What’s for dinner? Rapid evolution of non-native frugivores on O’ahu

Four introduced bird species show significant changes in size and beak morphology after a few decades on the island.

On the Hawaiian island of O’ahu you can find mostly non-native bird species. The native seed-dispersing birds have been replaced by many fruit-eating birds through introductions. These birds not only facilitate the invasion of exotic plants, they may also be the only dispersers of native plants. The non-native species have become integrated into the native ecosystem (see this Science paper for more details). But have these introduced bird species changed since they took over the island? A recent study in the journal Evolution tried to answer this question by studying four species that were introduced since the 1920s.

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The Japanese White-eye © Laitche | Wikimedia Commons

 

Museum Specimens

Jason Gleditsch and Jinelle Sperry focused on the following species: the Japanese White-eye (Zosterops japonicus), the Red-billed Leiothrix (Leiothrix lutea), the Red-wiskered Bulbul (Pycnonotus jocosus) and the Red-vented Bulbul (P. cafer). They compared museum specimens from these species’ native ranges with the living birds on O’ahu.

These comparisons revealed significant changes since the birds arrived on O’ahu. In general, the birds became shorter with more robust beaks. The changes in beak morphology are probably related to a more generalized diet, including many types of fruit.

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The morphological changes in the four species (from left to right: Japanese White-eye, the Red-billed Leiothrix, Red-wiskered Bulbul and Red-vented Bulbul) since they arrived on O’ahu. Body measurements (blue) tend to decrease while beak measurements (brown) mostly increase. Significant changes are indicated with stars (*) From: Gleditsch & Sperry (2019) Evolution

 

Natural Selection?

These clear changes took place in just a few decades, showing that under particular circumstances evolution can proceed rapidly. It does, however, raise the question of whether these changes are due to natural selection or non-adaptive processes. The researchers used Lande’s F statistic, a measure from quantitative genetics to determine the role of genetic drift (i.e. a non-adaptive process) in evolutionary changes. The analyses suggested that the observed patterns are due to a mixture of adaptive and non-adaptive processes.

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The Red-billed Leiothrix © Shrikant Rao | Wikimedia Commons

 

Founder Effects

However, the test does not address the role of founder effects or population bottlenecks, when a population is reduced to a small number of individuals . These events can have profound effects on the consequent evolutionary trajectory of a species. For example, the Red-billed Leiothrix went to a population bottleneck due to competition with another introduced species, the White-rumped Shama (Copsychus malabaricus). This event probably selected for certain traits that provided the starting point for further evolutionary change.

The morphological changes in these non-native frugivores have important implications for the future of this ecosystem. The authors conclude that “because all of the native frugivores have been extirpated from O’ahu, the ability of the nonnative community of frugivores to effectively disperse fruiting plants is crucial for the long‐term stability and functioning of the novel forest ecosystems on the island.”

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The Red-wiskered Bulbul © Satheesan.vn | Wikimedia Commons

 

References

Gleditsch, J. M., & Sperry, J. H. (2019). Rapid morphological change of nonnative frugivores on the Hawaiian island of O’ahu. Evolution.

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Genetic study of the Mallard complex reveals extensive hybridization with little recent gene flow

The genetic similarity between members of this complex is mainly due to ancestral variation and ancient gene flow.

The Mallard (Anas platyrhynchos) is the king of hybrids. This duck species has hybridized with at least 40 different species. Specifically, it regularly interbreeds with its closely related monochromatic (i.e. males and females have the same color) cousins, such the Mottled Duck (A. fulvigula), the Black Duck (A. rubripes) and the Spot-billed Duck (A. zonorhyncha). This extensive hybridization has raised conservationists warning flags. Is the Mallard driving these species to extinction by hybridization? A recent study in the journal Molecular Ecology explored the Mallard complex to answer this question.

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The Mallard hybridizes with numerous other duck species. © Richard Bartz | Wikimedia Commons

 

The Mallard Complex

Philip Lavrestsky and his colleagues sequenced the DNA of five members from the Mallard complex that occur in North America: the Mallard (obviously), the American Black Duck, the Mexican Duck (A. diazi), the Florida Mottled Duck and the West Gulf Coast Mottled Duck (for more information about population structure in the Mottled Duck you can read this blog post). Although these species are morphologically clearly distinct, it has been proven difficult – if not impossible – to distinguish between them genetically.

The genetic similarity of these duck species could be due to two processes: extensive gene flow through hybridization or retention of ancestral genetic variation (the technical term is incomplete lineage sorting). My money would be on the first possibility, but I am biased when it comes to avian hybrids…

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The monochromatic members of the Mallard complex: the Mexican Duck (topleft, © Gary L. Clark), the Mottled Duck (topright, © Dick Daniels) and the American Black Duck (bottom, © Dick Daniels)

 

Patterns of Gene Flow

Using a large genetic data set (with over 3000 loci), the researchers were able to assign individuals to their respective taxonomic groups. They also inferred gene flow between particular species, but the results varied depending on which genetic variants were analyzed. In general, they found the following patterns of gene flow (I added information about the variants that supported the findings, see also figure below):

  • From Mexican Ducks into West Gulf Coast Mottled Ducks (all variants)
  • From Black Ducks into Mottled Ducks (all variants)
  • From Mallard into Black Ducks (autosomal, non-outlier variants)
  • From Mallard into Mexican Ducks (Z-linked variants)
  • From West Gulf Coast Mottled Ducks (Z-linked, non-outlier variants)

However, the amount of gene flow was less than expected. The authors write that “although hybridization is known to occur between mallards and each of the monochromatic species, our results suggest that contemporary gene flow and introgression may be lower than assumed.” It seems that the genetic similarity between these species is mainly due to ancestral variation and ancient gene flow. However, it is good news for conservationists. The monochromatic species are not threatened by extinction due to hybridization with the Mallard.

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Patterns of gene flow (indicated by arrows) between the different duck species for particular genetic variants. From: Lavretsky et al. (2019) Molecular Ecology

 

Unexpected Results

Honestly, I am a bit disappointed with the results of this study. As loyal readers might know, I just published a review on “Multispecies Hybridization in Birds.” In that paper, I constructed a hybrid network for the Anseriformes. The Mallard was connected to numerous other species and I envisioned this species as a “genetic distribution center”, picking up genetic material from one species and siphoning it to another one. Whether this idea fits with reality remains to be determined, but this study questions it by showing that recent gene flow is lower than expected. As so often, more research is needed to figure this out.

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A hybridization network for the Anseriformes. The Mallard is the blue dot with many connections. Whether it functions as a genetic distribution center remains to be investigated. From: Ottenburghs (2019) Avian Research

 

References

Lavretsky, P., DaCosta, J.M., Sorenson, M.D., McCracken, K.G., & Peters, J.L. (2019). ddRAD‐seq data reveal significant genome‐wide population structure and divergent genomic regions that distinguish the mallard and close relatives in North America. Molecular ecology.

 

This paper has been added to the Anseriformes page.

 

A Brazilian Brain-teaser: How did the hybrid zone between Rufous-capped and Bahia Spinetail form?

Did it arise in situ or due to secondary contact? And did rivers play a role in the process?

There is more than one way to get a hybrid zone. Over the years, several models have been put forward to explain the formation of hybrid zones. The easiest model to envision is the origin of a secondary hybrid zone: two populations diverge in separate regions (i.e. allopatry) and establish secondary contact at some point. I have written about such hybrid zones (see for example here and here), which seem to be the most common type.

Hybrid zones can also arise in situ in response to environmental gradients. Imagine a population distributed across a landscape that changes from very wet to very dry . Over time, some individuals adapt to the wet areas whereas others learn to cope with dry conditions. The middle of the range contains a mixture of wet-adapted and dry-adapted individuals. This would be a primary hybrid zone. A possible example of this scenario is the Little Greenbul (Andropadus virens). You can read the whole story here.

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The Rufous-capped Spinetail © Dario Sanches | Wikimedia Commons

 

Atlantic Forest Spinetails

In a recent Heredity paper, Henrique Batalha-Filho and his colleagues set out to test whether a hybrid zone between two Atlantic Forest spinetail species (Synallaxis) is of primary or secondary origin. The two species – the Rufous-capped Spinetail (S. ruficapilla) and the Bahia Spinetail (S. cinerea) – hybridize in a contact zone along the Doce and Jequitinhonha Rivers in Brazil. The location of this hybrid zone suggests that the rivers might play a role.

The researchers collected samples from more than 100 birds and sequenced a handful of molecular markers. The genetic analyses confirmed the location of the hybrid zone and indicated gene flow mainly from the Bahia Spinetail into the Rufous-capped Spinetail. Moreover, these species diverged about one million years ago.

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The Bahia Spinetail © Hector Bottai | Wikimedia Commons

 

Forest Fragmentation

But is the hybrid zone primary or secondary? Ecological niche modelling provided some clues. This technique predicts the distribution of a species across geographic space and time using environmental data. The analysis indicated that both species were geographically isolated in the past and established secondary contact at some point. This suggestion is further supported by the genetic data, which showed signatures of population expansion.

This observation leads to another question: did the rivers play a role? The answer seems to be no. The researchers found genetic material of the two species on both sides of the river. If the river was a barrier, the species would be restricted to separate banks. Moreover, the ecological niche modelling indicated range fragmentation in the past, unrelated to the rivers. So, they conclude that “isolation and divergence due to forest fragmentation during the last ca. 2 mya is the most likely scenario explaining the evolution of S. ruficapilla and S. cinerea in the Atlantic Forest.”

 

References

Batalha-Filho, H., Maldonado-Coelho, M., & Miyaki, C.Y. (2019). Historical climate changes and hybridization shaped the evolution of Atlantic Forest spinetails (Aves: Furnariidae). Heredity, 1.

 

This paper has been added to the Furnariidae page.

 

 

Introducing another hybrid bird species: the Salvin’s Prion

Its intermediate beak morphology might result in a broader range of prey species.

Scientific progress does not stand still. A few months ago, I published a review paper on hybrid bird species (check the blog post here) in which I assessed the evidence for seven putative hybrid species. In addition, I proposed a scheme to classify these cases into two types: 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. Recently, a study in the journal Molecular Biology and Evolution added another case to the list: the Salvin’s Prion (Pachyptila salvini). Let’s have a look at the evidence and see if we are dealing with a type I or type II hybrid species.

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Salvin’s Prion, another hybrid bird species? © Duncan | Wikimedia Commons

 

Whale-birds

The prions (genera Pachyptila and Halobaena) are a group of small petrels. Their name comes from the Greek word for saw, referring to the serrated edges of their bills. Prions have a fringe of thin plate-like structures (so-called palate lamellae) in their beaks. In some species, such as the Broad-billed Prion (P. vittata), these lamellae are quite big and can be used for filter feeding, resulting in an exclusive diet of copepods. That is why they are sometimes called whale-birds. In other species, such as the Thin-billed Prion (P. belcheri), the lamellae are vestigial and not suitable for filter feeding. These species focus on other prey items, including small squids and fish.

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A prion bill showing the lamellae. © Colin Miskelly | Museum of New Zealand

 

Genetic and Morphological Evidence

An international team of researchers sequenced the DNA of six prion species. When they compared these species, they discovered that Salvin’s Prion was a genetic mixture of Antarctic Prion (P. desolata) and Broad-billed Prion. Further analyses indicated that an evolutionary model in which Salvin’s Prion was a hybrid between these two species was the most likely scenario.

The genetic analyses were complemented with morphological measurements. These showed that, in terms of beak morphology, Salvin’s Prion was intermediate between both parental species. This could potentially allow it to feed on a broad range of prey species by combining the filter feeding and non-filter feeding strategies described above.

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The beak morphology of Salvin’s Prion (grey) is intermediate between its parental species, the Antarctic Prion (green) and the Broad-billed Prion (orange). From: Masello et al. (2019) Molecular Biology and Evolution.

 

Breeding Times

There is clear genetic and morphological evidence for hybridization. But what about reproductive isolation? The authors suggest that Salvin’s Prion has an intermediate breeding schedule compared to the other two species. Antarctic Prions start laying eggs at the end of August, whereas Broad-billed Prion does this in December. Salvin’s Prion falls somewhere in the middle (early to mid-November). Because egg-laying correlates strongly with mating time, this pattern suggests that Salvin’s Prion is isolated from both parental species. However, this hypothesis remains to be tested.

This intermediate breeding schedule suggests a genetic basis of this behavior (similar to the intermediate beak morphology). If so, reproductive isolation is a direct consequence of hybridization. That would mean Salvin’s Prion is a type I hybrid species.

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One of the parental species, the Antarctic Prion. © Trevor Lancaster | Wikimedia Commons

 

References

Masello, J. F., Quillfeldt, P., Sandoval-Castellanos, E., Alderman, R., Calderón, L., Cherel, Y., Cole, T.L., Cuthbert, R.J., Marin, M., Massaro, M., Navarro, J., Phillips, R.A., Ryan, P.G., Shepherd, L.D., Suazo, C.G., Weimerkirch, H. & Moodley, Y. (2019). Additive traits lead to feeding advantage and reproductive isolation, promoting homoploid hybrid speciation. Molecular Biology and Evolution. msz090.

Ottenburghs (2018) Exploring the hybrid speciation continuum in birds. Ecology and Evolution. 8(24): 13027-13034.

 

This paper has been added to the Procellariiformes page.

Spooky introgression in the African jungle: Bonobos hybridized with a “ghost” ape

Genetic study finds traces of ancient introgression in bonobo genomes.

Gene flow between distinct species is a common phenomenon. Scientists often observe hybrids in the wild and find evidence for genetic exchange when they look into the genomes of the interbreeding species. But what about ancient hybridization with extinct species? It seems sensible to expect traces of ancient introgression from unknown species in the genomes of extant species. In fact, more and more studies are reporting genetic material from these so-called “ghost lineages” in present-day genomes. A recent paper in the journal Nature Ecology & Evolution found traces of a “ghost” ape in bonobos. Spooky!

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A bonobo contemplating the traces of ancient introgression in its genome… © Pierre Fidenci | Wikimedia Commons

 

S* statistic

Martin Kuhlwilm and his colleagues compared 69 genomes of chimpanzees and bonobos to explore patterns of ancient introgression. Chimpanzees have been split into several subspecies based on their distribution, namely the central, western, eastern and Nigeria-Cameroon populations. The map below gives an overview of these subspecies.

The researchers used the S* statistic to find signatures of old gene flow events (for more information about this statistic, you can check this review by Fernando Racimo and others). The analyses revealed an unexpected sharing of genetic variation between central chimpanzees and bonobos, which confirms previous findings.

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Distribution of chimpanzee subspecies – western (verus, blue), Nigeria-Cameroon (ellioti, red),
central (troglodytes, green), eastern (schweinfurthii, orange) – and bonobo (pink). Adapted from: de Manuel et al. (2016) Science

 

Ghost Ape

Some genomic regions in the bonobo genomes could not be assigned to any of the chimpanzee populations. Detailed analyses indicated that bonobos received 0.9 to 4.2 percent from an unknown extinct population. The haplotype network below nicely illustrates this finding. The bonobos (red) and chimpanzees (yellow, green, blue and purple) form clearly defined clusters. However, one bonobo haplotype (indicated with the black arrow) is very different from the rest and probably came from a ghost lineage. Inspection of these spooky genomic regions revealed genes involved in immune response and diet, but further research is needed to validate any functional consequences of ancient hybridization.

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The haplotype network shows that bonobos (red) and chimpanzees (yellow, green, blue and purple) form clearly defined clusters. However, one bonobo haplotype (indicated with the black arrow) is very different from the rest and probably came from a ghost lineage. Adapted from: Kuhlwilm et al. (2019) Nature Ecology & Evolution

 

Multispecies Introgression

This study shows that the evolutionary history of chimpanzees and bonobos was heavily influenced by introgressive hybridization. Not only did they interbreed among each other, bonobos even received genetic material from an extinct species. I would not be surprised if traces of ancient introgression are also present in the chimpanzee populations. It also illustrates that hybridization is not always restricted to two species, multiple species might be hybridizing. The ecological and evolutionary consequences of multispecies hybridization remain to be investigated. I just published a paper advocating a multispecies perspective on avian hybridization (see blog post here and paper here). Clearly, this perspective is not restricted to birds…

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The evolutionary history of chimpanzees and bonobos has been shaped by introgressive hybridization. From: Kuhlwilm et al. (2019) Nature Ecology & Evolution

 

References

de Manuel, M. et al. (2016). Chimpanzee genomic diversity reveals ancient admixture with bonobos. Science, 354(6311), 477-481.

Kuhlwilm, M., Han, S., Sousa, V. C., Excoffier, L., & Marques-Bonet, T. (2019). Ancient admixture from an extinct ape lineage into bonobos. Nature Ecology & Evolution, 3(6), 957.

Ottenburghs, J. (2019). Multispecies hybridization in birds. Avian Research, 10(1), 20.

Racimo, F., Sankararaman, S., Nielsen, R., & Huerta-Sánchez, E. (2015). Evidence for archaic adaptive introgression in humans. Nature Reviews Genetics, 16(6), 359.

The more the merrier? My review on multispecies hybridization in birds

A summary of my recent paper in Avian Research.

Traditionally, hybridization has been studied by comparing species pairs. But what happens when multiple species are interbreeding? This question led me to write a review on multispecies hybridization in birds, which was recently published in the journal Avian Research. In this blog post, I will walk you through the review and highlight some interesting results and discussion points. But I encourage you to read the whole paper.

 

How Common Is It?

First of all, how common is multispecies hybridization? To quantify this phenomenon, I used records from the Serge Dumont Bird Hybrid Database for six bird orders with the highest incidence of hybridization: Anseriformes (waterfowl), Galliformes (wildfowl), Charadriiformes (waders, gulls and auks), Piciformes (woodpeckers), Apodiformes (hummingbirds and swifts) and Passeriformes (songbirds). This analysis revealed that most species hybridize with only one other species, but multispecies hybridization does occur frequently. In some bird orders, there are clear outliers (i.e. species that hybridize with many other species) such as Mallard (Anas platyrhynchos) Anseriformes, Common Pheasant (Phasianus colchicus) and European Herring Gull (Larus argentatus). These outliers have a large distribution range, providing ample opportunity to interact and potentially interbreed with several other species.

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The incidence of multispecies hybridization in six bird orders.

 

Networks!

I have been advocating the use of phylogenetic networks for some time (see for example this paper in The Auk). In the current review paper, I introduce networks as a way to visualize patterns of hybridization. In its simplest form, you just connect all the species that are known to have produced hybrid offspring. Next, you could adjust the width or color of the connections based on other parameters, such as frequency, fertility, gene flow, … These hybrid networks are an important starting point for further exploration of multispecies hybridization.

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A hybrid network for the Anseriformes. Dots represent species (colored according to different genera) while connections indicate that hybrid offspring have been observed.

 

Multispecies Introgression

Hybridization does not necessarily result in introgression (i.e. the exchange of genetic material from one (sub)species into the gene pool of another by means of hybridization and backcrossing). Although most statistical methods have been developed to quantify introgression between two hybridizing species, some approaches can be transferred to a multispecies setting. I will not cover all these methods here, but I will highlight a few interesting studies.

 

Clustering Methods

Model-based clustering methods, such as STRUCTURE and ADMIXTURE , are often used to visualize the genetic ancestry of individuals. This approach can be used to pinpoint individuals whose genomes show signs of ancestry from multiple sources. For example, Thies et al. (2018) uncovered a putative hybrid zone between three subspecies of the Common Ringed Plover (Charadrius hiaticula) based on STRUCTURE analyses. However, the output from these model-based clustering methods should not be taken at face value. Other processes can produce similar ancestry plots. That is why it is important to present the results of other analyses alongside these plots.

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A STRUCTURE plot for the Common Ringed Plover. Three locations (in red box) point to a hybrid zone between three subspecies. Adapted from: Thies et al. (2018) Ardea.

 

The D-statistic and Phylogenetic Networks

The D-statistic was originally developed to infer gene flow between humans and Neanderthals. This approach was hidden deep in the supplementary material of a Science paper, but emerged as one of the most often used statistics to infer introgression. There are several bird studies that have applied the D-statistic in a multispecies setting. For instance, I used this method in my goose work to show that there is a lot of genetic exchange between several species, as exemplified by the phylogenetic network below. Did I mention that I like networks?

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A phylogenetic network of the True Geese. From: Ottenburghs et al. (2017) BMC Evolutionary Biology

 

Modelling Introgression

There are numerous ways of modelling the demographic history of a species pair, ranging from Approximate Bayesian Computation (ABC) to diffusion-based models (e.g., DADI). Most of these models have not been applied in a multispecies setting yet. One notable exception concerns Isolation-with-Migration models that have been used to infer gene flow between three Acrocephalus warblers and between three Icterus orioles. New methods are being developed (as I witnessed at a recent Speciation Genomics workshop), but each of these approaches comes with its own assumptions (e.g., selective neutrality or small changes in allele frequency) and one should be aware of these when inferring patterns of gene flow.

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An Isolation-with-migration model for three oriole species. The circles indicate effective population sizes and the arrows show the amount of gene flow. Adapted from: Jacobsen & Omland (2012) Ecology and Evolution

 

Adaptive Introgression

In his book on introgressive hybridization, Edgar Anderson (1949) already stated that “raw material brought in by introgression must greatly exceed the new genes produced directly by mutation.” Indeed, adaptive introgression has been documented in several bird groups, such as Darwin’s finches and sparrows (see this blog post for more details). When multiple lineages are interbreeding, the adaptive alleles can flow in from multiple sources. This is nicely illustrated by a recent study on the diversification and domestication of the bovine genus Bos. Comparing the genomes of members of this genus – which includes taurine cattle, zebu, gayal, gaur, banteng, yak, wisent and bison – revealed complex patterns of introgression between several species. Interestingly, both gayal and bali cattle received genes from zebu cattle through introgressive hybridization. In both cases, the introgressed genes were related to a decrease in anxiety-related behaviour and could have played a role in the domestication of these animals. You can read the whole story on cow introgression here.

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The complex evolutionary history of cows with multiple introgression events. From Wu et al. (2018) Nature Ecology & Evolution

 

Conclusions

To end this blog post, I will copy the conclusions of my review paper below. I hope more people will adopt this multispecies perspective on hybridization.

Hybridization is generally studied in the context of species pairs, although multiple species might be interbreeding. Hence, a multispecies perspective on hybridization is warranted. In birds, an animal group prone to hybridization, multispecies hybridization is common. However, hybridization does not necessarily result in introgression. A broad range of tools are available to infer interspecific gene flow. The majority of these tools can be transferred to a multispecies setting. Specifically, model-based approaches and phylogenetic networks are promising in the detection and characterization of multispecies introgression. At the moment, we know that introgression is relatively common across the avian Tree of Life, but we do not have an estimate of how often introgression is adaptive. In addition, when multiple species are interbreeding, the impact on the build-up of reproductive isolation, adaptation to novel environments and the architecture of genomic landscapes remains elusive. Studying hybridization between multiple species and applying new network approaches will lead to important insights into the history of life on this planet.

 

References

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

Do grazing waterbirds surf the green wave?

This widely held idea about waterbird migration might not be so universal.

When you read “waterbirds surf the green wave”, you might think of ducks gently bobbing on the waves. In this case, however, the green wave does not refer to the sea but to a particular hypothesis that attempts to explain the migration patterns of grazing waterfowl. The migration routes of these birds tend to follow the emergence of green plants during spring. As the green wave of food resources rolls to the north, the birds catch up. This ensures plenty of food along the way to their breeding grounds. You can check the animation below to see how the world turns green over time.

 

Correlation is not causation!

The correlation between spring migration and vegetation indices has been confirmed for several grazing waterbirds, such as geese, ducks and swans. However, correlation does not necessarily mean causation. Other environmental factors might play a role. Perhaps birds are just tracking temperature or day length (variables that happen to be associated with the green wave). To disentangle these factors, a group of international waterfowl researchers teamed up and analyzed the migration strategies of 10 species. They compared the timing of migration with several stochastic models. Can the green wave hypothesis explain the patterns better than random movements?

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The ten waterbird species and their migration routes. From: Wang et al. (2019) Nature Communications. Click here for the full picture.

 

Mixed results

The simulations showed that only two species consistently follow the green wave: Barnacle Goose (Branta leucopsis) and Greater White-fronted Goose (Anser albifrons). These species are the most effective grazers – based on their beak morphology – and rely on fresh grass during their journey north. It thus makes sense that they track the green wave.

In East Asia, however, the Greater White-fronted Goose does not follow the green wave. The researchers suggest that this mismatch is due to human disturbance. Hunting pressure, land use changes and poisoning are complicating the migration of waterbirds in this part of the world. They might take a safer route and deviate from the expected pattern.

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A pair of Greater White-fronted Geese © Bill Bouton | Wikimedia Commons

 

Other Explanations

Human disturbance is not the only possible reason that certain species do not follow the green wave. Some species can eat other food types and do not solely depend on the fresh supply of grass. For example, Taiga Bean Goose (Anser fabalis), Tundra Bean Goose (Anser serrirostris) and Pink-footed Goose (Anser brachyrhynchus) often rely on grains during their migration.

Other species might just be unable to predict the progress of the green wave, especially when their migration route passes over the ocean. Birds can also deviate from their traditional migration routes due to unpredictable weather or climate change. Perhaps the latter resulted in a mismatch between migration timing and the start of spring in particular species. Finally, some birds might actively overtake the green wave in order to produce their eggs before the peak of food availability. This will ensure that their goslings have plenty of food when they grow up. Clearly, there is still a lot of work to be done here. I will be waiting for the next wave of waterbird migration papers.

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A group of Pink-footed Geese in Scotland © Duncan Brown | Flickr

 

References

Wang, X., Cao, L., Fox, A.D., Fuller, R., Griffin, L., Mitchell, C., Zhao, Y., Moon, O.-K., Cabot, D., Xu, Z., Batbayar, N., Kölzch, A., van der Jeugd, H.P., Madsen, J., Chen, L. & Nathan, R. (2019). Stochastic simulations reveal few green wave surfing populations among spring migrating herbivorous waterfowl. Nature communications, 10(1), 2187.

Hybridization with escaped falconry birds and nest poaching threaten Barbary Falcons on the Canary Islands

The exact (genetic) impact of these threats remains to be determined, but measures should be taken.

Hybrid falcons are common in captivity. Falconers regularly cross different species, often using artificial insemination. A survey of captive falcons in the United Kingdom noted no less than 11,778 registered hybrid falcons between 1983 and 2007. Falcons that escape from captivity can cause problems in wild populations (regardless of whether they are hybrids or “pure” species). They can interbreed with wild birds, compete for breeding territories or even prey on them. A recent study, published in the Journal of Raptor Research, assessed the impact of escaped falconry birds on the Barbary Falcon (Falco pelegrinoides) population on the Canary Islands.

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An adult male Barbary Falcon from Lanzarote, Canary Islands © Juan Sagardía | http://www.magornitho.org

 

A Pale Peregrine

The Barbary Falcon is a small peregrine-like falcon. Some ornithologists consider it a separate species, while others classify it as a subspecies of the Peregrine Falcon (F. peregrinus). Barbary Falcons can be distinguished from other falcon (sub)species by its pale plumage, slimmer body and characteristic head pattern. This falcon can be found from the Canary Islands to the Arabian Peninsula.

Although the population of Barbary Falcons on the Canary Islands is increasing, it is still listed as “Endangered” by the Spanish National and Regional catalogs of threatened species. The main threats for this (sub)species are collisions with human-made structures and shooting. Monitoring the population on Tenerife, Beneharo Rodríguez and his colleagues identified two new threats: hybridization with escaped falconry birds and poaching.

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Three wild falcons (A-C) and three lost falconry birds (D-F) on Tenerife. (A) presumed Peregrine Falcon, (B) dark Barbary Falcon, (C) pale Barbary Falcon, (D) Barbary Falcon, (E) Peregrine Falcon, (F) Saker Falcon. © Beneharo Rodríguez

 

Hybridization and Poaching

Between 1993 and 2017, the researchers observed 12 falcons that carried falconry equipment: seven Peregrine Falcons, three Saker Falcons (F. cherrug) and two unidentified individuals. Several of these birds paired up with native falcons and produced viable offspring. The genetic consequences of these hybridization events have not been studied.

Apart from hybridization with escaped birds, the researchers also recorded the poaching of nests. Of the 43 monitored nests during 2016 and 2017, six were poached. Three nesting sites were being poached each year. The picture below shows an incubating female on 5 March 2016 and an empty nest a few weeks later (1 April 2016). The damaged vegetation surrounding the nest (indicated with a white arrow) suggests that poachers climbed to the nest.

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An example of a poached nest on Tenerife © Beneharo Rodríguez

 

Recommendations

The exact impact of hybridization and poaching on the Barbary Falcon population remains to be investigated. But it is clear that measures have to be taken. The researchers provide the following recommendations.

We recommend that local authorities continue to assess the degree of genetic admixture that occurs in this population, modify the current falconry regulations, implement management actions to prevent new escapes, eradicate exotic raptors, and put a stop to illegal nestling harvests.

 

References

Fleming L.V., Douse A.F. & Williams N.P. (2011) Captive breeding of peregrine and other falcons in Great Britain and implications for conservation of wild populations. Endangered Species Research, 14:243-257.

Rodríguez, B., Siverio, F., Siverio, M., & Rodríguez, A. (2019). Falconry Threatens Barbary Falcons in the Canary Islands Through Genetic Admixture and Illegal Harvest of Nestlings. Journal of Raptor Research, 53(2), 189-197.

 

This paper has been added to the Falconiformes page.

The genetic legacy of population decline and recovery in the Red-cockaded Woodpecker

Genetic variation was lost, but no distinct mitochondrial lineages disappeared.

There used to be about 1.6 million Red-cockaded Woodpeckers (Dryobates borealis) in the southeastern United States. Their numbers dwindled when humans started changing the habitat (e.g., timber harvesting). The population decreased to less than 10,000 birds in 1978, after which it became one of the first species to be protected under the US Endangered Species Act. The implementation of management plans stabilized the population and eventually resulted in increasing numbers. By the early 2000s, there were approximately 14,000 woodpeckers hammering away. A recent study in the journal Ecology and Evolution reconstructed the genetic legacy of these population dynamics.

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A Red-cockaded Woodpecker bringing some food to the nest. © U.S. Fish and Wildlife Service Southeast Region | Wikimedia Commons

 

Genetic Variation

Mark Miller and his colleagues compared the genetic variation of Red-cockaded Woodpeckers from three time periods: before 1970, 1992-1995 and 2010-2014. The analyses showed that several mitochondrial variants (so-called haplotypes) have been lost over time. However, these variants did not represent distinct evolutionary lineages. It mainly concerned small offshoots of the most common variants. All in all, not much mitochondrial variation was lost.

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The haplotype networks for different time periods (pre-1970, 1992-1995 and 2010-2014) show that certain variants (depicted as circles) have been lost over time. However, it mainly concerns small offshoots of the most common variants (big circles). From: Miller et al. (2019) Ecology and Evolution.

 

Population Structure

The genetic diversity was similar in 1992-1995 and in 2010-2014, suggesting that little variation was lost between these time periods. This indicates that the management practices are bearing fruit. The population is recovering, not only in numbers but also in genetic diversity. However, the population structure did chang over time. Before 1970, the woodpecker population was panmictic; there were no identifiable subpopulations. The present study found low levels of population differentiation, suggesting that some subpopulations are relatively isolated from one another. This is probably a consequence of habitat fragmentation. Human impact has clearly left its mark in the genetic make-up of the Red-cockaded Woodpecker.

 

References

Miller, M.P., Vilstrup, J.T., Mullins, T.D., McDearman, W., Walters, J.R., & Haig, S.M. (2019). Changes in genetic diversity and differentiation in Red‐cockaded woodpeckers (Dryobates borealis) over the past century. Ecology and Evolution, 9(9):5420-5432.

Honey, I resolved the Meliphaga phylogeny!

A taxonomic revision of the Honeyeater genus Meliphaga.

Honeyeaters represent the largest radiation of birds in Australasia. The family (Meliphagidae) comprises more than 100 species that occupy a variety of ecological niches. Surprisingly, the evolutionary relationships within this family remain largely unresolved. Ornithologists are working on this species-rich group, one genus at a time. A recent study in the journal Zoologica Scripta focused on the genus Meliphaga.

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A Orange-cheeked Honeyeater in New Guinea. How is it related to species in the genus Meliphaga? © Paul van Giersbergen | http://www.hbw.com

 

Two Groups and an Additional Species

The species in the genus Meliphaga are listed below. Previous work suggested that these species can be divided into two groups: the aruensis-lewinii-notata group and the remaining species. The situation is complicated by the Orange-cheeked Honeyeater (Oreornis chrysogenys). This bird, which is endemic to the subalpine regions of New Guinea, might be related to some species in the Meliphaga-genus.

  • Puff-backed Honeyeater (M. aruensis)
  • Lewin’s Honeyeater (M. lewinii)
  • Yellow-spotted Honeyeater (M. notata)
  • White-lined Honeyeater (M. albilineata)
  • Kimberley Honeyeater (M. fordiana)
  • Scrub Honeyeater (M. albonotata)
  • Mimic Honeyeater (M. analoga)
  • Yellow-gaped Honeyeater (M. flavirictus)
  • Graceful Honeyeater (M. gracilis)
  • Elegant Honeyeater (M. cinereifrons)
  • Mottle-breasted Honeyeater (M. mimikae)
  • Forest Honeyeater (M. montana)
  • Mountain Honeyeater (M. orientalis)
  • Streak-breasted Honeyeater (M. reticulata)
  • Tagula Honeyeater (M. vicina)
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Lewin’s Honeyeater in New South Wales, Australia. © J.J. Harrison | Wikimedia Commons

 

Revising the Taxonomy

Jenna McCullough and her colleagues collected samples from all but two Meliphaga Honeyeater species (they did not include Mimic Honeyeater and Tagula Honeyeater) and the Orange-cheeked Honeyeater. Based on a set of thousands of ultra-conserved elements, they reconstructed the phylogeny of this genus. The results confirmed previous studies, indicating a clear split between the aruensis-lewinii-notata group and the remaining species. Moreover, the Orange-cheeked Honeyeater fell right into the other group of Honeyeaters.

This arrangement calls for a taxonomic revision. The authors propose to recognize the three species of the aruensis-lewinii-notata group (i.e. Puff-backed Honeyeater, Lewin’s Honeyeater and Yellow-spotted Honeyeater) as the only members of the genus Meliphaga. The rest of the species will be classified into two other genera: Territornis and Microptilotis. The Orange-cheeked Honeyeater remains in its own genus Oreornis.

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The phylogenetic relationships between members of the genus Meliphaga and the newly proposed classification. Adapted from McCullough et al. (2019) Zoological Scripta.

 

Work in Progress

Meliphaga is just one genus within the large Honeyeater family. Another recent study in the journal Emu presented a phylogenetic tree for the entire family. I won’t bother you with all the details (it would take too much time to discuss all the species), but it is clear that the taxonomy of this bird group needs work. Several genera, such as Gymnomyza and Melidectes, turned out to be paraphyletic. And although the Kadavu Honeyeater (Xanthotis provocator) looks like a Xanthotis, it does not belong to that genus. In fact, it clustered with the genus Foulehaio. Clearly, the Meliphagidae phylogeny is work in progress.

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A drawing of the Kadavu Honeyeater © Joseph Smit | Wikimedia Commons

 

References

Andersen, M. J., McCullough, J. M., Friedman, N. R., Peterson, A. T., Moyle, R. G., Joseph, L., & Nyári, Á. S. (2019). Ultraconserved elements resolve genus-level relationships in a major Australasian bird radiation (Aves: Meliphagidae). Emu-Austral Ornithology, 1-15.

McCullough, J. M., Joseph, L., Moyle, R. G., & Andersen, M. J. (2019). Ultraconserved elements put the final nail in the coffin of traditional use of the genus Meliphaga (Aves: Meliphagidae). Zoologica Scripta.