Off-color Orioles: Hybrids or abnormal plumage patterns?

A high frequency of aberrantly plumage orioles in Australia and New Guinea. 

Occasionally ornithologists come across a bird with aberrant plumage. Abnormal plumage patterns can be the result of problems with feather growth or issues with pigmentation. Alternatively, aberrant plumage can point to hybridization. In some cases, the bird shows characteristics of both parents. But how can you confidently discriminate between these explanations? One way is to sequence the DNA of the bird in question. And that is exactly what Leo Joseph and his colleagues did when they stumbled upon some orioles (genus Oriolus) with peculair plumage. Their findings recently appeared in the journal Emu – Austral Ornithology.

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An Olive-backed Oriole © Patrick_K59 | Wikimedia Commons

 

Noticeable, aberrant plumage

During field work in Papua New Guinea, three researchers (Leo Joseph, Ian Mason and Alex Drew) were collecting orioles. They captured four individuals, representing two species: the Olive-backed Oriole (O. sagittatus) and the Green Oriole (O. flavocinctus). Three of them were “unremarkable” members of their species. The fourth, however, caught the attention of its captors. This specimen, that received the name ANWC B56219, was “an adult female also from Bensbach [with] an easily noticeable, aberrant plumage unknown in Oriolus.”

 

Hypotheses

What could explain this remarkable plumage pattern? There are several explanations: (1) it could be a new, undescribed taxon, (2) an individual with aberrant plumage, or (3) a hybrid. Regarding the hybrid hypothesis, there were three possible species involved: the two species collected in the area (Olive-backed Oriole and Green Oriole) and a third species, the Brown Oriole (O. szalayi). To solve this mystery, the researchers sequenced the mitochondrial gene ND2, which has been shown to be diagnostic for the three species, for the aberrant specimen.

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A Green Oriole © Francesco Veronesi | Wikimedia Commons

 

Specimens A and B

The analyses revealed that ANWC B56219 had the ND2-gene of Olive-backed Oriole. The plumage trait resembled Green Oriole in the front and Olive-backed Oriole in the back. You can compare this bird in the large figure below (marked as specimen A) with the other species. Together, these observations point to a hybrid. Because of the mitochondrial gene (which is passed on through the female line), we can deduce that this bird is probably the result of hybridization involving a female Olive-backed Oriole followed by several generations of backcrossing with Green Orioles.

The genetic analyses also incovered an unexpected result. An unremarkable Olive-backed Oriole carried the mtDNA of Green Oriole. Close inspection of the plumage indicated that the dorsal parts (crown and mantle) do indeed resemble Green Oriole. Here, we have a bird that is the result of hybridization between a female Green Oriole and a male Olive-backed Oriole. It is marked as specimen B in the figure below.

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An overview of all two hybrids (Specimens A and B) in comparison to the “pure” species. From: Joseph et al. (2019) Emu – Austral Ornithology

 

More Hybrids?

A search through several museums uncovered more noticeable orioles. In the American Museum of Natural History (ANHM), they found an aberrant Olive-backed Oriole from 1937. The label indicated that it was a female from “Bugi, 3 miles east of mouth of Mai Kussa River.” The Port Moresby (PM) collection housed a bird that was previously described as a rare grey morph of the Green Oriole. Are these two specimens also hybrids? The plumage characteristics of the 1937 specimen suggest a hybrid origin. The grey morph, however, seems to be an aberrant individual and not a hybrid. Unfortunately, genetic data is missing for these birds. Perhaps they will figure it out in the future. And then you can read all about it at the Avian Hybrids Project.

 

References

Joseph, L., Dolman, G., Iova, B., Jønsson, K., Campbell, C. D., Mason, I., & Drew, A. (2019). Aberrantly plumaged orioles from the Trans-Fly savannas of New Guinea and their ecological and evolutionary significance. Emu-Austral Ornithology, 1-10.

 

This paper has been added to the Oriolidae page.

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Pecking in the Pyrenees: No gene flow across a hybrid zone between Eurasian and Iberian Green Woodpecker

Genetic study finds few admixed individuals in the contact zone.

Today I learned a new word: yaffling. It refers to the sound a Eurasian Green Woodpecker (Picus viridis) makes when it flies off (click here to listen to it). I have heard this loud “laughter” during countless forest walks. Indeed, this distinctive call echoes throughout most of the European forests because this species occurs from northern Sweden all the way down to southern France. When you cross the Pyrenees into Spain, however, you will meet another woodpecker species – the Iberian Green Woodpecker (P. sharpei) – that produces a similar call. A recent study in the Journal of Ornithology explored a contact zone between these two woodpeckers.

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Eurasian Green Woodpecker © MoniqueWN | Wikimedia Commons

 

Le Pic Vert

The Iberian and Eurasian Green Woodpecker might sound alike, they differ by several plumage features. The color of the face is most obvious: the Iberian birds have a grey face whereas the face is black in Eurasian woodpeckers. In the Pyrenees, which we crossed in the introduction of the blog post, individuals with intermediate plumage have been observed. Already in the 1920s, Jouard asked if there was “une nouvelle forme de Pic Vert” in the Pyrenees (For my non-French-speaking readers: a new form of Green Woodpecker).

 

No Gene Flow

This observation, along with more recent ones, suggests that there is a hybrid zone between these Woodpeckers. Jean-Marc Pons and his colleagues collected samples within and outside the putative contact zone. Based on a suite of 19 molecular markers, they characterized the genetic structure in this region. They found “no introgression of nuclear loci in allopatric populations located on both sides of the contact zone, which thus acts as an efficient barrier to gene flow.” This finding contributes to the idea that Eurasian and Iberian Green Woodpecker are distinct species.

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Iberian Green Woodpecker © Luis García | Wikimedia Commons

 

Tension Zone

In the contact zone, the researchers found several individuals that carried DNA from both species. This indicates that although hybridization occurs, the genes are not flowing between the species. There is probably some selection against hybrids. Based on these findings, the contact zone is most likely a so-called tension zone. This particular type of contact zone occurs when the effects of dispersal of parental species into the zone and selection against hybrids balance each other.

Cline analyses revealed that the hybrid zone is about 245 kilometers wide and centered in the area between Béziers and Montpellier (for more on cline theory, you can read this blog post). This seems like the perfect place for a hybrid zone, given that the University of Montpellier is known for its excellent evolutionary research.

 

References

Pons, J. M., Masson, C., Olioso, G., & Fuchs, J. (2019). Gene flow and genetic admixture across a secondary contact zone between two divergent lineages of the Eurasian Green Woodpecker Picus viridis. Journal of Ornithology, 1-11.

 

This paper has been added to the Piciformes page.

Back from the brink of extinction: The remarkable recovery of the Crested Ibis

The severe bottleneck has left its footprints in the genomes of these birds.

Seven. There were only seven individuals of the Crested Ibis (Nipponia nippon) in 1981. China and Japan started extensive captive breeding programs to save this species from extinction. Currently, the Crested Ibis is still endangered but the population has increased to over 2,000 individuals. A remarkable recovery. However, the severe bottleneck has left its traces in the genome of the Toki (its Japanese name). A study in the journal Current Biology documented the genetic legacy of this history by comparing contemporary samples with historical ones.

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A crested Ibis in the Xi’an QinLing Wildlife Park (China)  © Danielinblue | Wikimedia Commons

 

Low Genetic Diversity

An international group of scientists sequenced the genomes of 57 historical samples (dating between 1841 and  1922) and eight contemporary birds. These genomes revealed that the historical samples clustered according to their geographic origin. The contemporary samples formed a separate group, indicating that they can be traced back to a few individuals.

The clustering analyses suggests that a lot of genetic diversity has been lost over time. Indeed, nucleotide diversity is considerably lower in the contemporary samples. Moreover, several genetic loci that were polymorphic (i.e. had multiple variants) in the historical samples have gone to fixation (i.e. only one variant is left) in present-day birds. You can compare this situation with a pool filled with differently colored balls. Over time, more and more colors disappear until you are left with only one color. A dramatic loss in diversity.

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Nucleotide diversity across the genomes of historical (blue) and contemporary (red) samples. From: Feng et al. (2019) Current Biology

 

Inbreeding Depression

These results point to high levels of inbreeding. Not that surprising, given that the population was reduced to only seven individuals. To quantify the severity of this inbreeding, the researchers counted the number of homozygous deleterious mutations in the genomes of these birds. Homozygous indicates that both chromosomes have the same variant and deleterious refers to the negative effect on the fitness of the individual. Several diseases are caused by this type of mutation. This analysis revealed that the number of these mutations has doubled in less than 100 years. That is a severe bottleneck.

The seriousness of this situation is nicely exemplified by the Major Histocompatibility Complex (MHC). This set of genes is essential for the immune response that protects individuals from harmful bacteria and viruses. Ideally, the MHC is very variable, allowing it to swiftly react to a broad range of pathogens. In the Crested Ibis, however, the MHC has experienced a drastic decrease in genetic diversity. This makes these birds susceptible for different diseases.

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The genetic diversity of MHC is drastically reduced in contemporary birds (outer ring), as shown by the large stretches of low diversity regions (dark-blue). From: Feng et al. (2019) Current Biology

 

Conservation

Although the Crested Ibis has made a remarkable recovery, the footprints of the severe bottleneck are still visible in their genomes. The low genetic diversity renders these birds vulnerable to future perturbations. It is thus essential to continue the conservation efforts to ensure the survival of this beautiful bird.

 

References

Feng, S., Fang, Q., Barnett, R., Li, C., Han, S., Kuhlwilm, M., Zhou, L., Pan, H., Deng, Y., Chen, G., Gamauf, A., Woog, F., Prys-Jones, R., Marques-Bonet, T., Gilbert, M. T. P. & Zhang, G. (2019). The genomic footprints of the fall and recovery of the crested ibis. Current Biology29(2), 340-349.

How many species of Red-shouldered Hawk are there?

Which subspecies deserve a species status?

The taxonomy of raptors can be complicated as shown by several species complexes, such as the Northern Goshawk (Accipiter gentilis) and Palearctic buzzards (genus Buteo). Another example is the Red-shouldered Hawk (Buteo lineatus), a common sit-and-wait predator from North America. Currently, ornithologists recognize five subspecies. One subspecies (elegans) occurs in the west of the US, while the remaining four can be found in the east (alleni, extimus, lineatus, and texanus). Do some of these subspecies actually represent distinct species? A recent study in the journal Ecology and Evolution tried to figure it out.

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A Red-shouldered Hawk in Florida © Andy Morffew | Wikimedia Commons

 

East vs. West

George Barrowclough and his colleagues collected samples across the entire range of the Red-shouldered Hawk. They sequenced the mitochondrial ND2 gene and two nuclear introns. These genes revealed a clear difference between the eastern and western populations. The western birds in California – originally described as a distinct species Buteo elegans by Cassin (1855) – are clearly a separate species. This conclusion is corroborated by morphological data: California birds have a much richer rufous coloration and can be diagnosed using the number and size of tail bands.

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Morphological differences between western (elegans) and eastern (lineatus and extimus) populations of the Red-shouldered Hawk. From: Complete Birds of North America, 2006.

 

Florida Population

What about the four subspecies the eastern part of the range? Three subspecies – alleni, lineatus and texanus – were genetically similar. The fourth subspecies (extimus), however, was genetically distinct from the rest (see haplotype network below). This subspecies, which occurs in Florida, is substantially smaller and paler than the other three subspecies.

Does the Florida population represent a different species? The genetic analyses uncovered extensive gene flow between this population and the northern birds. Following the Biological Species Concept (which emphasizes reproductive isolation), the Florida birds would be a well-differentiated subspecies. However, the authors of the present study argue that the population in Florida should be considered a distinct species:

However, our opinion is that the Florida peninsula population has had a separate evolutionary history from that of the other eastern birds and consequently represents an appropriate unit for studies of diversification and historical biogeography; therefore, it represents a phylogenetic species. This would not be apparent were the taxon to be simply regarded as a subspecies in the B. lineatus complex.

In this regard, they follow the Evolutionary Species Concept, which states that “A species is an entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary fate and historical tendencies.” Hence, we can recognize three species in the Red-shouldered Hawk:

  • Red-shouldered Hawk (B. lineatus)
  • California Red-shouldered Hawk (B. elegans)
  • Florida Red-shouldered Hawk (B. extimus)

You can check this recent blog post for a longer discussion on avian species concepts.

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Haplotype network of the ND2 gene in the Red-shoulderd Hawk and a map of the geographic distribution of these haplotypes. This analysis suggests three separate species (represented in different colors). From: Barrowclough et al. (2019) Ecology and Evolution

 

References

Barrowclough, G. F., Groth, J. G., Mauck, W. M., & Blair, M. E. (2019). Phylogeography and species limits in the red‐shouldered hawk (Buteo lineatus): Characterization of the Northern Florida Suture Zone in birds. Ecology and Evolution, 9(11):6245-6258.

 

This paper has been added to the Accipitriformes page

It takes two: The evolution of duets in New World warblers

Ornithologists uncover relationship between duets and migration.

Everybody knows that birds sing. But did you know that some species sing duets? Males and females sometimes combine their songs into a harmonious melody. The Carolina Wrens (Thryothorus ludovicianus), for example, produce a primitive duet. The male sings a song that sounds like tea-kettle, tea-kettle, tea-kettle, and from time to time the female jumps in with a buzzy sound. You can listen to such a duet in the video below.

 

Pair Bonds

Ornithologists think that birds mainly sing these duets to defend their territories. In addition, duets might also play a role in strengthening pair bonds. The more time birds spend together, the more likely they will evolve the ability to produce duets. The duration of a pair bond can be limited by migration during which partners go their separate ways. So, you would expect that duets are negatively associated with migration. A recent study in the journal The Auk tested this prediction for the bird family Parulidae (the New World warblers).

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A singing Common Yellowthroat (Geothlypis trichas). However, this species does not duet. © Wolfgang Wander | Wikimedia

 

Evolution

Liam Mitchell and his colleagues investigated 107 species of warblers and found evidence for duets in 19 species. When they correlated this behavior with migratory strategies, they uncovered a significant relationship. In line with the prediction outlined above, birds that produce duets tend to be sedentary. The researchers also reconstructed the evolutionary history of duets, revealing that this behavior evolved several times (concentrated in particular genera, such as Myioborus and Myiothlypis). The ancestor of these birds probably did not sing duets.

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The evolutionary history of duets and migration in the New World warblers. Birds that sing duets (red dots on the left) tend to be sendentary (blue dots in the right). From: Mitchell et al. (2019) The Auk

 

A role for hybridization?

One big assumption in this macro-evolutionary study is that the traits under investigation (here, duets and migration) follow the species tree. But this does not have to be the case (see this commentary in Evolution). If traits have a genetic basis, they could be exchanged between species by hybridization. On the evolutionary tree, it might seem like an independent origin, while in reality the trait was transferred during a hybridization event.

I don’t know whether this scenario applies to the duet behavior in New World warblers. These birds are known to hybridize extensively (see here for an overview), but the genetic basis of duets is – to my knowledge – still a mystery. It would be interesting to pinpoint the genes underlying this behavior and reconstruct their evolutionary history. Will they follow the species tree?

 

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A duetting species from Costa Rica, the Collared Whitestart (Myioborus torquatus) © Cephas | Wikimedia Commons

 

References

Hahn, M. W., & Nakhleh, L. (2016). Irrational exuberance for resolved species trees. Evolution, 70(1), 7-17.

Mitchell, L. R., Benedict, L., Cavar, J., Najar, N., & Logue, D. M. (2019). The evolution of vocal duets and migration in New World warblers (Parulidae). The Auk: Ornithological Advances, 136(2), ukz003.

 

A philosopher claims species do not exist. He is wrong.

What is a species?

Philosophers think a lot. Sometimes they think so much that they get entangled in their own thoughts and do not see the flaws in their reasoning (theology is the perfect example of this). Henry Taylor – a philosopher from the University of Birmingham – wrote an article on The Conservation in which he claims that species do not exist. He argues that we should scrap the idea of a species and “think of life as one immense interconnected web.” His article has already been criticized by evolutionary biologist Jerry Coyne. I recently published a book chapter on avian species concept, so this is a nice opportunity to summarize this chapter and correct the sloppy reasoning of this philosopher.

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There are about 10,000 bird species. And they do exist. From: https://www.trendhunter.com/

 

We know what a species is…in theory

After a general introduction about taxonomy, Taylor writes the following sentence: “So, what even is a species? The truth is, we don’t really have any idea.” Actually, we do have an idea. But to understand the solution to this species problem, we need to make a distinction between the theoretical question of what species are (i.e. species concepts) and the ways in which species can be delimited in practice (i.e. species criteria). From a theoretical point of view, we know what species are. This part of the species problem was independently solved by Richard Mayden and Kevin De Quieroz.

Richard Mayden examined 22 distinct species concepts and proposed a hierarchy of species concepts, with a primary theoretical species concept and several secondary operational species concepts. He argued that only one concept is suitable as primary concept, namely the Evolutionary Species Concept: “A species is an entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary fate and historical tendencies”. The remaining, secondary concepts function as guidelines that are essential for the study of species in practice. So, they are actually species criteria instead of concepts.

Similarly, Kevin De Queiroz reviewed several existing species concepts and argued that all existing species concepts are variants of a single general concept, which he dubbed the General Lineage Concept. Species are considered separately evolving metapopulation lineages. A lineage indicates an ancestor-descendant series, and metapopulation refers to an inclusive population made up of connected subpopulations.

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The hierarchy of species concepts with an overarching primary concept (i.e. the Evolutionary Species Concept) and several secondary concepts that are used as criteria. From: Mayden (1997) Species: The units of diversity.

 

A Life History Approach

So far, so good. From a theoretical point of view, we can define species according to the Evolutionary Species Concept and the General Lineage Concept. But what about species delimitation in practice? Which secondary concepts should we use? That choice will depend on the evolutionary history of the species in question. Here, a “life history approach” is warranted, in which different species concepts correspond to different stages in the life history (i.e. speciation process) of a species.

It is important to keep in mind that the order in which species concepts arise is contingent upon the speciation process. In some cases, morphological differentiation might evolve first, followed by reproductive isolation. In other cases, it might be the other way around. And in yet other cases, some species concepts will not apply.

Moreover, during the speciation process, there will be a grey zone in which different species criteria come into conflict. For example, several putative species of Redpoll Finches (genus Acanthis) are morphologically different despite largely undifferentiated genomes. There is a conflict between a morphological and a genetic species concept. What should taxonomists do now? The answer is integrative taxonomy.

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This simplified diagram represents a single lineage splitting into two independently evolving lineages (or species). The horizontal lines represent the times at which the lineages acquire different properties (e.g. they become phenetically distinguishable, reproductively isolated, reciprocally monophyletic, etc.). This set of properties (SC species concept) coincides with a grey zone in which alternative species concepts come into conflict. On either side of the grey zone, there is agreement on the number of species. Adapted from De Queiroz (2007) Systematic Biology.

 

Integrative Taxonomy

The rationale behind integrative taxonomy is quite straightforward: different taxonomic concepts and methods are integrated in the delimitation and description of species. A recent paper on taxonomic bird studies found that nearly half (46.5%) applied multiple criteria in species delimitation. Within this context, two general frameworks have been advocated: integration by congruence and integration by cumulation.

The congruence approach to species delimitation entails that different data sets, such as molecular and morphological characters, support the decision to recognize certain taxa as valid and distinct species. For example, Per Alström and his colleagues used congruence between plumage, biometrics, egg coloration, song, mitochondrial DNA and distribution to draw species limits in the Bradypterus thoracicus complex. The main advantage of this approach is that most taxonomists will agree on the validity of a species supported by several independent data sets, leading to taxonomic stability.

The alternative framework, integration by cumulation, is based on the assumption that any of the data sets can be used as evidence for the delimitation of species. Congruence is desired but not necessary. In practice, evidence from different data sets is cumulated, concordances and conflicts are explained within the specific evolutionary context of the taxa under study, and based on the available evidence a decision is made. An advantage of this approach is that species delimitation is not restricted by one particular biological property.

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Species limits in the Bradypterus thoracicus complex were determined using an integrative approach based on several criteria. From: http://www.hbw.com/

 

An Interconnected Web

I hope that the explanations above have convinced you that species do exist and that we can define them in practice (although drawing species limits is not always straightforward). What about the statement by Henry Taylor that “we should think of life as one immense interconnected web.” I agree with this statement but it does not lead to the conclusion that species do not exist.

Hybridization is a common phenomenon (as this website clearly shows for birds). Numerous species are known to exchange genetic material through hybridization. But does that invalidate their rank as distinct species? No, despite the occasional genetic contribution of other taxa, species tend to maintain their own identity with their own independent evolutionary fate and historical tendencies (as stated by the Evolutionary Species Concept).

In the end, Taylor writes that “there is no such thing as ‘the human species’ at all.” He does not explain the exact reasoning behind this bold statement, but I assume he refers to the interbreeding between humans, Neanderthals and Denisovans. Does gene flow between these members of the genus Homo result in the disappearance of the species Homo sapiens? Of course not! The genes that we obtained from Neanderthals and Denisovans have certainly influenced our consequent evolution but overall the human species has maintained its own identity with its own independent evolutionary fate and historical tendencies (sorry to repeat this concept, but I really want to drive the point home here).

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The complex evolutionary history of humans with multiple events of interbreeding with Neanderthals and Denisovans. From: Callaway (2016) Nature

 

Avian Species Concepts in the Light of Genomics

I will end my discussion of the species problem here. To summarize, the species problem can be partly resolved by theoretical monism (the evolutionary species concept or general lineage concept) in combination with practical pluralism, in which different species criteria correspond to different stages during the speciation process.

In my book chapter on species concept, I focused on the role of genomics in this debate. If you are interested in this question, feel free to contact me for a PDF of the chapter. I will paste the main points from the abstract below:

In this chapter, I argue that genomics provides another line of evidence in this pluralistic approach to species classification. Indeed, genomic data can be combined with classical species criteria, such as diagnosability, phylogeny and reproductive isolation. First, genomic data can provide an extra diagnostic feature in species delimitation. Compared to ‘old-school’ genetic markers, the use of genome-wide markers leads to a significant rise in statistical power. Second, phylogenomic analyses can resolve the evolutionary relationships within rapidly diverging or hybridizing groups of species while taking into account gene tree discordance. Third, genomic data can be used to pinpoint the genetic basis of reproductive isolation and provide a detailed description of the speciation process. All in all, the genomic era will supply avian taxonomists with a new tool box that can be applied to old concepts, leading to better informed decisions in cataloguing biodiversity.

 

References

Ottenburghs, J. (2019). Avian species concepts in the light of genomics. In Avian Genomics in Ecology and Evolution (pp. 211-235). Springer, Cham.

 

Can Mandarin Ducks hybridize with other duck species?

Send me your pictures of Mandarin Duck hybrids!

This week, Jan Harteman (from Harteman Wildfowl) asked me if Mandarin Ducks (Aix galericulata) hybridize with other duck species. Someone once told him that this duck species cannot interbreed with other ducks because of a difference in chromosome number. My overview of duck hybrids on the Anseriformes page lists six captive crosses with Mandarin duck:

  • Wood Duck (Aix sponsa)
  • Laysan Duck (Anas laysanensis)
  • Mallard (Anas platyrhynchos)
  • Gadwall (Anas strepera)
  • Redhead (Aythya americana)
  • Long-tailed Duck (Clangula hyemalis)
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The colorful Mandarin Duck © Alexandra Sora | Wikimedia Commons

 

Reliable Sources?

These hybrid records are based on the Serge Dumont Bird Hybrid Database, which provides references for all the listed hybrids. Most of the reported hybrids can be traced back to the Hybrid Ducks inventory of Eric and Barry Gillham. A few hybrids are supported by older articles in the Avicultural Magazine, namely “On Mandarin Duck Hybrids” by Prestwich (1960) and “Reference to possible Mandarin x Wood (Carolina) Duck and Wood Duck x Mandarin Hybrids Bred at Tracy Aviaries, Salt Lake City” by Anon (1965). Unfortunately, I cannot access these articles and judge their reliability.

There is, however, a paper by Paul Johnsgard on these putative hybrids. He provides good evidence for the Mandarin Duck x Laysan Duck hybrid but casts some doubt on the crosses with Long-tailed Duck and Redhead. In addition, he described some possible hybrids between Mandarin Duck and Wood Duck. It seems that these two species can interbreed.

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A putative hybrid between Mandarin Duck and Wood Duck © Quartl | Wikimedia Commons

 

Chromosomes

The idea that Mandarin Ducks cannot hybridize with other species due to a chromosomal difference has a long history. Johnsgard writes the following.

Prestwich (1960) and Gray (1958) have concluded, mirroring Delacour and Mayr (1945) and Seth-Smith (1922), that the Mandarin Duck is unable to hybridize, even with its nearest living relative the Wood Duck. The explanation usually advanced for this seemingly unique situation is a reportedly an aberrant chromosomal condition of the Mandarin Duck (Yamashima 1952).

I could not access the paper by Yamashima, but I did find the karyotype for the Mandarin duck in another study. This duck has 84 chrosomosomes whereas other ducks have 80. This difference could prevent the production of hybrids. However, horses (with 64 chromosomes) and donkeys (with 62) can interbreed and the resulting offspring, mules and hinnies, have 63 chromosomes. Perhaps other chromosomal rearrangements might have occurred in the Mandarin Duck genome? Indeed, in the book “The Cell Nucleus“, I found that all the chromosomes of the Mandarin Duck are acrocentric, meaning that one chromosomal arm is much shorter than the other. In other duck species, the chromosomes can also be submetacentric (i.e. one chromosome arm is somewhat shorter than the other). This different chromosomal shape could explain the difficulty of producing hybrids with this species.

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The different types of chromosomes. Mandarin Ducks only have acrocentric chromosomes (highlighted with black box). From: http://www.liberaldictionary.com/

 

References

Anon (1965) Reference to possible Mandarin x Wood (Carolina) Duck and Wood Duck x Mandarin Hybrids Bred at Tracy Aviaries, Salt Lake City. Avicultural Magazine, 71(3), 96-97.

Johnsgard, P. A. (1968). Some putative Mandarin Duck hybrids. Bulletin of the British Ornithologists Club, 88, 140-148.

Prestwich A. A. (1960) On Mandarin Duck Hybrids. Avicultural Magazine, 66(1), 5-8.

Shields, G. F. (1982). Comparative avian cytogenetics: a review. The Condor, 84(1), 45-58.

 

Thanks to Jan Harteman for bringing this mystery to my attention. If you have any burning questions about avian hybrids, feel free to contact me.

The social lives of quails: Why do California and Gambel’s quail hybridize?

Social network analyses might provide the answer to this question.

Why do some birds choose a partner from a different species? Couldn’t they find a member of their own species? Did they want to try something new? Or did they just make a mistake? These questions come to mind when you observe a hybrid individual in the wild. A recent study, published in The American Naturalist, tries to solve this conundrum for a pair of quail species (genus Callipepla).

 

Hybrid Zone

California Quail (C. californica) and Gambel’s (C. gambelii) Quail are sister species that hybridize along a contact zone in Southern California. Previous work showed that these species can discriminate between each other in captivity. In the wild, however, mating seems random, resulting in mixed pairs and consequently hybrids. These patterns raise the question how quails choose a partner.

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A California Quail © Alan Schmierer | Flickr

 

Social Interactions

To investigate the mating patterns of these quails, David Zonana and his colleagues turned to social network analyses. They equipped several birds with automated radio-frequency identification tags that generate detailed data on which individuals interact with one another. These interactions can be visualized in social networks in which the individuals represent dots and the connecting lines (or edges) indicate the strength of interaction. The figure below shows the social network for the quails.

Analyses of these social networks can tell us more about the way quails select their partner. Surprisingly, they do not use species-specific plumage during mate choice. These plumage patterns did not correlate with the structure and strength of the associations in the network. Instead, the birds focus on two other characteristics: body mass and monomorphic plumage (i.e. the same in both sexes).

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A social network for the California Quail and Gambel’s Quail. The dots represent individuals (blue = male, orange = female) and the lines indication interactions. The thicker the lines, the stronger the association. From: Zonana et al. (2019) The American Naturalist

 

Body Mass and Shared Plumage Patterns

Pairing up by body mass is consistent with a previous study on Gambel’s Quail. This study found that larger males are more likely to win competitions with other males. Consequently, these males have first pick with the females and they settle down with large females that are more experienced. This choice increases the chances of a successful brood.

The second cue for mate choice is monomorphic plumage. Given that quails can discriminate between species in captivity, it is surprising that they pair up with individuals that look similar to them. This pattern suggests that quails are sexually imprinted in particular plumage traits that are shared by males and females. Which specific traits are used by the quails remains to be determined.

 

Mate Choice in the Contact Zone

These finding can explain the occurrence of hybridization in the contact zone. Quails pair up with individuals that share particular plumage patterns with them. Outside the contact zone, this strategy works perfectly because they only encounter members of their own species. In the contact zone, however, they run into quails from another species. But instead of focusing in the species-specific differences, they keep using the shared plumage traits to pick their partner. And voila, hybrids!

 

Callipepla_gambelii_male.jpg

A Gambell’s Quail © Alan D. Wilson | NaturesPicsOnline

 

References

Gee, J. M. (2003). How a hybrid zone is maintained: behavioral mechanisms of interbreeding between California and Gambel’s quail (Callipepla californica and C. gambelii). Evolution, 57(10), 2407-2415.

Hagelin, J. C. (2003). A field study of ornaments, body size, and mating behavior of the Gambel’s Quail. The Wilson Bulletin, 246-257.

Zonana, D. M., Gee, J. M., Bridge, E. S., Breed, M. D., & Doak, D. F. (2019). Assessing Behavioral Associations in a Hybrid Zone through Social Network Analysis: Complex Assortative Behaviors Structure Associations in a Hybrid Quail Population. The American Naturalist193(6), 852-865.

 

This paper has been added to the Galliformes page.