Looks can be deceiving: Parallel evolution of plumage coloration in wheatears

The similar plumage patterns can be partly explained by hybridization.

Closely related species tend to look alike. So, it is no surprise that scientists have been inferring evolutionary relationships based on external characters. However, parallel evolution can complicate matters. This process – which can be defined as “similar development of a trait in distinct species which are not not closely related, but share a similar original trait in response to similar evolutionary pressure” – can lead to wrong conclusions about how species are related to one another. A recent study in Journal of Evolutionary Biology explores this issue in wheatears (Oenanthe).


Problematic Plumage Patterns

Consider the following four species. How would you classify them based on plumage coloration?

  • Cyprus wheatear (O. cypriaca)
  • Pied wheatear (O. pleschanka)
  • Western black-eared wheatear (O. hispanica hispanica)
  • Eastern black-eared wheatear (O. hispanica melanoleuca)

Four species of wheatear. How would you group them?

Solely using morphology, it makes sense to group Cyprus and pied wheatears and western and eastern black-eared wheatears. Genetic analyses, however, tell a different story.


Genetics vs. Morphology

Manuel Schweizer, Vera Warmuth, Niloofar Alaei Kahki (I mention these three authors because they contributed equally to the study) and their colleagues collected samples of these four species and performed genetic analyses using about 20,000 markers. The results are not in line with the morphological classification: Cyprus wheatear is more closely related to eastern black-eared wheatear, followed by pied wheatear and western black-eared wheatear. Not what you would expect based on their plumage patterns.

wheatear tree

The unexpected evolutionary tree for these four wheatear species. The circles indicate the plumage patterns (from: Schweizer et al. 2018)


Parallel Processes

The evolution of plumage coloration seems to be the result of parallel evolution. But what is the exact mechanism? Parallel evolution can be the outcome of several biological processes. Unrelated species might use genetic material that was already present in a distant ancestor. Or traits might be transferred from one species to another by hybridization. Or similar mutations might have arisen independently.  Unfortunately, the present study cannot confidently discriminate between these scenarios.

Finally, the genetic analyses uncovered gene flow from pied into eastern black-eared wheatear. This is not that surprising, because a hybrid zone between these species has been described before. Whether hybridization has influenced the evolution of plumage patterns in these species remains to be investigated.



Schweizer, M., Warmuth, V., Kakhki, N.A., Aliabadian, M., Forschler, M., Shirihai, H., Suh, A. & Burri, R. (2018) Parallel plumage colour evolution and introgressive hybridization in wheatears. Journal of Evolutionary Biology.


This paper has been added to the Muscicapidae page.


Hybridization leads to emergence of a new sex chromosome

A surprising finding in a long-term hybridization experiment with swordtail fish. 

When I say sex chromosomes, you say XY (or ZW if you are a bird or butterfly person). But there are many more sex-determining systems than you can imagine. Take the fish genus Xiphophorus for example. Among the 26 species in this genus there are species with both XY and ZW sex chromosomes, but also some with other peculiar systems. The Southern Platyfish (X. maculatus) has three different sex chromosomes (X, Y and W), while the Green Swordtail (X. hellerii) has a so-called polyfactorial sex-determining system. In this species, the genes that determine the sex are scattered across the genome (the technical term is autosomal modifiers). A recent study in the journal Nature Communications makes the situation even more confusing.

southern platyfish

A Southern Platyfish (from: http://www.wikipedia.com/)


Long-term Experiment

The evolutionary history of the genus Xiphophorus is littered with hybridization events. Some species – such as the Southern Mountain Swordtail (X. monticolus) and the Yellow Swordtail (X. clemenciae) – are even thought to be of hybrid origin (similar to some bird species). To better understand the impact of hybridization in this group of fish, scientists started a long-term hybridization experiment.

First, they mated a female Southern Platyfish with a male Green Swordtail. Interestingly, these two species have different sex-determining systems as described above. From the resulting hybrid offspring, the researchers selected females with two color patterns: a red coloration (caused by the gene Dr, dorsal red) and black spots (caused by the gene Sd, spotted dorsal). These genes are closely linked on the X-chromosome of the Southern Platyfish and are thus excellent markers to check if hybrids carry genetic material from this species. The selected females were then backcrossed to male Green Swordtails for 100 generations (more than 30 years!).


A Green Swordtail (from: http://www.wikipedia.com:)


A New Sex Chromosome

After patiently waiting for more than 30 years (I already get anxious when I have to run an analysis overnight), the researchers were in for a surprise. During the hybridization experiment, the sex-determining region of the X-chromosome from the Southern Platyfish was translocated to another chromosome in the backcrosses. It concerns a large block of about 10 Megabases (1 Mb is 1 million nucleotides) on linkage group 2. Essentially, a new sex chromosome originated through selection and hybridization.

Based on the outcome of this experiment and the widespread occurrence of hybridization during the evolution of Xiphophorus fish, the researchers conclude that “hybridization may be a key contributor to the evolutionary history of this group of fishes.”



Franchini, P., Jones, J.C., Xiong, P., Kneitz, S., Gompert, Z., Warren, W.C., Walter, R.B., Meyer, A. & Schartl, M. (2018) Long-term experimental hybridisation results in the evolution of a new sex chromosome in swordtail fish. Nature Communications, 9:5136.

A hybrid zone between subspecies of the Common Ringer Plover?

Genetic analyses support subspecies classification in the Common Ringed Plover.

“Of what use are subspecies?” asked biologist Ernst Mayr in a 1982 paper. The concept of a subspecies has generally been used to subdivide the geographical distribution of a species into meaningful units that differ in some morphological characters. Most subspecies, however, have not been assessed with genetic data (but see e.g., Wagtails). A recent study in the journal Ardea checks how well subspecies of Common Ringed Plover (Charadrius hiaticula) are supported by genetics.


How Many Subspecies?

The number of Common Ringed Plover subspecies ranges from two to seven, but most authors recognize three subspecies:

  • hiaticula (southern Scandinavia and the Baltic)
  • tundrae (northern Scandinavia and Russia)
  • psammodromus (Canada, Greenland, Iceland and Faeroe Islands)

Birds from Chukotka, in the Russian far east, differ from the other subspecies in this region (tundrae) and might represent a distinct subspecies: kolymensis.


A Common Ringed Plover of the subspecies tundrae (from: http://www.wikipedia.com/)


No Population Structure

Leon Thies (University of Graz) and his colleagues collected samples across the range of the Common Ringed Plover and genotyped them using microsatellites. In general, the genetic analyses revealed no pronounced population structure. This observation can be explained by the dispersal patterns of these birds: juveniles do not breed at the site where they were born (i.e. low natal philopatry). The dispersal between different breeding sites results in gene flow that counteracts population differentiation.

However, when prior knowledge about the sampling location is included in the models, the three main subspecies are supported by the genetic data. There was no evidence for the putative kolymensis subspecies from Chukotka.


A Hybrid Zone

Admixture analyses (using the software package Structure) suggests a hybrid zone between all three subspecies, running from Northern Scandinavia to Belarus. The authors speculate that the subspecies diverged in allopatry during the ice ages and came into secondary contact. However, a more thorough genetic analysis is needed to characterize this putative hybrid zone.


A juvenile Common Ringed Plover (from: http://www.hbw.com/)


North vs. South

Finally, one paragraph in the discussion caught my attention:

In Dunlins Calidris alpina and Purple Sandpipers Calidris maritima, two sandpipers that breed at similar latitudes as Ringed Plovers, subspecies delineation based on phenotypic characters is poorly supported by genetic markers. In contrast, in several temperate or tropic waders, sub species delineation is in agreement with patterns of genetic differentiation.

Why do mismatches between genetic and phenotypic data occur more frequently in high latitude species? The classical explanation is that climatic oscillations had a stronger impact at higher latitudes. But is that really so?



Thies, L., Tomkovich, P., dos Remedios, N, Lislevand, T., Pinchuk, P., Wallander, J., Dänhardt, J., þórisson, B., Blomqvist, D & Küpper, C. (2018) Population and Subspecies Differentiation in a High Latitude Breeding Wader, the Common Ringed Plover Charadrius hiaticula. Ardea 106(2), 163-176.


This paper has been added to the Charadriiformes page.

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: http://www.birdphoto.nl/)


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: http://www.wikipedia.com/).


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

Going with the flow: Gene flow from Chinese Spot-billed Ducks into Mallards

Genetic analyses show that Spot-billed Duck and Mallard diverged recently and hybridized in the process.

Telling male and female Mallards (Anas platyrhynchos) apart is not difficult. The male has an obvious fluorescent-green head, while the female is…well…brown (but nonetheless very beautiful). But did you know that most species in the Mallard complex are monochromatic (i.e. males and females have the same color)? Just have a look at the American Black Duck (A. rubripes) or the Hawaiian Duck (A. wyvilliana).

Most waterbird researchers argue that these monochromatic species originated from a dichromatic Mallard-like ancestor (see for example Omland 1997). A recent study in Current Zoology explored the evolutionary history of another monochromatic duck, the Spot-billed Duck (A. zonorhyncha).

mallard couple.jpg

Mallard couples are dichromatic: the brown female and the colorful male (from: http://www.wikipedia.com/).


Mixed Signals

Mallards and Spot-billed Ducks are difficult to separate with genetic data. When you construct a family tree for these duck species, you will arrive at clusters that contain both species. In technical terms, they do not form monophyletic groups. This pattern can be the outcome of two processes: recent divergence or hybridization.

Previous work in Russia suggested that hybridization was the main cause for this non-monophyly. Wenjuan Wang and colleagues expanded the sampling to China and reconstructed the evolutionary history of the Spot-billed Duck in more detail.


Mallards (black squares) and Spot-billed Ducks (white circles) do not form separate groups when you construct their family tree. They are not monophyletic (from: Kulikova et al. 2004 The Auk)


Too Little Time

The genetic analyses revealed that the non-monophyly between Mallard and Spot-billed Duck can be explained by both recent divergence and hybridization. Both species went their own evolutionary ways about 40,000 years ago. There has been too little time for the genetic variation to sort among the lineages.

In addition, hybridization between both species prevented faster differentiation. Isolation-with-migration analyses of mitochondrial DNA (mtDNA) indicated that gene flow was mainly from Spot-billed Ducks into Mallards. Given that mtDNA is maternally inherited, this suggests that female Spot-billed Ducks mostly mated with male Mallards.

This study nicely shows that ducks are a goldmine for those who want to understand the dynamics of hybridization.

spot-billed duck.jpg

A couple of Spot-billed Ducks (from: https://www.hbw.com)



Wang, W., Wang, Y., Lei, F., Wang, H. & Chen, J. (2018) Incomplete lineage sorting and introgression in the diversification of Chinese spot-billed ducks and mallards. Current Zoology

This paper has been added to the Anseriformes page.

Studying sperm success: A possible reproductive barrier between subspecies of the Long-tailed Finch

Interactions between sperm and egg might act as a reproductive barrier between two subspecies of an Australian finch.

Life as a sperm cell is not easy. In order to reach your goal (i.e. fertilizing the egg), you need to overcome numerous barriers. In birds, this includes among others passing the vagina, enter the female sperm storage tubules and reaching the site of fertilization. These obstacles can be especially problematic when you are a sperm cell in the body of a female from another species.


The different barriers a sperm cell has to cross before fertilizing an egg (from Birkhead & Brillard, 2007 Trends in Ecology and Evolution).

The challenges between copulation and fertilization of the egg are called postmating prezygotic barriers and might lead to reproductive isolation between two species. Because this process is difficult to study, it has been largely overlooked in birds. A recent study in Ecology and Evolution explored this reproductive barrier between subspecies of the Long-tailed Finch (Phoephila acuticauda).


Bill Colors

The Long-tailed Finch is endemic to Northern Australia. Based on bill color, you can distinguish between two subspecies: the red-billed hecki and the yellow-billed acuticauda. Interestlingly, hybrids between both subspecies have orange bills. A survey of bill color across their range indicated that there is selection against hybrids. Could this selection be due to postmating prezygotic barriers?


The two subspecies of Long-tailed Finch: the yellow-billed acuticauda and the red-billed hecki (from: https://griffithecology.com/research/long-tailed-finch/)


Sperm-Egg Interactions

To answer this question, Laura Hurley (Macquarie University) and her colleagues created captive crosses between the subspecies. They collected the eggs and extracted the perivitelline layer (PVL) of each egg. Next, they counted the number of sperm cells that managed to reach this layer. In birds, successful fertilization requires multiple sperm cells to penetrate the PVL. So, the number of sperm in the extracted PVL can be used as a proxy for fertilization success.



There was no significant difference in the number of sperm between pure crosses and hybrid pairs. However, when backcrossing hybrids with one of the parental species less sperm reached the PVL. This suggests that first generation hybrids have lower fertilization success. Similar results have been documented in wildfowl (order Galliformes) where later generation hybrids and backcrosses showed fertility problems. The authors indicate that their study “supports the role of [postmating prezygotic barriers] in avian specation, even in recently diverged taxa, that may not yet be fully genetically incompatible.”


A red-billed Long-tailed Finch (from: http://www.wikipedia.com/)



Griffith, S.C. & Hooper, D.M. (2017) Geographical variation in bill colour in the Long-tailed Finch: evidence for a narrow zone of admixture between sub-species. Emu – Austral Ornithology, 117: 141-150.

Hurley, L.L., Rowe, M. & Griffith, S.C. (2018) Differential sperm-egg interactions in experimental pairings between two subspecies and their hybrids in a passerine bird. Ecology and Evolution.


The papers have been added to the Estrildidae page.

Exploring the genomic landscape of wood-warblers

A more technical story on the evolution of genomic landscapes in wood-warblers.

Any birdwatcher can list the morphological characters that he or she uses to discriminate between closely related species. But how is this variation reflected on the genetic level? The possibility to sequence the entire genome of an individual has allowed biologists to explore this question. In general, they scan across the genome looking for differentiated regions (based on a collection of summary statistics). These ‘genomic landscapes’ hold to key to understand how bird species diversify. A recent study in the journal Molecular Ecology applied this approach to several members of the Parulidae family (New World warblers or wood-warblers).


Townsend’s Warbler (from: https://www.allaboutbirds.org/)


Some Summary Statistics

Darren Irwin (University of British Columbia) and his colleagues used three summary statistics: Fst, π(between) and π(within). What do these statistics actually represent?


The American geneticist Sewall Wright introduced Fst to explore how genetic variation is structured between populations by comparing the genetic variation within and between these populations. Fst ranges from 0 to 1, where zero implies that the two populations are freely interbreeding and where one implies that all genetic variation is explained by the population structure. This statistic can also be used to pinpoint genomic regions that are different between two populations.


This statistic is also known as Dxy and indicates the absolute genetic distance between two populations (in contrast to Fst, which is a relative measure). It counts the number of differences between two DNA sequences from distinct populations. For example, the sequences AATTCC and AATTGG differ in two positions. You can calculate π(between) by dividing the number of differences and the total sequence length: 2/6 = 1/3 or 33%.


The last summary statistic is similar to π(between) but instead of comparing individuals of two populations, you calculate it between individuals from the same population.


Mourning Warbler (from https://www.allaboutbirds.org/)


Four Models

By studying these three summary statistics, you can infer the evolutionary history of bird populations. For example, if two species diverged with some gene flow, you would expect that genomic regions that contribute to reproductive isolation – and are thus not exchanged between species – become different over time. So, both Fst and π(between) should be higher in these regions.

If, on the other hand, the species diverged without gene flow but were subject to different selection pressures, regions of high Fst are expected to show lower π(within), because selection tends to lower the genetic variation within a population.

Another pattern that is commonly observed in regions of high Fst and low π(between). This can be explained by a ‘recurrent selection’ model where these divergent genomic regions experienced strong selection before the species split, followed by recurrent selection after the split.

The authors of this study introduce a fourth model – ‘geographic-sweep-before-selective-differentiation’ – in which advantageous genetic variants spread through a geographically structured species complex. The outcome would be reduced π(between) and π(within) in regions of high Fst.

I can imagine that the four models and their predictions make your head spin. Luckily, the researchers provided a graphical representation of these models. Take your time to study this figure and see if you understand the expected patterns.


The four models and their predictions (From: Irwin et al. 2018, Molecular Ecology)


Three Species Pairs

The authors calculated the summary statistics for three species pairs in the Parulidae family:

  • MacGillivray’s Warbler (Geothlypis tolmiei) and Mourning Warbler (G. philadelphia)
  • Townsend’s Warbler (Setophaga townsendi) and Black-throated Green Warbler (S. virens)
  • Audubon’s Warbler (S. auduboni) and Myrtle Warbler (S. coronata)

In all three comparisons, the genetic patterns point to a model of recurrent selection or sweep-before-differentiation (models c and d in the figure). A more thorough modelling study will be needed to discriminate between these two models.

Interestingly, most of the genomes show little differentiation, only a small fraction is different. This suggests that reproductive isolation is caused by a few highly divergent regions, a pattern that has been observed in several other bird species, such as Wagtails and Nightingales. Moreover, these highly divergent regions – or ‘islands of differentiation’ –  were not shared between the species pairs, indicating that the causes of genomic differentiation are specific to each speciation event.


Myrtle Warbler (from http://www.wikipedia.com/)



Irwin, D.E., Mila, B., Toews, D.P.L., Brelsford, A., Kenyon, H.L., Porter, A.N., Grossen, C., Delmore, K.E., Alcaide, M. & Irwin, J.H. (2018) A comparison of genomic islands of differentiation across three young avian species pairs. Molecular Ecology.


This paper has been added to the Parulidae page.

Newly discovered hybrid zone between Colima and Virginia’s Warbler in Texas

Another case of hybridization in Wood-warblers.

If you want to study avian hybridization, the Parulidae family (Wood-warblers) is an obvious choice. Since 1980, hybrids have been documented in 24 out of 45 species (based on a 2014 paper). A recent study in Texas adds another hybrid to the list: Colima Warbler (Oreothlypis crissalis) x Virginia’s Warbler (O. virginiae).

virginias warbler.jpg

A Virginia’s Warbler (from: http://www.audubon.org/)


Surveys in the Davis Mountains

In June 1999, extensive surveys in the Davis Mountains revealed the presence of singing Colima Warblers, the first documented occurrence outside of its main breeding distribution in the Chisos Mountains. More efforts were put into locating additional territories of this species. During this search it became apparent that some Colima Warblers showed plumage characteristics and song patterns that were similar to Virginia’s Warbler. Could these be hybrids?


A Colima Warbler (from: http://www.audubon.org/)


Intermediate Plumage and Song Patterns

Several birds were caught on camera or trapped with mist nests. Morphological analyses confirmed previous observations: they exhibited plumage patterns that were intermediate between both parental species. Overall, the body plumage is darker than the gray of a typical Virginia’s Warbler, but the yellow tail coverts and the lighter gray head and underparts lead to a more Virginia-like appearance. There is, however, considerable variation in plumage patterns, suggesting some degree of backcrossing.

The morphological evidence was corroborated with acoustic data. Between 1999 and 2006, the songs of several birds were recorded. Interestingly, putative hybrids responded more strongly to ‘hybrid songs’ compared to recordings of pure species.

The morphological and acoustic data already point to hybrids in the Davis Mountains, but genetic analyses will be necessary to characterize this newly discovered hybrid zone in more detail.

2070-84858 hybrid warbler AHY M return on 91918 copy.jpg

A hybrid Virginia’s x Colima Warbler (picture courtesy of Kelly B. Bryan)


Bryan, K.B. & Lockwood, M.W. (2018) Plumage Characteristics and Song Patterns of Presumed Colima x Virginia’s Warbler Hybrids in the Davis Mountains of Texas. North American Birds, 70(2): 142-154.


This paper has been added to the Parulidae page.

Thanks to Darren Irwin for pointing this study out to me and to Kelly Bryan for sending me the PDF and pictures!

What selection pressure is acting on hybrids between Audubon’s and Myrtle Warbler?

Another attempt to characterize the selection pressure acting on warbler hybrids.

What form of selection is acting on hybrids between Audubon’s Warbler (Setophaga coronata auduboni) and Myrtle’s Warbler (S. c. coronata)? It is a question that is haunting several American ornithologists. Genetic analyses of a hybrid zone between these species indicate that there is some selection against hybrids, but the exact mechanism remains a mystery.

Some weeks ago I wrote about a paper that tested whether parasites might be involved. The answer was negative, so the search for the exact selection mechanism continues. The latest attempt was recently published in the journal The Auk.


An Audubon’s warbler (from: https://www.allaboutbirds.org/)


Comparing Classes

David Toews and his colleagues wanted to know if there are any differences in viability between male and female hybrids or between different age classes. If they uncover differences, this might provide important insights into what kind of selection is acting on the hybrids.

Females might be less fit compared to males. This pattern – known as Haldane’s Rule – has been observed in several bird species. If Haldane’s Rule also applies to these warblers, we would expect to find more early-generation male compared to female hybrids.

There might also be differences between age classes due to variation in migration strategies. If difference species follow distinct migration routes, hybrids might opt for an intermediate – and possibly sub-optimal – route. This could increase the mortality rate among hybrids. If this is the case, we would expect more early-generation hybrids before their migration.


A female Myrtle warbler (from: https://www.allaboutbirds.org/)



To test these two expectations, the researchers used triangle plots (I have written about these before). Here is the crystal-clear explanation from the paper:

“In these plots, individuals near the top of the triangle are first-generation hybrids (i.e. they are heterozygous at nearly all the highly divergent sites). Individuals falling along the right and left edges of the triangle are backcrosses to the parental forms; individuals within the center of the triangle are F2, F3, and subsequent hybrid classes.”


A triangle plot to deduce the genetic ancestry of hybrids (from Toews et al. 2018, The Auk)


And the selection pressure is …

These triangle plots were compared between the different classes (males vs. females and pre-migration vs. post-migration) and revealed…no differences. There seems to be no selection against a particular sex or age class. Hence, the quest for the hybrid warbler selection pressure will continue.



Toews, D.P.L., Lovette, I.J., Irwin, D.E. & Brelsford, A. (2018) Similar hybrid composition among different age and sex classes in the Myrtle-Audubon’s warbler hybrid zone. The Auk, 135: 1133-1145.


The paper has been added to the Parulidae page.

Crisscrossing Europe: The genetics of crossbills in the western Palearctic

What drives genetic differentiation in European crossbills?

Crossbills are a textbook example of how adaptation to different resources can result in genetic differentiation. I remember reading a 2003 paper by Craig Benkman during my Masters in Antwerp. This article, entitled ‘Divergent selection drives the adaptive radiation of crossbills‘, featured a figure showing how groups of crossbills with different beak morphologies are adapted to different species of pine (see below). Later research showed that these distinct beak shapes result in several call types, which consequently leads to assortative mating (i.e. birds prefer a partner with the same call). In the end, this culminates in the build-up of genetic differentiation between the call types, the onset of ecological speciation!

This fascinating system has been studied for years in North America. But what about the crossbills in Europe? Do they show similar patterns? Thomas Parchman and his colleagues crossed the Atlantic to figure this out. Their findings recently appeared in the Journal of Evolutionary Biology.


Different groups of crossbills are adapted to different species of pine (from: Price 2008).


Should I stay of should I go now?

The European subspecies of the common crossbill (Loxia curvirostra) mostly feed on the Aleppo pine (Pinus halepensis), which occurs around the Mediterranean. Three subspecies are sedentary: balearica on Mallorca, poliogyna in northern Africa and hispana in Spain. The northern subspecies curvirostra, however, often undertakes long-distance movements when food resources are sparse. These nomadic ventures can be quite impressive, as noted by the English monk Matthew Paris:

“In 1254, in the fruit season, certain wonderful birds, which had never before been seen in England, appeared, chiefly in the orchards. They were a little bigger than Larks, and eat the pippins of the apples [pomorum grana] but no other part of them… They had the parts of the beak crossed [cancellatas] by which they divided the apples as with a forceps or knife. The parts of the apples which they left were as if they had been infected with poison.”


Geographical Isolation

The occasional movements of curvirostra into the distribution of other subspecies could potentially lead to hybridization and gene flow. To test this idea, the researchers compared genetic data from these European subspecies using a genotyping by sequencing (GBS) approach. The results indicated that balearica and poliogyna were clearly different from the other subspecies, probably because they are geographically isolated in Mallorca and northern Africa, respectively.


Two crossbills on a pine tree (from: http://www.wikipedia.com/)


Resource Competition

The degree of genetic differentiation between the Spanish hispana and the northern curvirostra suggests that other factors than geographical isolation are at play. The Spanish birds differ in beak morphology from their northern relatives because they are adapted to the local pine trees, which provide a stable food source. When nomadic curvirostra crossbills arrive in Spain, they would be outcompeted by the locally adapted hispana birds. This might prevent interbreeding and thus promote genetic divergence.


Parrot Crossbill

Finally, the researchers also compared the northern subspecies with the parrot crossbill (L. pytyopsittacus), a species that occurs in the same area. Previous analyses found no clear genetic differences between these species. The present study did uncover some genetic differentiation, suggesting that a few genomic regions are responsible for the morphological differences between common crossbill and parrot crossbill. A pattern that has been observed in other bird species as well (see for example wagtails and crows). Pinpointing these differentiated regions – and checking if they are related to adaptation to local pine species – is a promising next step. Fingers crossed!


A parrot crossbill – picture by Tom Melling (from: http:www.flickr.com/)



Parchman, T.L., Edelaar, P., Uckele, K., Mezquida, E.T., Alonso, D., Jahner, J.P., Summers, R.W. & Benkman, C.W. (2018) Resource stability and geographic isolation associated with genome divergence in western Palearctic crossbills. Journal of Evolutionary Biology