The evolution of bird song: gradual or punctuated?

Testing different evolutionary models in the Eastern Double-collared Sunbird species complex.

Evolution is often depicted as the slow accumulation of genetic differences, potentially giving rise to new species when two lineages diverge. However, evolution can also proceed quite rapidly. Long periods of stability – or stasis – are punctuated by bursts of evolutionary change (I covered a possible genetic mechanism for this model of punctuated equilibria in this blog post). We now know that both models of evolution – gradualism and punctuated equilibria – occur, but the relatively contribution of these models remains a matter of debate. Moreover, they might just represent the extremes on a continuum of evolutionary speed. For each and every trait, we can investigate whether their evolution was gradual or punctuated. A recent study in the journal Proceedings of the Royal Society B took a closer look at the evolution of certain song characteristics.

Sunbird Songs

Before we dive into the study, let’s see if we can deduce the most likely evolutionary model for bird song. In most species, song is learned by imitating conspecifics. Small mistakes during this learning process can result in the development of new song variants or dialects. These mistakes might accumulate gradually, giving rise to almost continuous variation. This scenario has been documented in the Greenish Warbler (Phylloscopus trochiloides, see also this blog post). Alternatively, female birds might not be impressed by the new songs and select for males with a more conservative repertoire. This stabilizing selection can lead to long periods of stasis. When a new song variant pops up that is favored by females, it will quickly spread through the population. A punctuated change. This might have happened in some populations of White-throated Sparrows (Zonotrichia albicollis). Clearly, both gradual and punctuated changes are possible when it comes to bird song.

In the current study, Jay McEntee and his colleagues focused on members of the Eastern Double-collared Sunbird (Cinnyris mediocris) species complex. These colorful birds can be found in the mountainous areas of East Africa where different populations have evolved distinct songs. The researchers quantified the variation in 14 song traits across the distribution of these sunbirds. After checking that each genetic lineage produces its own distinct song (which was indeed the case), different evolutionary models were tested on the dataset. These analyses revealed that seven song traits followed a pattern of punctuated equilibria, while the remaining traits could not be assigned to a specific evolutionary model. Interestingly, the bursts of evolutionary change for four traits were all located on the branch leading to one set of populations belonging to the Forest Double-collared Sunbird (Cinnyris fuelleborni).

Acoustic analyses indicated that different populations of the species complex produce distinct songs. From: McEntee et al. (2021).

Speculating about Mechanisms

These findings suggest that some song traits in the Double-collared Sunbird species complex show punctuated patterns. However, it remains to be determined which mechanisms underlie periods of stasis and bursts of rapid evolutionary change. When it comes to the stasis of songs, the researchers offer two possibilities: stabilizing selection (as discussed above) or occasional gene flow between the populations. The latter explanation seems unlikely, because most populations are isolated on distant mountains. And although some populations are interbreeding – such as C. fuelleborni and C. moreaui – the levels of genetic and cultural exchange are probably to low to have a significant stabilizing effect on the evolution of song.

And what about the punctuated changes? One hypothesis states that these changes are a by-product of morphological evolution. Indeed, previous work has shown that the evolution of bird song is tightly connected to changes on morphology (see for example this blog post). However, the researchers deem this explanation unlikely, because “there is limited morphological evolution” in this species complex. Moreover, the largest species – the Loveridge’s Sunbird (C. loveridgei) – produces the highest frequency songs, while you would expect lower frequency songs based on its body size. However, I find this reasoning unconvincing. Besides body size, there are numerous other morphological changes that can have an impact on song frequency. Detailed morphological analyses are needed here.

An alternative possibility concerns rapid range expansion in which a series of founder effects introduces new song variants throughout the range of another population. Selection for certain variants can then result in the rapid rise of a new song. Reconstructing the evolutionary history of the Double-collared Sunbird species complex can reveal whether this scenario is plausible or not.

Punctuated changes in seven song traits (indicated by vertical bars) across the phylogeny of the Double-collared Sunbird species complex. Notice that most changes occured on the branch leading to the C. fuelleborni populations. From: McEntee et al. (2021).

References

McEntee, J. P., Zhelezov, G., Werema, C., Najar, N., Peñalba, J. V., Mulungu, E., Mbilinyi, M., Karimi, S., Chumakova, L., Burleigh, G. J. & Bowie, R. C. (2021). Punctuated evolution in the learned songs of African sunbirds. Proceedings of the Royal Society B, 288(1963), 20212062.

Featured image: Eastern Double-collared Sunbird (Cinnyris mediocris) © Nigal Voaden | Wikimedia Commons

How was the genome of non-avian dinosaurs arranged?

A high-quality Emu genome helps to reconstruct the evolution of avian chromosomes.

Although the non-avian dinosaurs receive a lot of media attention – especially the iconic Tyrannosaurus rex – their genetic evolution is largely ignored by journalists. However, we can probe the fascinating genomic history of non-avian dinosaurs by studying their modern descendants, the birds (see for example this book chapter). Most bird species have about 10 large macrochromosomes, accompanied by 30 smaller microchromosomes. Over time, numerous chromosomal sections have been rearranged, both within and between chromosomes (as I recently described in sandpipers). Hence, if we want to reconstruct a genomic picture of the non-avian dinosaurs, we need to look at the bird species with the slowest chromosomal evolution. Generally, evolutionary biologists have assumed that ratites – a group of flightless birds, including the ostrich and kiwis – show the slowest rate of evolution on the avian phylogeny. But is that really the case?

Reptilian Rearrangements

In a recent study in the journal Genome Research, researchers produced a high-quality assembly of the Emu (Dromaius novaehollandiae) genome. To assess the rate of chromosomal evolution in this species, they compared its genome and several other avian genomes with a set of reptiles: the Green Sea Turtle (Chelonia mydas), the Prairie Rattlesnake (Crotalus viridis), and the American Alligator (Alligator mississippiensis). This comparison revealed that the Emu underwent fewer chromosomal rearrangements than the other bird species, suggesting that it is a good proxy for the ancestral condition in birds. In other words, the genome of non-avian dinosaurs probably looked like the Emu genome.

Chromosomal comparisons between the Emu and several reptiles (top figures) reveals that this species experienced fewer genomic rearrangements than other bird species (bottom figure). From: Liu et al. (2021).

Nuclear Puzzle

Now that we have established that the genome of non-avian dinosaurs is comparable with that of the Emu, we can move one step further. How are the chromosomes arranged in the nucleus? Studies in Chicken (Gallus gallus) showed that microchromosomes are huddled together in the center while macrochromosomes are located at the periphery (owls, by the way, are an interesting exception). This arrangement probably relates to the activity of the genes on these chromosomes. Microchromosomes have a higher gene density and are transcriptionally more active. Macrochromosomes are less active and tend to be tightly packed (i.e. as heterochromatin). That is the situation in Chicken, what about the Emu?

To solve this mystery, the researchers turned to Hi-C sequencing, which is a technique to identify contacts within (cis-contacts) and between (trans-contacts) different chromosomes. If the Emu genome is similarly organized in the nucleus, we would expect to see more trans-contacts between the active microchromosomes in the center than between the inactive macrochromosomes at the periphery. And that was indeed the case. Because the genome of the Emu is close to the ancestral condition in birds, the genomes of non-avian dinosaurs were probably arranged in a similar way.

A simplified overview of the organization of micro- and macrochromosomes in the nucleus of the Emu. Notice the higher number of trans-contacts between the microchromosomes. From: Liu et al. (2021).

Non-avian Dinosaur DNA

Can we now conclude that the genomes of all non-avian dinosaurs, from Ankylosaurus to Zapalasaurus, were arranged in this way? Not exactly, the findings of this Emu study only provided insights into the ancestral chromosomal condition of birds. To reconstruct the genome of the common ancestor of avian and non-avian dinosaurs we would have to go back further in time. Moreover, numerous other species split off from that distant ancestor and probably evolved their specific chromosomal differences. The genome of a Tyrannosaurus likely looked quite different from the genome of a Triceratops. Each species has a unique evolutionary trajectory with its own peculiarities. An important nuance to keep in mind (and that is often neglected by journalists). Nonetheless, it seems reasonable to assume that the genome of non-avian dinosaurs was packed into the nucleus with microchromosomes in the center and macrochromosomes at the periphery. There will undoubtedly have been some exceptions. Whether we can ever identify these outliers is another question.

References

Liu, J., Wang, Z., Li, J., Xu, L., Liu, J., Feng, S., … & Zhou, Q. (2021). A new emu genome illuminates the evolution of genome configuration and nuclear architecture of avian chromosomes. Genome Research, 31(3), 497-511.

Featured image: Emu (Dromaius novaehollandiae) © H. Zell | Wikimedia Commons

A mitochondrial mystery involving two Monarch Flycatchers

Why are there two mitochondrial haplotypes in the Spectacled Monarch?

When researchers took a closer look at the mitochondrial DNA of the Spectacled Monarch (Symposiachrus trivirgatus), they were in for a surprise. This species contained two distinct haplotypes that were not even closely related. One of these haplotypes clustered with a non-sister species: the Spot-winged Monarch (S. guttala). What is going on here? A recent study in the journal Molecular Ecology tried to solve this mitochondrial mystery by testing several hypotheses:

1. Cryptic speciation: the Spectacled Monarch is actually comprised of two species.
2. Neutral gene flow: the mtDNA introgressed from the Spot-winged into the Spectacled Monarch by neutral processes (e.g., genetic drift).
3. Adaptive introgression: the mitochondrial variant confers an adaptive advantage to the Spectacled Monarch.

To discriminate between these explanations, Michael Andersen and his colleagues sampled birds across the range of both species (i.e. Australia and New Guinea) and analyzed thousands of genetic markers. Let’s see if they could figure out what happened with these Monarch Flycatchers…

Cryptic Species?

First, the cryptic speciation hypothesis: could it be that the Spectacled Monarch is not one, but two species? If so, we would expect to find similar patterns of genetic divergence in the nuclear genome. This was, however, not the case. Genetic analyses of the nuclear markers clearly separated the Spot-winged and the Spectacled Monarch into two distinct groups. No sign of hidden species in nuclear genome of the Spectacled Monarch. Hence, we can already cross out cryptic speciation from our list.

The presence of mtDNA from the Spot-winged Monarch in some Spectacled Monarch individuals clearly suggests that introgression occurred. And indeed, demographic analyses pointed to ancient introgression events. The exact timing of these events was more difficult to determine, because these species probably established secondary contact multiple times when lower sea levels allowed exchange between Australia and New Guinea. But the main question is: was the introgression of mtDNA adaptive or not?

The mitochondrial phylogeny (figure b) reveals two distinct haplotypes in the Spectacled Monarch (blue and orange groups). These birds cluster together in the nuclear phylogeny (figure a), rejecting the hypothesis of cryptic speciation. From: Andersen et al. (2021) Molecular Ecology.

Positive Selection

To examine the possibility of adaptive introgression, the researchers performed several tests of selection on the genetic data (including the McDonald-Kreitman test). These analyses provided “little to no evidence for positive selection acting on the mitochondria since the capture event.” These results are thus consistent with neutral processes. It seems that genetic drift alone might be sufficient to explain the mitochondrial mystery in these birds. Genetic drift is especially prevalent in small populations. It is easy to imagine that the population size of the Spectacled Monarch diminished when sea levels rose and these birds became isolated again. However, more detailed demographic analyses are needed to test this scenario.

Finally, there is one explanation that the researchers did not explicitly consider: ghost introgression (as was probably the case in other species, such as Phylloscopus warblers and the Red-billed Chough, Pyrrhocorax pyrrhocorax). Perhaps the mtDNA did not introgress from the Spot-winged Monarch, but rather from an extinct species. Although this scenario does not provide an answer to the neutral vs. adaptive debate, it raises an interesting hypothesis that remains to be explored (see my review for possible analyses). As if the situation was not complicated enough.

References

Andersen, M. J., McCullough, J. M., Gyllenhaal, E. F., Mapel, X. M., Haryoko, T., Jønsson, K. A., & Joseph, L. (2021). Complex histories of gene flow and a mitochondrial capture event in a nonsister pair of birds. Molecular Ecology, 30(9), 2087-2103.

Featured image: Spectacled Monarch (Symposiachrus trivirgatus) © J.J. Harrison | Wikimedia Commons

How did rails spread across the globe?

The importance of dispersal-related traits in historical biogeography.

“The distribution of species on islands and continents throughout the world is exactly what you’d expect if evolution was a fact.” This quote from evolutionary biologist Richard Dawkins nicely illustrates the importance of evolution to understand the present and past distribution of species on our planet. Historical biogeography is the discipline that deals with the interface of evolutionary history and the changing distributions of species. Using probabilistic models, scientists try to reconstruct the journey of a group of species across space and time. Most of these models do not take biology into account, but consider all species as interchangeable units. This is obviously not the case. Species differ in many traits, which could impact the way they spread across islands and continents. A recent study in the journal Molecular Phylogenetics and Evolution provides a nice example on how to include species-specific traits into biogeographical models.

Flightless Rails

Juan Garcia-R and Nicholas Matzke focused on the evolution of rails (family Rallidae). Numerous rail species have lost the ability to fly (see for example here and here). Being able to fly or not obviously affects a species’ capacity to disperse. Hence, the researchers included this trait in their biogeographical models. First, they build an evolutionary tree for the rails, using morphological and genetic data. This phylogenetic framework – containing 129 extant and 29 extinct taxa – provided the basis for a comparison of several biogeographical models. The model including trait-dependent dispersal outperformed all the other models. Clearly, the ability to fly matters.

Time-calibrated phylogeny and ancestral range estimation based on total-evidence data of the family Rallidae. From: Garcia-R & Matzke (2021).

Ancestral Area

The final model provided some interesting insights into the evolution of flightlessness in rails. The ability to fly was lost at least 22 times independently. And the earliest transition to a non-flying lifestyle occurred about 12 million years ago, which overlaps with the age of the flightless Litorallus and Priscaweka from New Zealand. However, including trait-dependent dispersal did not allow the researchers to pinpoint the exact origin of the rails. The most likely ancestral areas are the Afrotropical and Neotropical regions (see figure above), but the uncertainty surrounding this ancestral distribution is quite large. This biogeographical mystery can perhaps be solved by including even more species-specific traits.

References

Garcia-R, J. C., & Matzke, N. J. (2021). Trait-dependent dispersal in rails (Aves: Rallidae): Historical biogeography of a cosmopolitan bird clade. Molecular Phylogenetics and Evolution, 159, 107106.

Featured image: Buff-banded Rail (Gallirallus philippensis) © J.J. Harrison | Wikimedia Commons

Are Quail hybrids more susceptible to avian malaria?

Studying the interactions between parasites and hybridization.

The relationship between parasites and hybrids is complex, to put it mildly. On the one hand, hybrids might be more prone to infection because of genetic or developmental defects. On the other hand, they might be able to fend of parasites due to hybrid vigor. Or perhaps they show intermediate infection rates compared to their parental species. In addition to these three broad scenarios, the likelihood and severity of a parasite infection can be influenced by several ecological and behavioral factors. In short, it’s complicated. A recent study in the journal Ecology and Evolution investigated the interplay between parasites and hybrids in a contact zone between California Quail (Callipepla californica) and Gambel’s Quail (Callipepla gambelii). Let’s see if they could untangle this complex interplay.

Infection Rates

Allison Roth and her colleagues tested 193 quails (72 California Quail, 27 Gambel’s Quail and 94 hybrids) for the presence of Haemoproteus lophortyx, a parasite that can cause avian malaria. The screening indicated that Gambel’s Quail were more often infected compared to California Quail and hybrids. However, the intensity of infection was higher in the latter two groups. In other words, California Quail and hybrids were infected less often, but when experiencing an infection they had more parasites in their blood. A counterintuitive result that requires further explanation.

Gambel’s Quail showed the highest prevalence of parasites (left figure), but California Quail and hybrids had the more parasites when infected (right figure). From: Roth et al. (2021).

Two Scenarios

The researchers describe two possible scenarios to explain the discrepancy between infection rate and intensity. First, the ecology or behavior of Gambel’s Quail might increase the likelihood of encountering biting midges (the vector of the parasites), resulting in a higher infection rate. In addition, the social interactions between individual Gambel’s Quail might be conducive for the exchange of parasites (see this blog post on the social network of quails). The higher infection rates can drive co-evolution between the Gambel’s Quails and their parasites, culminating in a higher parasite resistance in this species. Hence, Gambel’s Quail will show lower infection intensities.

The second scenario suggests that both quail species are equally likely to become infected, but Gambel’s Quail have more difficulty in completely clearing parasites from their system. This might explain the less intense infection patterns in these birds. California Quail, however, might be able to recover from mild infection. Only the most intense infections will leave their signature in the blood.

Hybrid Patterns

The similarity between the California Quail and the hybrids suggests that hybrids are genetically and/or behaviorally similar to this species. However, the researchers analyzed the hybrids as one homogenous group, although they probably represent a range of first-generation hybrids and backcrosses. More detailed analyses, taking into account the genetic make-up of the hybrids, are warranted. Moreover, several other details remain to be determined, such as the fitness effects of parasite infections and the possibility of multiple parasite lineages that are circulating in these quails. Despite these open questions, the researchers indicate that “these findings suggest that infection by H. lophortyx has the potential to influence species barrier dynamics in this system.” Parasite-driven speciation, wouldn’t that be awesome?

References

Roth, A. M., Keiser, C. N., Williams, J. B., & Gee, J. M. (2021). Prevalence and intensity of avian malaria in a quail hybrid zone. Ecology and Evolution, 11(12), 8123-8135.

Featured image: Gambel’s Quail (Callipepla gambelii) © Alan D. Wilson | NaturesPicsOnline

Stopping the decline of the Bearded Vulture

Genetic analyses point to two conservation units that need protection.

The number of Bearded Vultures (Gypaetus barbatus) is drastically decreasing. In recent decades, several local extinctions occurred, such as the disappearance of populations on the Italian island Sardinia and in the Balkans. The main drivers of this decline include unintentional poisoning of birds, habitat loss, collisions with energy infrastructure and use of vulture body parts in traditional medicine (which is obviously based on pseudoscientific and misguided beliefs). There is, however, hope: in 1986, Bearded Vultures from a captive breeding program were reintroduced into the Alps. The current population is estimated to ca. 300 individuals. In order to extend this success story to other areas and protect this charismatic species, proper conservation measures will need to be implemented. In addition, a genetic perspective on the population structure of the Bearded Vulture can help conservationists to focus on genetically impoverished regions and take action quickly.

Gene Flow

In a recent study in the journal BMC Ecology and Evolution, researchers used a set of 14 microsatellites to determine the genetic population structure of the Bearded Vulture. An impressive dataset of 236 individuals covered the entire range of the species, from Europe and Asia to the southern tip of Africa. The genetic analyses revealed little gene flow between four geographic populations: Europe, Asia, northern Africa and southern Africa. The highest level of genetic exchange occurred between Europe and Asia (28%), suggesting that there used to be a connection across the Balkans. There has been minimal gene flow with the population in southern Africa, which only received about 7% of genetic variation from northern Africa (and nearly nothing from Europe or Asia). In all analyses, the researchers noted that “the isolated southern African bearded vulture population is genetically distinct from all other bearded vulture populations.”

Hence, the Bearded Vulture populations can best be managed as two separate conservation units (similar to the Pink Cockatoo in Australia). The three northern populations – Europe, Asia and northern Africa – are connected by occasional gene flow, while the population in southern Africa is genetically isolated.

Patterns of gene flow between different populations of the Bearded Vulture. The thickness of the lines indicates the level of gene flow. From: Streicher et al. (2021).

Subspecies

Apart from conservation, these findings might also have consequences for the taxonomy of the Bearded Vulture. Based on the current study, the authors advocate the recognition of G. b. barbatus in Eurasia and northern Africa and G. b. meridionalis in eastern and southern Africa. These subspecies probably became isolated by the expansion of the Sahara desert, followed by a stepping-stone connection between southern and eastern Africa (which explains the low level of gene flow uncovered in this study). Regardless of the taxonomic division, the researchers highlight “the need for conservation programmes to effectively manage populations of this species and maintain extant genetic diversity.”

References

Streicher, M., Krüger, S., Loercher, F., & Willows-Munro, S. (2021). Evidence of genetic structure in the wide-ranging bearded vulture (Gypaetus barbatus (Linnaeus, 1758)). BMC ecology and evolution, 21(1), 1-11.

Featured image: Bearded Vulture (Gypaetus barbatus) © Richard Bartz | Wikimedia Commons

First-generation hybrids between two highly divergent Wren species

Despite four million years of divergence, Winter Wren and Pacific Wren can still interbreed.

When you look at the Winter Wren (Troglodytes hiemalis) and the Pacific Wren (T. pacificus), you might think that they belong to the same species. Professional ornithologists were also tricked by these morphological similarities. Until 2010, these wrens were considered conspecific with the Eurasian Wren (T. troglodytes). However, genetic analyses revealed that we are dealing with cryptic species that diverged millions of years ago. In particular, Winter Wren and Pacific Wren have been on separate evolutionary trajectories for over 4 million years (according to mitochondrial DNA). This deep divergence suggests that hybridization between these species is unlikely. But unlikely is not the same as impossible. Indeed, a recent study in the journal Molecular Ecology reports a few first-generation hybrids between Winter Wren and Pacific Wren.

Genetic Analyses

Because these wren species are morphologically difficult to tell apart, Else Mikkelsen and Darren Irwin turned to genetic data. The analyses of DNA samples from 76 individuals revealed two clear genetic clusters, corresponding to the two species. Interestingly, two samples showed genetic signatures that had all the hallmarks of first-generation (F1) hybrids. In the phylogenetic network and in the principal component analyses, these samples ended up in intermediate positions. And their genetic ancestry was roughly 50% Winter Wren and 50% Pacific Wren. A textbook example of how to spot F1-hybrids in genetic data.

Clear genetic differentiation between Winter Wren and Pacific Wren in (b) principal component analysis, (c) phylogenetic network and (d) STRUCTURE analyses. Two individuals were identified as F1-hybrids.

Tension Zone

There were only two F1-hybrids among the 76 samples and no signs of backcrosses. An interesting observation that leads to several new questions. Indeed, the researchers noted that:

Despite production and viability of F1 hybrids, we saw no evidence for recent backcrossing or other reproduction of hybrids, suggesting that F1 hybrids suffer greatly reduced fitness relative to parental birds. The most plausible explanation for our results is that F1 hybrids currently have low (virtually zero) reproductive success.

The exact reason for the lack of reproductive success of these hybrids remains to be determined. It could be that these individuals are sterile, or they might be unable to attract a partner. This situation – where F1-hybrids have extremely low reproductive success – can give rise to an extreme version of a “tension zone”. This specific hybrid zone model involves a balance between selection against hybrids and dispersal of parental species into the contact zone. Very strong selection against hybrids leads to a narrow hybrid zone, as seen in other old species pairs that still interbreed. Although the hybrid zones might be narrow, their implications for understanding the speciation process can be broadly applied.

References

Mikkelsen, E. K., & Irwin, D. (2021). Ongoing production of low‐fitness hybrids limits range overlap between divergent cryptic species. Molecular Ecology, 30(16), 4090-4102.

Featured image: Winter Wren (Troglodytes hiemalis) © Fyn Kynd | Wikimedia Commons

Delineating conservation units in the Pink Cockatoo

How many independent populations should conservationists consider?

The Pink Cockatoo (Lophochroa leadbeateri) – one of the most iconic species in Australia – is of conservation concern. This colorful species is threatened by the intensification of agriculture, habitat loss and the illegal pet trade. In order to safeguard the future of the Pink Cockatoo, we need to have a clear overview of the populations that require protection. Here, conservation units and evolutionary significant units (ESU) are useful concepts. Conservation units refer to demographically independent units of genetic variation, whereas ESUs correspond to independently evolving units of genetic variation. Currently, the Pink Cockatoo is divided into two subspecies – leadbeateri and mollis – mainly based on morphological differences, such as the color and pattern of the crest. A recent study in the Biological Journal of the Linnean Society investigated whether these subspecies correspond to conservation units or ESUs, which would facilitate conservation efforts to protect this species.

Nuclear and mitochondrial patterns

Kyle Ewart and his colleagues took a closer look at the genetic make-up of 87 individuals across the range of the Pink Cockatoo. Analyses of nuclear markers revealed two clear genetic clusters that correspond to the two known subspecies. A third cluster was composed of several specimens of the subspecies leadbeateri and probably represents an artifact from analyzing related individuals. Therefore, we will focus on the two main clusters. Additional analyses indicated “relatively low, but significant genetic differentiation” between the two subspecies.

The population structure in the nuclear markers was not apparent in the mitochondrial DNA. Here, the researchers could not discriminate between the two subspecies. This lack of reciprocal monophyly is probably the result of a recent population expansion in which a common mitochondrial variant spread across the range of the Pink Cockatoo.

Distribution of Lophochroa leabeateri leadbeateri (blue) and Lophochroa leabeateri mollis (orange) in Australia. Genetic analyses revealed two clear clusters that correspond to both subspecies. However, the genetic differentiation is not clear cut. From: Ewart et al. (2021).

Conservation Units

Based on the genetic patterns described above, the researchers argue that the two subspecies should be considered as separate conservation units. However, the lack of mitochondrial differentiation and the subtle differences in the nuclear markers suggest that they are not independent evolutionary units. Although interesting from a fundamental perspective, these labels are less relevant within a conservation context. In the end, both subspecies might require different conservation measures.

Moreover, the researchers developed a set of molecular markers that can be used to confidently discriminate between the subspecies. This technique will be extremely useful in identifying source populations and combating illegal trade in these birds. Hopefully, these efforts will ensure a bright future for these beautiful birds.

References

Ewart, K. M., Johnson, R. N., Joseph, L., Ogden, R., Frankham, G. J., & Lo, N. (2021). Phylogeography of the iconic Australian pink cockatoo, Lophochroa leadbeateri. Biological Journal of the Linnean Society, 132(3), 704-723.

Featured image: Pink Cockatoo (Lophochroa leadbeateri) © JJ Harrison | Wikimedia Commons

Morphological and genetic evidence for hybrids between Magnolia Warbler and American Redstart

But why would these two clearly distinct species interbreed?

The bird family Parulidae – the wood warblers – is a hybrid hotspot, with more than 50% of species known to hybridize. Indeed, numerous species combinations have been reported (check out this page for an overview), providing ornithologists with valuable insights into the origin and evolution of species (see for example here and here). You would think that by now, we would have uncovered all hybrid combinations among these warblers. However, a recent study in The Wilson Journal of Ornithology adds another hybrid to the list: Magnolia Warbler (Setophaga magnolia) x American Redstart (S. ruticilla). Using morphological and genetic analyses, ornithologists managed to confirm the hybrid nature of two peculiar individuals.

Plumage and Genetics

It is interesting that although this hybrid combination had not been reported before, two independent observations are described in the paper. The first hybrid was found by two birdwatchers – Daniel Néron and Joël Coutu – in Quebec, Canada. They detected a Magnolia Warbler-like individual that produced the song of an American Redstart. This aberrant bird was later captured and banded. The second hybrid was caught during a routine ringing session of the Black Swamp Bird Observatory in Ohio, USA. One of the staff members took detailed measurements and collected feathers from this bird. In both cases, the hybrid showed plumage characteristics of both species, namely:

Magnolia Warbler plumage features include white eye arcs, yellow wash on the belly, narrow white wing bars, and black lores. American Redstart plumage features include a dark yellow-orange coloration on the sides of the breast and under the wing, yellow on the base of most of the secondaries, and pale yellow patches on the outer tail feathers.

The morphological suggestions of hybridization were confirmed with genetic data. The researchers obtained DNA sequences for three genes: the mitochondrial ND2, the Z-linked MUSK and the nuclear MYO2. For both hybrids, analyses of these markers pointed to a Magnolia Warblers as the mother and American Redstart as the father.

Two hybrid Magnolia Warbler x American Redstart. Images to the left (a–d) show the male hybrid discovered in Laval, Québec, Canada (photos courtesy of Simon Duval). Images to the right (e–h) show the female hybrid discovered at the Navarre Banding Station, Ottawa County, Ohio, USA (photos courtesy of Ryan Jacob). From: Brennan et al. (2020).

A Rare Occasion

The strikingly different plumage patterns of these two species raise the question why they would engage in hybridization. Surely, they look different enough to avoid any confusion. The researchers speculate that the immature plumage of young male American Redstarts is similar to that of Magnolia Warblers. Moreover, both species sing similar songs which could interfere with species recognition. In most cases, hybridization will be unlikely. But under particular circumstances – such as a lonely male unable to locate a conspecific partner – interbreeding might occur. And hopefully, an observant ornithologist will be around to spot such a rare sight.

References

Brennan, C. L., Boulanger, E., Duval, S., Frei, B., Gorbet, A., Head, J., Shieldcastle, M. & Jones, A. W. (2020). Two cases of a previously undocumented New World warbler hybrid (Setophaga magnolia x S. ruticilla) in eastern North America. The Wilson Journal of Ornithology, 132(3), 537-547.

Featured image: Magnolia Warbler (Setophaga magnolia) © William H. Majoros | Wikimedia Commons