A hybrid in the making? Attempted mating between Sanderling and Broad-billed Sandpiper

Could this mating result in a hybrid?

It is always nice to receive pictures from readers (see for example here and here). Mostly, the pictures contain a peculiar hybrid, but this week another kind of picture ended up in my mailbox. Jaysukh Parekh Suman from India send me a photograph of a Sanderling (Calidris alba) mounting a Broad-billed Sandpiper (Calidris falcinellus). Could this unexpected mating result in a hybrid?

A Sanderling (Calidris alba) mounting a Broad-billed Sandpiper (Calidris falcinellus). Picture taken in Great Rann of Kutch,Gujarat, India on 6 October 2019. © Jaysukh Parekh Suman

 

No Reports

To my knowledge, no hybrids between these two species have been documented. In fact, no hybrids with Broad-billed Sandpiper have been reported so far. But that doesn’t mean they don’t occur. Hybrids between Sanderling and Broad-billed Sandpiper might be rare or they are difficult to identify (see Randler 2004 for more on hybrid detectability).

 

Viable or Not?

Alternatively, hybrids between these waders might not be viable. TimeTree dates the divergence between these species to about 26 million years ago (range: 18.8 – 38.3). Benjamin Fitzpatrick estimated that birds can still hybridize after an average of 21 million years of divergence. So, in theory, Sanderling and Broad-billed Sandpiper could produce viable offspring.

However, things can go awry at an earlier stage. Possibly, the sperm and egg cells of these species are not compatible. Hence, the Sanderling sperm will not be able to fertilize the Broad-billed Sandpiper egg. But we can only guess about the compatibility of their gametes. For more about these so-called postcopulatory prezygotic barriers to hybridization, I can recommend this review by Tim Birkhead and Jean-Pierre Brillard.

 

Conclusion

All in all, we cannot make a confident statement about the possible outcome of this unusual mating attempt. It seems unlikely that this copulation will result in a hybrid. First, the sperm and egg have to be compatible (and fertilization has to occur). Then, the hybrid embryo will need to develop properly. And finally, it will have to be viable and crawl out of the egg. Never say never, though…

 

References

Birkhead, T. R., & Brillard, J. P. (2007). Reproductive isolation in birds: postcopulatory prezygotic barriers. Trends in Ecology & Evolution, 22(5), 266-272.

Fitzpatrick, B. M. (2004). Rates of evolution of hybrid inviability in birds and mammals. Evolution, 58(8), 1865-1870.

Randler, C. (2004). Frequency of bird hybrids: does detectability make all the difference? Journal of Ornithology, 145(2), 123-128.

 

Pinning down the Shrike phylogeny

Molecular results are largely incongruent with morphological and behavioral data.

“I don’t trust the molecules,” said António Frias Martins during a field trip on the Azores (the Portuguese island group). We were looking for several snail species of the genus Plutonia that I would use for my Master thesis. António had been studying these slimy creatures for years and used morphological characters (mainly from reproductive organs) to reconstruct their evolutionary history. My goal was to use DNA to build a phylogenetic tree. I completely disagreed with him: genetic markers can be more reliable than morphology because phenotypes are vulnerable to convergent evolution (i.e. independent evolution of similar features). Convergent evolution can lead to homoplasy, a shared character between two or more species that did not arise from a common ancestor.

Red-backed Shrike © Chris Romeiks | Wikimedia Commons

 

Shrikes

The difficulty of homoplasy is nicely illustrated by a recent paper in the journal Zoological Scripta that focused on the bird family Laniidae. This group of birds contains about 30 species of shrike, divided over four genera. Shrikes are also known as “butcherbirds” because they impale their prey on thorns and spikes. The evolutionary relationships within this family have been a matter of debate. Two studies used morphological and behavioral data to sort things out. Panov (2011) combined zoogeographical, morphological and behavioral characters to divide the shrikes into seven groups. Harris and Franklin (2000) used a different approach and recognized two main groups based on alarms calls. They identifed a “keer alarm group” and a “chatter alarm group”. Both classifications are depicted in the figure below.

Two possible evolutionary trees for the Laniidae: (a) based on zoogeographical, morphological and behavioral characters by Panov (2011) and (b) based on alarm calls by Harris and Franklin (2000).

 

Enter Genetics

As I mentioned in the beginning, morphological characters can be unreliable because of convergent evolution and consequent homoplasy. But this process can also occur in molecular data: the same nucleotide can evolve independently at the same position in the genome. However, because there are so many genetic characters in a sequence, it is unlikely that it happens very often. Moreover, there are several tools to detect homoplasy in DNA sequences and account for them (see for example this recent paper). In the end, homoplasy is less of a problem for molecular analyses. Hence, Jérôme Fuchs and his colleagues used three genes (one mitochondrial gene and two nuclear regions) to reconstruct the evolutionary history of the shrikes.

The phylogenetic tree of the Laniidae family based on molecular data. Adapted from: Fuchs et al. (2019) Zoologica Scripta

 

Differences

The results from the molecular analyses were drastically different from the previous studies (you can compare the three trees above). I will not discuss all the incongruences, but let’s have a look at one major difference. Based on their alarm calls, Souza’s Shrike (L. souzae) and Emin’s Shrike (L. gubernator) were placed in the “chatter alarm group”. However, the molecular tree groups them with several species from the “keer alarm group”, such as Newton’s Fiscal (L. newtoni) and Northern Fiscal (L. humeralis). The authors write that “Apparently, all of the traits analysed by earlier authors are highly homoplastic, with many cases of parallel evolution as well as cases of strongly divergent morphological differentiation.”

Northern Fiscal © Dominic Sherony | Wikimedia Commons

 

Taxonomic Rearrangements

The genetic analyses also revealed some other interesting patterns. First, the genus Eurocephalus – which holds the Northern White-crowned Shrike (E. ruppelli) and the Southern White-crowned Shrike (E. anguitimens) – does not belong to the Laniidae family. Instead it is more closely related to a member of the Corvidae family: the Crested Jay (Platylophus galericulatus). The genus Eurocephalus should probably be placed in its own family. Second, the Yellow-billed shrike (Corvinella corvina) and the Magpie shrike, (Urolestes melanoleucus) are nested within the genus Lanius. It makes sense to include them in the Lanius-genus.

Northern White-crowned Shrike © Jente Ottenburghs

 

Trust the Molecules?

This study nicely shows that molecular data can be more reliable than morphological analyses. But this doesn’t mean that we should blindly go where the molecules take us. Genetic data comes with its own set of problems, such as discordance between gene trees, and we should be careful with drawing conclusions.

You might be wondering about my Master thesis. Well, I used two genes to construct a phylogenetic tree which agreed with the morphological analyses from António. But he still did not completely trust the molecules. So, we agreed to disagree…

The evolutionary tree of the genus Plutonia that I constructed for my Master thesis.

 

References

Fuchs, J., Alström, P., Yosef, R., & Olsson, U. (2019). Miocene diversification of an open‐habitat predatorial passerine radiation, the shrikes (Aves: Passeriformes: Laniidae). Zoologica Scripta, 48(5), 571-588.

Harris, T., & Franklin, K. (2000). Shrikes and Bush‐shrikes: Including
wood‐shrikes, helmet‐shrikes, flycatcher‐shrikes, philentomas, batises
and wattle‐eyes. London, UK: Helm Editions.

Ottenburghs, J. (2011) Molecular relationships of endemic landsnails of the genus Plutonia (Pulmonata, Gastropoda) on the Azores (Macaronesia – Portugal). Master Thesis, University of Antwerp (Belgium).

Panov, E. N. (2011). The True Shrikes (Laniidae) of the world: Ecology,
behavior and evolution. Sofia and Moscow: Pensoft.

Testing for gene flow between Torrent Ducks in Peru and Argentina

Is there a hybrid zone between two subspecies of the Torrent Duck?

A few months ago, I wrote a blog post about the Torrent Duck (Merganetta armata). A genetic study found that this beautiful duck – which swims in the rapid waters of the Andes – is comprised of three distinct lineages, corresponding to three subspecies: colombiana, leucogenis and armata. I ended the blog post with the following paragraph:

The three mitochondrial lineages correspond to three previously described subspecies. A morphological analysis of these birds showed that the Northern Andes group (colombiana) is highly differentiation from the other two groups. The differences between the Central (leucogenis) and the Southern (armata) groups are more subtle. Possibly, these birds represent the extremes of a range of phenotypes. There might even be a hybrid zone at some location. Denser sampling is required to figure this out. If it turns out that they hybridize, you will definitely read it on this blog.

Believe or not, but a recent study in the journal Ecology and Evolution did just that. Luis Alza and his colleagues sampled individuals from these two subspecies (leucogenis and armata) and performed several genetic analyses. Is there a hybrid zone between these subspecies?

A pair of Torrent Ducks in Colombia © Alejandro Bayer Tamayo | Wikimedia Commons

 

The Dry Diagonal

Based on an extensive data set of mitochondrial and nuclear markers, the researchers reconstructed the evolutionary history of these ducks. The analyses showed that both subspecies formed distinct clusters (i.e. they are monophyletic) and that there was no significant gene flow between them. So, unfortunately for this website, no hybrid zone between leucogenis and armata.

The lack of gene flow can be explained by the habitat specialization of Torrent Ducks. These birds rely on fast-flowing water and cannot cope with dry conditions. And the regions between the two subspecies are especially dry: the Atacama Desert in Chile, the Monte Desert in Argentina and the dry Puna in Bolivia. These regions are part of the so-called South American Dry (or Arid) Diagonal and present a formidable barrier for the Torrent Ducks.

The genetic population structure of leucogenis (in Peru) and armata (in Argentina). The Arid Diagonal presents a geographical barrier between these subspecies. Adapted from: Alza et al. (2019) Ecology and Evolution

 

Metapopulation

A closer look at the genetic structure within each subspecies reveals some interesting patterns. Populations inhabiting shorter rivers have lower levels of genetic diversity compared to their cousins that swim in longer rivers. This might be the result of recent founder events: a small population colonizes a new region resulting in a decrease in genetic diversity. Such small populations are vulnerable to extinction and might disappear quickly. This interplay between colonization and extinction (or metapopulation dynamics) might have shaped the present-day genetic patterns in Andean Torrent Ducks.

These metapopulation dynamics have important implications for the conservation of these ducks. The authors advocate “to protect and manage the subspecies and riverine populations as separate conservation units due to the strong genetic structure and very low gene flow.”

 

References

Alza, L., Lavretsky, P., Peters, J. L., Cerón, G., Smith, M., Kopuchian, Astie, A. & McCracken, K. G. (2019). Old divergence and restricted gene flow between torrent duck (Merganetta armata) subspecies in the Central and Southern Andes. Ecology and Evolution, 9(17), 9961-9976.

 

This paper has been added to the Anseriformes page.

Separating Sparrows: Species limits in the striped-sparrows (genus Rhynchospiza)

How many species of Rhynchospiza Sparrows are there?

Twenty-two. That is the number of species concepts listed by Richard Mayden in 1997. Since that overview, more concepts have been added to the list (for example, the mitonuclear compatibility species concept by Geoffrey Hill). However, proposing new species concepts is a futile endeavor. A thorough analysis of the species problem reveals that there is no silver-bullet species concept. Each case should be considered separately by investigating different criteria, such as morphology, song, behavior and DNA. These multiple lines of evidence can then be combined into a taxonomic decision. A comprehensive explanation of this process is outlined in another blog post: “A philosopher claims species do not exist. He is wrong.” Or you can check my recent book chapter on avian species concepts (available here).

An illustration of the Tumbes Sparrow by Joseph Smit | Wikimedia Commons

 

Striped Sparrows

A paper published in the Journal of Ornithology nicely exemplifies the use of different species concepts to draw species limits. Juan Areta and his colleagues focused on Rhynchospiza sparrows, a group of striped passerines that used to be included in the genus Aimophila. Currently, the genus Rhynchospiza comprises two species: the Tumbes Sparrow (R. stolzmanni) and the Stripe-crowned Sparrow (R. strigiceps). The latter is divided into two subspecies: strigiceps and dabbenei. This leaves us with three taxa: stolzmanni, strigiceps and dabbenei. Do these three taxa represent three distinct species? Let’s have a look at several features…

 

Head Patterns

The striped-sparrows won’t come to mind when you think about tropical species. These species are not brightly colored, but mainly differ in their brownish head patterns. There are, however, some notable differences between the three Rhynchospiza taxa. The subspecies dabbenei has a diagnostic black area in front of the eyes and distinct chestnut shoulders and head-stripes. These features are reduced in the other subspecies (strigiceps). This subspecies resembles the Tumbes Sparrow, but you can identify the latter by its darker head-stripes.

The sparrows also differ in size. The subspecies dabbenei is bigger than strigiceps in several measurements, such as wing and tarsus length. And the Tumbes Sparrow is characterized by a larger bill compared to the other two taxa. Morphological differences? Check.

The three Rhynchospiza taxa. From: Areta et al. (2019) Journal of Ornithology

 

Singing Sparrows

What about song? A comparison of the spectrograms shows that the three taxa produce distinct songs. The song of dabbenei mainly consists of a series of chirping notes that are repeated in long bouts. This has resulted in its local names “Chisca” (an onomatopoeia) and “Charlatan” (i.e. that who talks to much with a tendency to lie or exaggerate). The songs of strigiceps and stolzmanni are more melodic, but differ in the repetition speed: stolzmanni sings faster phrases. In addition, the three taxa produce different calls. Vocal differences? Check.

However, to conclude that these different songs play a role in species recognition more research is needed. For instance, playback experiments can reveal how the birds respond to each others’ songs.

The songs of the three Rhynchospiza taxa. From: Areta et al. (2019) Journal of Ornithology

 

Genetics

The three taxa differ in morphology and song, but can they also be separated genetically? The researchers used the mitochondrial gene NADH. Phylogenetic analyses of this gene showed that the Rhynchospiza sparrows form a distinct group in which the Tunges Sparrow is sister to both subspecies of the Stripe-crowned Sparrow. However, it would be interesting to perform more detailed genetic analyses and determine whether there is (or has been) gene flow between the different taxa. Ecological niche modelling showed that the ranges of strigiceps and dabbenei are parapatric which could result in occasional hybridization. Nonetheless, genetic differences? Check.

Furthermore, the ecological niche modelling revealed that the three taxa occupy distinct geographic areas: the Stripe-crowned Sparrow can be found in Argentina, Bolivia and Paraguay but the subspecies have distinct ranges: dabbenei occurs at high altitude in the Andes, while strigiceps lives in the lowlands. The Tunges Sparrow resides in Ecuador and Peru. These distinct distribution patterns suggest differences in habitat use and ecology.

Phylogenetic relationships between the three Rhynchospiza taxa. From: Areta et al. (2019) Journal of Ornithology

 

Conclusion

Let’s put all the evidence together. The three taxa differ in:

• Head plumage patterns
• Size
• Songs and calls
• Distribution
• Egg color (I did not mention this in the blog post)

This list of differences prompts the researchers to recognize three distinct species. They propose to use English names that reflect the geographic distribution of these sparrows:

• Tumbes Sparrow Rhynchospiza stolzmanni (Taczanowski 1877)
• Yungas Sparrow Rhynchospiza dabbenei (Hellmayr 1912)
• Chaco Sparrow Rhynchospiza strigiceps (Gould 1839)

 

References

Areta, J. I., Depino, E. A., Salvador, S. A., Cardiff, S. W., Epperly, K., & Holzmann, I. (2019). Species limits and biogeography of Rhynchospiza sparrows. Journal of Ornithology, 1-19.

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

Ecological speciation in the White-tipped Plantcutter

Recent study provides several lines of evidence that natural selection is driving divergence in this tropical species.

Ecological speciation is still a controversial idea. This process describes the evolution of reproductive isolation between populations due to divergent natural selection. Environmental gradients provide an ideal setting for ecological speciation to take place: populations adapt to the changing conditions along this gradient and diverge in certain characters, possibly culminating in the origin of new species. A recent study in the Journal of Ornithology argues that this is happening in the White-tipped Plantcutter (Phytotoma rutila).

This bird species can be found across the Chaco-Andes dry forest belt in South America. It is divided into two subspecies that occupy different altitudes: rutila occurs in the lowlands, while angustirostris lives in forests between 2000 and 3900 meters. Could these subspecies be the products of divergent natural selection? Let’s have a look at the evidence.

A White-tipped Plantcutter in Argentina © Hector Bottai | Wikimedia Commons

 

Body Size

First, there is a significant correlation between body size and elevation: larger birds tend to live at higher altitude. This is probably an adaptation to cope with particular conditions in the mountains, such as low temperatures and low levels of oxygen. However, larger body size can also be the outcome of neutral processes: birds at higher altitude just happen to be bigger by chance.

To rule out the possibility of neutral evolution, the researchers used the Pst-Fst comparison. Pst measures the differentiation in certain phenotypes, whereas Fst captures genetic differentiation. If morphology is evolving by neutral processes, Pst and Fst should be similar. If the two measures are different, adaptive processes might be involved. The analyses of several genetic markers revealed that Pst was consistently higher than Fst, supporting the idea that natural selection is driving body size evolution.

A positive correlation between elevation and body size supports the idea of ecological speciation. Blue circles represent lowland birds and red triangles highland birds. Adapted from: Rodríguez-Cajarville et al. (2019) Journal of Ornithology

 

Lowland Ancestor

More detailed genetic analyses provided further evidence for incipient speciation in the White-tipped Plantcutter. The highland birds formed a clade that was nested within the lowland birds, indicating that a lowland ancestor probably colonized higher altitudes at some point. Moreover, the researchers found gene flow from highland into lowland birds. This pattern also fits with the scenario of ecological speciation where ecological divergence occurs in the face of gene flow.

The White-tipped Plantcutter provides a nice example of ecological speciation. However, other processes than natural selection have probably contributed to the observed divergence. For example, the Yungas forests seem to present a partial barrier between lowland and highland birds. More research will be needed to sort out the details of this interesting case of incipient speciation.

Genetic analyses show that highland birds are nested with the lowland birds, suggesting that a lowland ancestor gave rise to the population at higher elevation. Adapted from: Rodríguez-Cajarville et al. (2019) Journal of Ornithology

 

References

Rodríguez-Cajarville, M. J., Calderón, L., Tubaro, P. L., & Cabanne, G. S. (2019). Body size and genetic variation in the White-tipped Plantcutter (Phytotoma rutila: Cotingidae) suggest ecological divergence across the Chaco–Andes dry forest belt. Journal of Ornithology, 1-15.

 

This paper has been added to the Cotingidae page.

How far did the hybrid zone between Hermit Warbler and Townsend’s Warbler move?

It turns out that this moving hybrid zone didn’t move very far.

Hybrid zones can move. In 2007, Richard Buggs presented evidence for 25 hybrid zones that have shifted over time. Since then, at least 11 more hybrid zones have been added to that list. Perhaps hybrid movement is more common than stability?

An often-cited example of such a moving zone is the contact zone between Hermit Warbler (Setophaga occidentalis) and Townsend’s Warbler (S. townsendi). These songbirds interbreed in three hybrid zones: two in Washington and one in Oregon. A genetic study found Hermit mtDNA in phenotypically pure Townsend’s populations up to 2000 km north along the Pacific coast. This was interpreted as a genetic wake left behind by the moving hybrid zone. A recent study in the Journal of Evolutionary Biology focused on one of the Washington hybrid zones (in the Cascade Mountains) and compared samples from 1987-1994 with samples from 2015-2016 to reconstruct the movement of this hybrid zone.

Hermit Warbler © Berichard | Wikimedia Commons

 

Clines

Silu Wang and her colleagues measured eight species-specific plumage characters to create a hybrid index score. A low score points to Hermit Warbler, while a high score indicates Townsend’s Warbler. Hybrids can be detected by intermediate scores. When you plot these hybrid indices on a map, you can see a gradual transition from Hermit Warbler-like birds to Townsend’s Warbler-like birds. The resulting figure is a so-called cline (for a quick lesson in cline theory, you can read this blog post). By comparing the cline from 1987-1994 with the one from 2015-2016, you can deduce how much the hybrid zone has moved. The result from this analysis was quite surprising: the Setophaga hybrid zone did not move…

The plumage clines from 1987-1994 (blue) and 2015-2016 (blue) are not significantly different, suggesting that the hybrid zone did not move. Adapted from: Wang et al. (2019) Journal of Evolutionary Biology

 

A New Scenario

Genetic analyses (based on 21,852 SNPs) confirmed the plumage patterns: the hybrid zone did not move. So, what happened then? The authors propose a new biogeographic scenario to explain these results. During the last glacial maximum, Hermit Warblers resided in various refugia along the coast. After this cold period, inland Townsend’s Warblers (from Idaho or Montana) moved to coastal area and made contact with the Hermit Warblers. This ancient hybridization event could have occurred “along a long front paralleling the coastal mountains, just inland from the coastline.” Later, the hybrid zones moved southwest to their current location. This scenario only requires slow movement of the hybrid zones and is more consistent with the stability observed in this study. We can thus remove this hybrid zone from Richard Buggs’ list…

The current distribution of Hermit Warbler (lightblue) and Townsend’s Warbler (magenta) and their hybrid zones. The black arrow indicates the potential origin of Townsend’s Warblers (from Idaho or Montana) that moved to the coastal areas. Adapted from: Wang et al. (2019) Journal of Evolutionary Biology

 

References

Buggs, R. J. A. (2007). Empirical study of hybrid zone movement. Heredity, 99(3), 301.

Krosby, M., & Rohwer, S. (2008). A 2000 km genetic wake yields evidence for northern glacial refugia and hybrid zone movement in a pair of songbirds. Proceedings of the Royal Society B: Biological Sciences, 276(1657), 615-621.

Wang, S., Rohwer, S., Delmore, K., & Irwin, D. E. (2019). Cross‐decades stability of an avian hybrid zone. Journal of Evolutionary Biology.

 

This paper has been added to the Parulidae page.

The Caracara Conundrum: Where does the extinct Creighton’s Caracara fit in?

How does it relate to other Caracara species?

It could be a question during a pubquiz: How many species of Caracara are there? The answer to that question depends on your timeframe. Are you thinking about extant species, then the answer is two: the Northern Crested Caracara (Caracara cheriway) and the Southern Crested Caracara (C. plancus). If you extend the count to extinct species, the number quickly rises. Multiple extinct species have been described:

• C. prelutosa (although it is difficult to distinguish from C. cheriway)
• C. seymouri
• C. major
• C. creightoni
• C. latebrosa
• C. tellustris
• C. lutosa

A recent study, published in the journal Molecular Phylogenetics and Evolution, focused on one of these extinct species – the Creighton’s Caracara (C. creightoni) – and figured out how it relates to the two living species.

A Southern Crested Caracara in Bolivia © Jente Ottenburghs

 

Blue Hole

The researchers were able to extract DNA from a 2500-year-old fossil of Creighton’s Caracara. This fossil was found in a blue hole (i.e. water-filled sink-hole) on the Bahamas. Fossils in these holes are well-preserved because the water is largely anoxic and sunlight does not penetrate deeply. This lack of oxygen and UV radiation minimizes the amount of DNA damage in the fossils. This allowed the reseachers to reconstruct the nearly complete mitochondrial genome of this species.

Comparing the mitochondrial DNA of Creighton’s Caracara with other species revealed that it is sister to the two living species (see phylogeny below). The divergence occurred between 1.13 and 0.41 million years ago.

Creighton’s Caracara is the sister lineage of both extant Caracara species. From: Oswald et al. (2019) Molecular Phylogenetics and Evolution

 

Extinction

Why the Creighton’s Caracara went extinct is not known. It might have to do with the extinction of other animals on Cuba and the Bahamas. These islands used to contain a variety of large reptiles and mammals, which provided a rich source of carrion for Caracaras (and other scavengers, such as the extinct Condor Gymnogyps varonai). When these large animals disappeared, the Creighton’s Caracara lost an important food source and consequently went extinct.

 

References

Oswald, J. A., Allen, J. M., Witt, K. E., Folk, R. A., Albury, N. A., Steadman, D. W., & Guralnick, R. P. (2019). Ancient DNA from a 2,500-year-old Caribbean fossil places an extinct bird (Caracara creightoni) in a phylogenetic context. Molecular Phylogenetics and Evolution, 140, 106576.