lark E. alpestris sensu stricto (E. a. alpestris and many other American subspecies, E. a. flavaE. a. brandti) from the Northern Palearctic and North and northern South America

The species tree for the genus Eremophila points to four distinct lineages. From: Ghorbani et al. (2020) Journal of Ornithology


It’s still complicated

Interestingly, the morphological data and plumage patterns are not in line with the genetic data. Only the Temminck’s Lark (E. bilopha) and one subspecies of the Mountain Horned Lark (E. penicillata) could be confidently separated from the rest. This indicates that the evolutionary history of these larks has been shaped by substantial convergent evolution and perhaps interbreeding. Perhaps hybridization between different subspecies has led to the exchange of “head pattern genes”, similar to the situation in wagtails. Stay tuned for more interesting studies on larks!


Morphological data could only discriminate he Temminck’s Lark (green circle on the right) and one subspecies of the Mountain Horned Lark (blue circle on the left) from the other taxa. From: Ghorbani et al. (2020) Journal of Ornithology 



Drovetski, S. V., Raković, M., Semenov, G., Fadeev, I. V., & Red’kin, Y. A. (2014). Limited phylogeographic signal in sex-linked and autosomal loci despite geographically, ecologically, and phenotypically concordant structure of mtDNA variation in the Holarctic avian genus Eremophila. PLoS One9(1).

Ghorbani, F., Aliabadian, M., Olsson, U., Donald, P. F., Khan, A. A., & Alström, P. (2019). Mitochondrial phylogeography of the genus Eremophila confirms underestimated species diversity in the Palearctic. Journal of Ornithology, 1-16.

Genomic data uncover subtle population structure in the White-chinned Petrel

This wide-ranging seabird can be divided into three distinct groups.

The Southern Ocean is a vast expanse of water without any obvious barriers for widely wandering seabirds. So, you would expect to find little or no genetic differentiation between different island populations around Antarctica. The White-chinned Petrel (Procellaria aequinoctialis), for example, breeds on several subantarctic islands and is generally considered as one global population. However, some studies (based on isotopes and tracking data) found some differences in diet and foraging locations between birds from islands in the Atlantic and Indian Ocean. Could there be more population structure than meets the eye? A recent paper in the journal Molecular Ecology investigated the situation in greater detail.


A White-chinned Petrel, skimming the waters east of Tasmania © JJ Harrison | Wikimedia Commons


Molecular Markers

Kalinka Rexer‐Huber and her colleagues collected samples from 220 white-chinned petrels, covering all known breeding populations. Next, they characterized the birds in two ways: (1) by sequencing two mitochondrial markers and (2) by sequencing more than 60,000 genome-wide markers with a genotyping-by-sequencing approach. Comparing the results from both data sets revealed some interesting patterns.

First, the mtDNA pointed to two distinct groups, namely birds breeding in New Zealand and birds breeding in the Indian and Atlantic Ocean. This subdivision confirmed previous studies using single markers. However, the genome-wide data uncovered more fine-scale population structure, discriminating between three groups: New Zealand, the Atlantic populations (South Georgia and Falklands) and the Indian populations (Prince Edward, Crozet and Kerguelen). This finding shows the importance of genomic data in pinpointing subtle population boundaries (you can check my recent book chapter for more details).


The genomic data could discrimate between three groups, corresponding to New Zealand (Antipodes, Campbell and Auckland), the Atlantic populations (South Georgia and Falklands) and the Indian populations (Prince Edward, Crozet and Kerguelen). From: Rexer‐Huber et al. (2019) Molecular Ecology



What can explain the genetic population structure of the White-chinned Petrel in a largely undifferentiated Southern Ocean? One possibility is isolation-by-distance: although the birds travel widely when foraging, they are very loyal to their birthplace (experts call this natal philopatry). Hence, there is little exchange between breeding colonies that diverge over time.

A second reason could be the Antarctic Polar Front, a thermal barrier for some marine species. However, this seems unlikely because White-chinned Petrels can just fly over this barrier. Moreover, there is gene flow between islands on either side of the Antarctic Polar Front (Falklands and South Georgia).


A White-chinned Petrel resting on the surface. © Ed Dunens | Wikimedia Commons



The results from this study have important implications for the conservation of the White-chinned Petrel, which is listed as “Vulnerable” by the IUCN. Based on mtDNA, this species would be split into two evolutionary significant units (ESUs): New Zealand and the Atlantic-Indian populations. But this arrangement would ignore the subtle – but significant – genetic differences between populations in the Atlantic and Indian Oceans. Hence, the authors argue that three distinct units will need to be taken into account to devise conservation strategies for the White-chinned Petrel.



Rexer‐Huber, K. et al. (2019). Genomics detects population structure within and between ocean basins in a circumpolar seabird: The white‐chinned petrel. Molecular Ecology28(20), 4552-4572.

Where did the Ecuadorian Creole Chicken come from?

Genetic study traces the origin of this South American chicken.

The Creole chicken is an important food source in Ecuador. But where did this breed come from? Some scientists argue that these chickens trace their origin to European animals that were introduced during the Spanish colonization in the 15th century. Others think that these chickens already roamed the countrysides in pre-Columbian times. Indeed, the Spanish conquistador Francisco Pizarro mentions chickens in his descriptions of indigenous settlements. Moreover, several South American chicken breeds lay blue eggs, a typical Asian feature. Perhaps the chickens arrived in South America from Polynesia (which raises another question: why did the chicken cross the Pacific Ocean?). A recent study in the journal Animals sampled more than 200 chickens across Ecuador to unravel the history of this breed.


The Araucana chicken breed from Chile © Melani Marfeld | Pixabay


An Admixed History

The researchers used 30 microsatellites and a mitochondrial marker (the D-loop) to probe the genetic diversity in Ecuadorian Creole chickens. The analyses revealed a complex history with multiple admixture events. The mitochondrial haplotypes demonstrated that the Ecuadorian chickens originated from at least two sources: a European and an Asian lineage. This suggests that the Creole chickens probably arrived from Polynesia (through Chile) and where later influenced by the introduction of Spanish breeds.

The link with Chile is supported by the genetic influence of the Araucana chicken, a common breed in southern Chile. These chickens produce eggs with blue shells (an Asian feature, remember?) which are highly valued in Ecuador because people associate it with the traditional breeding system.


The genetic network of chicken breeds shows that the Ecuadorian Creole breed (ECU) has been influenced by the Chilean Araucana chicken (ARAU) and the Spanish Castellana Negra breed (CASN). For other the chicken breeds, you can consult the original paper. Adapted from Toalombo Vargas et al. (2019) Animals


The Spanish Link

What about the Spanish influences? The Spanish colonizers probably brought breeds from southern Spain. And indeed several Spanish breeds from this region, such as the Castellana Negra (CASN in the network), are closely related to the Ecuadorian chickens. In addition, the researchers noticed some genetic signatures from a breed of fighting cocks (so-called combatiente español). These roosters are most likely reared near Ecuadorian chickens, resulting in occasional interbreeding.



Apart from the origins of the Creole chickens in Ecuador, the researchers also reported some good news for the local people. The Ecuadorian chickens show high levels of genetic diversity and no influence from commercial breeds. This indicates that they are relatively “pure” and represent an important aspect of Ecuadorian culture.


The mitochondrial network shows 24 different haplotypes in the Ecuadorian Creole chickens, revealing a high level of genetic diversity. From Toalombo Vargas et al. (2019) Animals



Toalombo Vargas, P. A., León, J. M., Fiallos Ortega, L. R., Martinez, A., Villafuerte Gavilanes, A. A., Delgado, J. V., & Landi, V. (2019). Deciphering the Patterns of Genetic Admixture and Diversity in the Ecuadorian Creole Chicken. Animals9(9), 670.


This paper has been added to the Galliformes page.

A mosaic hybrid zone between Scissor-tailed Flycatcher and Western Kingbird

Both species are expanding their range and hybridizing in the periphery.

In March 2018 I received a message from Alexander Worm. He had just completed his Master thesis and informed me that “Scissor-tailed Flycatcher (Tyrannus forficatus) and the Western Kingbird (T. verticalis) are able to hybridize and produce viable offspring.” Recently, the genetic analyses behind this statement were published in the journal Ibis. Let’s have a look at the results!


A Scissor-tailed Flycatcher © Gary Kramer | Wikimedia Commons



The Scissor-tailed Flycatcher and the Western Kingbird are widely distributed across North America and their ranges overlap in several regions. Some putative hybrids have been reported in Texas, Oklahoma, Colorado and California. To see if hybrids actually occur and if they are fertile, Alexander Worm and his colleagues collected samples from a contact zone combined with several museum specimens, culminating in a final data set of 84 individuals.


The distribution of Western Kingbird (dark blue) and Scissor-tailed Flycatcher (light blue) in North America. Highlighted counties reflect sampling locations. From: Worm et al. (2019) Ibis


Mosaic Hybrid Zone

Based on eight microsatellites and one mitochondrial marker, the researchers found convincing evidence for gene flow between these species. The analyses did not uncover first generation hybrids, but there were several backcrosses (in both directions).

The system can be regarded as a mosaic hybrid zone where two sympatric species interbreed in a patchy distribution. Moreover, both species are expanding their range (mostly it is only one species expanding into the range of another) and hybridizing at the periphery. This unusual situation offers many opportunities for further research.

San Luis National Wildlife Refuge, Los Banos, California

A Western Kingbird © Becky Matsubara | Flickr



Worm, A. J., Roeder, D. V., Husak, M. S., Fluker, B. L., & Boves, T. J. (2019). Characterizing patterns of introgressive hybridization between two species of Tyrannus following concurrent range expansion. Ibis161(4), 770-780.


This paper has been added to the Tyrannidae page.


Explaining the pantropical distribution of kingfishers, bee-eaters, rollers, motmots, and todies

Scientists reconstruct the evolutionary history of the Coraciiformes.

Some bird groups have a peculiar distribution pattern. Take the order Coraciiformes, for example. This colorful bird order (which includes kingfishers, bee-eaters, rollers, motmots, and todies) has representatives in all equatorial regions, from South America over Africa to East Asia. Biogeographers refer to this distribution as pantropical and have formulated two main hypotheses to explain these patterns.

The first hypothesis states that the continental break-up of the southern supercontinent Gondwana divided widespread populations over the current tropical regions. The second hypothesis proposes that pantropical clades originated on the northern supercontinent Laurasia and later moved to southern regions, following the spread of tropical forests (that is why this hypothesis is also known as the Boreotropics hypothesis).


A Lilac-breasted Roller (Coracias caudatus) in Tanzania. © Jente Ottenburghs


Ultraconserved Elements

Jenna McCullough and her colleagues investigated which of these two hypotheses explains the distribution of the Coraciiformes. The obtained DNA from all extant coraciiform species and sequenced more than 5000 ultraconserved elements (UCE). These conserved DNA sequences are found in distantly related animal genomes and are probably involved in controlling gene expression. Their slow evolution can be used to probe evolutionary relationships in the distant past. Using several well-known fossils, the researchers managed to construct a time-calibrated evolutionary tree for the Coraciiformes. In addition, they reconstructed the ancestral distributions of these birds to pinpoint the location of their origin: Gondwana or Laurasia?


The evolutionary history of the order Coraciiformes. From: McCullough et al. (2019) Proceedings of the Royal Society B



The analyses revealed that the major coraciiform lineages originated on Laurasia about 57 million years ago. As the climate changed, the birds tracked the tropical conditions and forest habitats southwards, ending up in their current pantropical positions. These results support the Boreotropics hypothesis outlined above. During the Miocene and Pliocene, a second burst of diversification occurred, giving rise to the genera we know today.

Reading this study, reminded me of a blog post that I wrote for the BOUblog (the official blog of the ornithological journal Ibis) about the evolution of Trogons. Here is an excerpt from that post:

These analyses indicated that Eurasia is the most likely site of origin for these birds. The ancestors of present-day trogons were probably distributed across Laurasia (i.e. Eurasia and North America). During the transition from the Oligocene to the Miocene, about 23 million years ago, the climate cooled, forcing the trogons to move to tropical forests in the southern latitudes of the Neotropics, Africa and Asia. There, they thrived while their relatives disappeared from the Northern Hemisphere, culminating in the widespread distribution we observe today.

Another example of the Boreotropics hypothesis!



McCullough, J. M., Moyle, R. G., Smith, B. T., & Andersen, M. J. (2019). A Laurasian origin for a pantropical bird radiation is supported by genomic and fossil data (Aves: Coraciiformes). Proceedings of the Royal Society B, 286(1910), 20190122.

Oliveros, C. H., Andersen, M. J., Hosner, P. A., Mauck III, W. M., Sheldon, F. H., Cracraft, J., & Moyle, R. G. (2020). Rapid Laurasian diversification of a pantropical bird family during the Oligocene–Miocene transition. Ibis, 162(1), 137-152.