Body size and phylogenetic history constrain the evolution of bird song

Although there seems to be a minor role for sexual selection as well.

Different bird species produce sounds across a wide range of frequencies. From the low-frequency booming calls of the Eurasian Bittern (Botaurus stellaris) to the high-pitched sounds of the Common Firecrest (Regulus ignicapilla). Ornithologists have formulated several hypotheses to explain the evolution of these frequency differences. The Acoustic Adaptation Hypothesis, for instance, suggests that acoustic signals are adapted to maximize the effectiveness of their transmission in a particular environment. Because low-frequency sounds travel better in dense vegetation than high-frequency sounds, bird species in forests are expected to produce calls at lower frequencies because of physical constraints. The Morphological and Phylogenetic Constraint Hypothesis, however, states that body size and/or evolutionary history limit the range of frequencies that a species can reach. Larger species tend to produce lower-frequency sounds. Finally, the Sexual Selection Hypothesis posits that frequency might act as an indicator of an individual’s size, dominance or fighting ability. Low-frequency sounds give the impression of a larger individual and might thus be selected for in the context of male-male competition.

Explaining Variation

To discriminate between these possible explanations, a recent study in the journal Ecology Letters took a closer look at the songs of more than 5000 songbirds. The researchers used the public database xeno-canto to collect information on the highest frequencies in different bird songs. Next, they correlated these peak frequencies with several evolutionary and ecological variables. The analyses revealed that evolutionary history (43-56%) and body mass (10-15%) explained the largest proportion of variation in peak song frequency. Heavier birds sang at lower frequencies. After accounting for these two factors, there was also a significant association with sexual size dimorphism (although the explained variation was very low: 1-3%). Peak frequencies were lower in species where males are larger than females, suggesting strong sexual selection. Song frequency might thus be a proxy for the competitive ability of males.

These patterns indicate that phylogenetic history and body size constrain the range of frequencies that a species can produce. Within the consequently limited range of frequencies, sexual selection appears to influence the peak frequency in particular species.

Most of the variation in song peak frequency was explained by body mass. Sexual size dimorphism accounted for a small fraction of the remaining variation, while there was no clear effect of tree cover. From: Mikula et al. (2021) Ecology Letters.

No Acoustic Adaptation?

Interestingly, there was no support for the Acoustic Adaptation Hypothesis. Although this hypothesis makes intuitive sense, there was no clear effect of tree cover (a proxy for habitat density) in the large dataset. The researchers propose several explanations for this negative result:

• The analyses lacked information about bird behavior or habitat at the recording location. These factors could influence the produced frequency.
• The estimates of habitat density were quite crude. Better measurements might pick up subtle signals.
• Other biotic and abiotic factors that might influence background noise were not included in the analyses.

More research is thus needed to explore how important acoustic adaptation is in the evolution of bird song. Nonetheless, phylogenetic constraints seem to restrict the available evolutionary pathways (similar to genomic constraints in hybrid species). Evolution is a powerful process, but that doesn’t mean that anything is possible.

References

Mikula, P., Valcu, M., Brumm, H., Bulla, M., Forstmeier, W., Petrusková, T., Kempenaers, B. & Albrecht, T. (2021). A global analysis of song frequency in passerines provides no support for the acoustic adaptation hypothesis but suggests a role for sexual selection. Ecology letters, 24(3), 477-486.

Featured image: European Robin (Erithacus rubecula) © Charles J. Sharpe | Wikimedia Commons

Rampant introgression across the evolutionary tree of suboscine birds

An extensive genomic study detects numerous cases of interspecific gene flow.

Introgression – the exchange of genetic material through hybridization and backcrossing – seems to be an integral part of avian evolution. In 2017, I published a review paper on this phenomenon, trying to provide an overview of the known cases and the methods to detect introgression. At the time, my search uncovered 165 genetic studies that were published between 1987 and 2017. Most of these papers focused on members of the Passeriformes (songbirds), Galliformes (gamefowl), Anseriformes (waterfowl) and Charadriiformes (gulls, waders and auks), bird orders that display high levels of hybridization. However, I did not attempt to quantify the incidence of introgression, because this would probably be a gross underestimate. Numerous bird species have not been studied with genetic tools yet. Such as quantification would involve a large genomic data set, comprising many species. And that is exactly the approach of a study that recently appeared in the journal Evolution Letters.

D-statistics

Sonal Singhal and her colleagues focused on the suboscines, a clade of passerine birds that comprises more than 1000 species, most of which can be found in South America. Using a genomic dataset of 2389 loci (consisting of ultraconserved elements and exons) for 1306 species, the researchers looked for signatures of introgression. They relied on one of my favorite methods: the D-statistic (also known as the ABBA-BABA test). I have explained this approach so many times that I regularly dream about it (and no, the dream does not involve ABBA-songs). Here is the short explanation from my review:

The D-statistic considers ancestral (‘A’) and derived (‘B’) alleles across the genomes of four taxa. Under the scenario of incomplete lineage sorting without gene flow, two particular allelic patterns ‘ABBA’ and ‘BABA’ should occur equally frequent. An excess of either ABBA or BABA, resulting in a D-statistic that is significantly different from zero, is indicative of gene flow between two taxa.

The researchers calculated this statistic for numerous trios of species. They found that 49 out of 130 trios (38%) had a significant D-statistic. The highest five D-statistics were detected in Dendrocolaptes woodcreepers, Smithornis broadbills, Neopelma manakins, Grallaria antpittas, and Phlegopsis antbirds. I have described hybridization in some of these taxa on their family pages at Avian Hybrids Project (you can click the links if you want to learn more).

An example of the ABBA-BABA test for a trio of species (plus an outgroup). From: Singhal et al. (2021) Evolution Letters.

An Underestimate?

To understand the mechanisms underlying the introgression patterns, the researchers correlated the D-statistics with several environmental variables. These analyses revealed some expected relationships. For example, introgression was most common between species that were geographically close and that experienced higher rates of climate change (which moves species around and might lead to secondary contact). In addition, introgression occurred more between species at lower latitudes. It could be that tropical regions provide more opportunities for introgressive hybridization due to the higher species richness in these areas. It would be interesting to repeat these analyses with more species-specific traits to better understand the drivers of introgression in suboscine birds.

Although an introgression incidence of 38% seems quite high, I think it is probably even an underestimate. The analyses only included trios of closely related species and might thus have missed introgression between more distantly related species. Moreover, the D-statistic does detect gene flow between sister species. It only reports only gene flow between the third species and one of the first two in the phylogenetic tree. The researchers did also not take ghost introgression into account: the exchange of genetic material with unsampled or extinct lineages (see my recent review on this topic for more details). And finally, the analyses involved only a subset of the genome. Who knows how many introgressed regions are hiding in the rest of the genome?

An overview of significant D-statistics (highlighted in orange) across the phylogeny of the suboscines. From: Singhal et al. (2021) Evolution Letters.

References

Singhal, S., Derryberry, G. E., Bravo, G. A., Derryberry, E. P., Brumfield, R. T., & Harvey, M. G. (2021). The dynamics of introgression across an avian radiation. Evolution letters, 5(6), 568-581.

Featured image: Green-and-black Fruiteater (Pipreola riefferii) © Francesco Veronesi | Wikimedia Commons

How did the population of Sussex Peregrine Falcons recover?

Searching for the source population of these UK birds.

The history of the Peregrine Falcon (Falco peregrinus) is a string of ups and downs. This enigmatic falcon species has been prosecuted numerous times and suffered from pollution, only to bounce back afterwards. During the Second World War, Peregrine Falcons were shot because they predated on homing pigeons, which carried important military messages. After the World War, they were targeted by grouse-moor gamekeepers (i.e. the hunting of Red Grouse, a field sport in the UK) and pigeon fanciers. In addition, the use of pesticides – specifically DDT – posed another threat. These chemicals flow through the food chain, eventually accumulating to high doses in predators where they lead to the thinning of eggshells and a reduction in breeding success.

Luckily, the use of these chemicals has been banned and the prosecution of Peregrine Falcons has diminished. In several regions, such as Scandinavia and North America, re-introduction programs have restored the breeding populations of Peregrine Falcons. In the southern part of the UK, however, the number of Peregrine Falcons seems to have recovered without any human assistance. In Sussex, the number of breeding pairs increased from ca. 10 in 1954 to more than 40 by 2016. This remarkable population growth raises an important question: where did these birds come from?

Source Populations

In a recent Conservation Genetics study, Angela Weaving and her colleagues attempted to determine the origin of the Sussex Peregrine Falcons. Using a combination of microsatellites and mitochondrial markers, they tested several hypotheses about the identity of the founding birds:

• Migrants from the UK
• Migrants from mainland Europe
• Escaped captive birds (possibly including hybrids)

To discriminate between these possible source populations, the researchers compared the genetic material of the Sussex birds with a historical sample (before the 1950s) of Peregrine Falcons from the UK, domestic-bred birds from the UK, and wild birds from Germany, the Republic of Ireland and the Mediterranean.

Microsatellites

Analyses of the mitochondrial DNA were not very helpful to solve this mystery. One main haplotype was shared by all the sampled populations. This low genetic diversity is probably a remnant from a rapid population expansion from a small population (with limited genetic diversity) at the end of the Pleistocene. The microsatellites, on the other hand, did provide insights into the origin of the Sussex population. The researchers found that “the contemporary wild Peregrine population in Sussex is genetically similar, but not identical, to the pre-pesticide UK population, and it is genetically different from other European populations and from the domestic stock.” It thus seems that the Sussex population increased through the influx of other Peregrine Falcons from the UK. More sampling is needed to pinpoint the exact locations where these birds came from.

The contemporary population (dark-blue) clusters with the UK population from the pre-pesticide period (light-blue). There is a clear difference with the domestic birds (orange). From: Weaving et al. (2021) Conservation Genetics.

Hybrid Falcons

Hybrid falcons, such as Peregrine Falcon x Gyrfalcon (Falco rusticolus), are quite common in captivity and regularly escape into the wild. Vincent Fleming and his colleagues estimated that since 1980 at least 30 Peregrine Falcon hybrids per year have been reported as lost. This is probably an underestimate because it is now no longer required to report escapees. The findings from the Sussex study suggest that escaped hybrids did not markedly influence the genetic make-up of the wild population. It could be that these hybrids might have lower survival rates or difficulties in finding a partner. However, these birds could still affect the wild population by occupying breeding territories or engage in failed nesting attempts with “pure” Peregrine Falcons. More research is thus needed to understand the fate of escaped hybrids, and their impact on wild populations of Peregrine Falcons.

The number of falcon individuals that were reported as lost in the UK. From: Fleming et al. (2011).

References

Weaving, A., Jackson, H. A., Nicholls, M. K., Franklin, J., & Vega, R. (2021). Conservation genetics of regionally extinct peregrine falcons (Falco peregrinus) and unassisted recovery without genetic bottleneck in southern England. Conservation Genetics, 22(1), 133-150.

Featured image: Peregrine Falcon (Falco peregrinus) © Mosharaf Hossain Ce | Wikimedia Commons

The Warbling Vireo comprises of two cryptic species

Genetic analyses confirm behavioral differences between two main groups.

There is more to avian diversity than meets the eye. Some species might look very similar although they are genetically and behaviorally distinct. An example that most European birders will be familiar with concerns the Chiffchaff (Phylloscopus collybita) and the Willow Warbler (Phylloscopus trochilus). Morphologically, these passerines are difficult to tell apart. But when they start singing, it is easy to discriminate between the two-syllable song of the Chiffchaff and the string of notes produced by the Willow Warbler. Recently, a study in the journal Ornithology (previously The Auk) described a similar case in North America where the Warbling Vireo (Vireo gilvus) might actually comprise two cryptic species.

Genetic Markers

Currently, the Warbling Vireo is classified into four subspecies that can be assigned to two main groups. The gilvus group houses only one subspecies (gilvus) and can be observed in deciduous forests across eastern North America. The swainsoni group holds three subspecies (swainsoni, victoriae, and leucopolius) that occur in the deciduous and mixed forests of western North America. Both groups meet along a contact zone in central Alberta (Canada). Some ornithologists suggested that these two groups might represent distinct species, but the evidence supporting this taxonomic arrangement is limited. Hence, Scott Lovell and his colleagues decided to unravel the genetics of this species complex by sequencing some mitochondrial and nuclear DNA.

The mitochondrial gene cytochrome b (cytb) supported the two groups described above, separated by 35 mutations. The genetic divergence between these mitochondrial clades amounted to 4% which translates into a divergence time between 1.9 and 2.5 million years ago (depending on the chosen mutation rate). The nuclear markers corroborated this pattern, clearly dividing most individuals into the gilvus and swainsoni groups. I write “most individuals”, because the genetic analyses uncovered some hybrids. Nine out of 145 individuals showed signs of mixed ancestry. These hybrids were detected within and near the contact zone, suggesting limited introgression outside of this area.

The mitochondrial DNA clearly separates the two groups. From: Lovell et al. (2021) The Auk.

Migratory Divide

Genetic data is just one line of evidence to support a taxonomic split. The two groups do show some minor differences in morphology and song, but the main difference probably relates to their migratory behavior. Individuals from the gilvus and swainsoni groups follow distinct migration routes: gilvus individuals fly through the mid-western and eastern USA whereas swainsoni individuals follow a route across the western USA and Mexico. Moreover, the two taxa arrive on the breeding grounds at different times: swainsoni individuals settle about two weeks earlier. This mismatch in timing reduces the possibility of mixed couples, allowing these taxa to diverge genetically. Hence, the authors argue to recognize these two groups as distinct species. In addition, they write that “future work may determine that additional cryptic species occur within the swainsoni group.” Who know what taxonomic discoveries lie ahead?

Analyses of the nuclear DNA discriminated between the two groups, but also uncovered some hybrids (green squares). Yellow triangles represent individuals with mismatches between nuclear and mitochondrial assignment. From: Lovell et al. (2021) The Auk.

References

Lovell, S. F., Lein, M. R., & Rogers, S. M. (2021). Cryptic speciation in the Warbling Vireo (Vireo gilvus). The Auk, 138(1), ukaa071.

Featured image: Warbling Vireo (Vireo gilvus) © Francesco Veronesi | Wikimedia Commons

Hybridization helps to determine the genetic basis of head patterns in the White Wagtail

This plumage traits is associated with two small genomic regions.

Subspecies of the White Wagtail (Motacilla alba) show a large variety of head patterns: from the black cheeks of personata to the completely white face of dukhunensis. In a previous blog post, I described how some researchers suspect that these plumage patterns can be explained by a small toolkit of genes that is being shuffled around by hybridization. To test this idea, we first need to unravel the genetic basis of these plumage patches. And that is exactly what a recent study in the journal Nature Communications did. Georgy Semenov and his colleagues focused on the Siberian hybrid zone between the subspecies alba and personata. The hybrids from this region show a range of plumage combinations, making it easier to link particular genomic regions to these plumage traits. Recent studies on warblers and woodpeckers took a similar approach and successfully identified several “plumage genes”. What about the wagtails?

An overview of the plumage patterns in the subspecies alba, personata and their hybrids. From: Semenov et al. (2021) Nature Communications.

Interacting Genes

The researchers sequenced the genomes of 10 individuals from the alba range, 10 individuals from the personata range, and 62 birds from the hybrid zone. Patterns of genetic differentiation within the hybrid zone uncovered two small genomic regions, located on chromosomes 1A and 20. The region on chromosome 20 contained three genes, including the well-known agouti signaling protein (ASIP) which is involved in the regulation of melanogenesis (i.e. production of the pigment melanin). The other region overlapped with a non-coding sequence between two genes (NT5D3 and CCDC91).

To figure out how these two genomic regions determine the development of different head patterns, the researchers took a closer look at the inheritance patterns in the hybrids. The genetic variants at the ASIP-region were classified into an alba-variant (A) and a personata-variant (P). Individuals with two different ASIP-variants (i.e. heterozygotes, AP) showed alba-like plumage patterns, suggesting that the alba-variant is dominant. More detailed analyses indicated that the expression of the other genomic region (on chromosome 1A) depends on the ASIP-genotype. This genetic architecture of two interacting genomic regions is reminiscent of the situation in Carrion Crow (Corvus c. corone) and Hooded Crow (C. c. cornix), which I described in detail in another blog post. The exact mechanisms underlying this interaction in the wagtails remain to be determined.

Comparing the genomes of the subspecies outside (top figure) and within (bottom figure) the hybrid zone pointed to two differentiated genomic regions on chromosomes 1A and 20. From: Semenov et al. (2021) Nature Communications.

Asymmetric Introgression

The story does not end here though. Previous work found that the transition from alba-like to personata-like heads was located about 300 kilometers northwest from the genetic center of the hybrid zone. Additional analyses of the two genomic regions described above revealed that they introgressed from personata into alba. The explanation for this asymmetric introgression is not clear yet. The head plumage could represent the leading edge of a moving hybrid zone, or there might be selection for or against certain phenotypes outside the hybrid zone. Whatever the mechanism, hybridization seems to play a pivotal role in the evolution of head patterns in the White Wagtail (similar to patterns described in wood-warblers). Indeed, the authors conclude:

The reticulate nature of phenotypic variation in head and neck plumage suggests that variation in a small number of genes may underlie the rich phenotypic diversity in wagtails. Our study suggests that only two loci contribute to head plumage differences in alba and personata subspecies. Differentiation at the two loci is retained in allopatry despite hybridization and these loci, located on different chromosomes, introgress asymmetrically together from one population into another. These results suggest epistatic interactions contribute to the evolution of sexual signals and that the genetic architecture of a trait is an important determinant in introgression.

The location of the hybrid zone (left figure) and the displacement of the head plumage (yellow line) compared to the hybrid zone (green line). From: Semenov et al. (2021) Nature Communications.

References

Semenov, G. A., Linck, E., Enbody, E. D., Harris, R. B., Khaydarov, D. R., Alström, P., Andersson, L. & Taylor, S. A. (2021). Asymmetric introgression reveals the genetic architecture of a plumage trait. Nature Communications, 12(1), 1-9.

Featured image: White Wagtail (Motacilla alba) © J.M. Garg | Wikimedia Commons

How can Snowfinches and Tree Sparrows survive at high altitudes?

Experimental work points to a more efficient metabolism in highland birds.

Life in the mountains is not easy. At very high altitudes, the concentration of oxygen drops, potentially leading to severe health risks. Nonetheless, several animal species – including humans – manage to thrive in these extreme conditions, despite facing many physiological challenges. Apart from the low oxygen concentrations in mountainous areas, temperature can also significantly drop. One important hurdle for high altitude animals thus concerns keeping a stable body temperature in these cold environments. Moreover, you will also need to move around to gather food and find a mate. An efficient energy metabolism is thus crucial.

There are several ways to amp up your metabolism, such as bigger muscles to do the work, more capillaries to transport the little oxygen that is available, and having more mitochondria (the powerhouse of the cell) in your muscle fibers. A recent study in the journal PLoS Genetics took a closer look at three high altitude bird species: the White-rumped Snowfinch (Onychostruthus taczanowskii), the Rufous-necked Snowfinch (Pyrgilauda ruficollis), and the Tree Sparrow (Passer montanus). It turned out that these species do indeed have bigger pectoral muscles and more mitochondria in their muscle fibers. But these phenotypic features were just the tip of the iceberg.

---

Glucose

Experimental work on these species revealed that they showed more efficient use of glucose (i.e. blood sugar). After a meal, the level of glucose in the blood increases. This rise in blood sugar triggers the production of the hormone insulin which converts some glucose into glycogen for later use and makes body cells take up the circulating glucose. These processes bring the blood sugar level down to stable levels. The researchers compared the processing of glucose in highland and lowland birds. The experiments indicated that “highlanders exhibited a more rapid normalization of blood glucose.” The difference between highland and lowland birds was even more pronounced when they were injected with insulin (an extra trigger to convert the glucose), suggesting that highland birds are more sensitive to this hormone. The extra insulin sensitivity can be regarded as an adaptation to life at high altitudes.

The left figure showed that Highland species (indicated in red and blue) exhibited more efficient glucose processing compared to lowland species (in green). In addition, the highland species were more sensitive to insulin than the lowland species (right figure). From: Xiong et al. (2020) PLoS Biology.

---

Candidate Genes

Next, the researchers focused on the genetic basis of these adaptations. They correlated the muscle phenotypes with genetic data in search of candidate genes, and they looked at gene expression data to see which genes were active in the muscle tissue. These analyses uncovered two interesting genes: EPAS1 and MEF2C. Readers familiar with the literature on hybridization might recognized the first gene. EPAS1 has been found in several human populations that adapted to high altitudes. Moreover, one variant of this gene probably introgressed from the extinct Denisovans into Tibetans. EPAS1 becomes active in low oxygen conditions. The other gene – MEF2C – ensures the maintenance of muscle mass and healthy glucose levels, important features when living in the mountains.

Finally, more detailed genomic analyses on highland and lowland populations of the Tree Sparrow revealed additional candidate genes. Several of these genes have also been identified in other high altitude species, such as the Andean House Wren (Troglodytes aedon) and the Band-winged Nightjar (Hydropsalis longirostris), pointing to convergent evolution. The most important ones were:

• HBB (involved in oxygen affinity)
• BNIP3L (breakdown of mitochondria when oxygen levels are low)
• METTL8 (associated with metabolic diseases in humans)

The last gene in this list (METTL8) is especially interesting, because certain variants of this gene were significantly associated with the expression of MEF2C (the gene found in the previous analyses). Additional experiments showed that birds with an particular mutations at sites 326 and 395 in this gene developed larger muscles and expressed more MEF2C. A few genes might thus make life at high altitudes a little bit easier.

Two mutations in the METTL8-gene (highlighted with red arrows) result in higher muscle mass (left figure H) and more production of the gene MEFC2 (right figure H). From: Xiong et al. (2020) PLoS Genetics.

---

References

Xiong, Y., Fan, L., Hao, Y., Cheng, Y., Chang, Y., Wang, J., Lin, H., Song, G., Qu, Y. & Lei, F. (2020). Physiological and genetic convergence supports hypoxia resistance in high-altitude songbirds. PLoS Genetics, 16(12), e1009270.

Featured image: Rufous-necked Snowfinch (Pyrgilauda ruficollis) © Dibyendu Ash | Wikimedia Commons

Does sexual selection promote speciation?

Male-male competition might be a driving factor of speciation in birds.

When I write that “sexual selection speeds up speciation”, you might nod your head in agreement. It sounds logical that sexual selection on certain traits (e.g., song or plumage color) can contribute to reproductive isolation, culminating in the origin of new species. However, several studies that approximated the strength of sexual selection as the degree of sexual dichromatism (i.e. the differences in color between males and females) found no supporting evidence for this hypothesis (see Huang & Rabosky 2014 and Cooney et al. 2017). There could be numerous explanations why these studies failed to detect a connection between sexual selection and speciation rates. Perhaps sexual dichromatism is not a reliable proxy for the strength of sexual selection? Or maybe certain environmental factors influence the relationship between sexual selection and diversification? Indeed, theoretical models suggest that sexual selection can speed up local adaptation in variable environments. A recent study in the journal Evolution revisited this evolutionary conundrum and investigated the link between sexual selection and speciation rates, taking into account environmental factors.

---

Measures of Sexual Selection

Justin Cally and his colleagues collected data on more than 5800 passerine species. They estimated sexual selection in two ways: (1) the degree of sexual dichromatism and (2) an index of male-biased sexual selection. This index is associated with information on sexual size dimorphism, social polygyny (i.e. male with multiple females) and the lack of parental care. For readers interested in the technical details, the index corresponds to the first principal component of a phylogenetic PCA that is correlated with the three features listed above. The two measures of sexual selection were then correlated with speciation rates across the phylogeny of the passerines. In addition, the researchers accounted for the possible effects of several ecological and environmental variables, such the seasonality of temperature and rainfall.

An overview of the parameters used in this study: the researchers tested for a relationship between diversification rate (in green) and two measures of sexual selection: sexual dichromatism (in brown) and male-biased sexual selection (in grey). From: Calley et al. (2021) Evolution.

---

Male-male competition

The analyses revealed that “the composite index of male-biased sexual selection, but not measures of sexual dichromatism, is correlated with the rate of speciation in passerine birds.” These findings are in line with previous studies that found no relationship between sexual dichromatism and diversification. The authors of the current study argue that this measure is not a good proxy for sexual selection, because males and females can evolve different plumage patterns for other reasons, such as adaptation to distinct ecological niches (see for example the Sulawesi Babbler, Pellorneum celebense).

A detailed look at the positive effect of the male-biased sexual selection index indicated that sexual size dimorphism was the main driver. Differences in size mostly evolve due to male-male competition in which males compete for access to females. Because body size is often associated with other morphological and ecological traits, it could speed up the divergence between young evolutionary lineages. In fact, studies on mammals suggested that the evolution of large body size allowed for the diversification of ecological strategies, leading to higher speciation rates. Similar processes might have occurred in some bird groups. These patterns highlight the potential role of male-male competition in generating diversity. Sexual selection can thus promote speciation, but maybe not in the way we might have expected.

---

References

Cally, J. G., Stuart‐Fox, D., Holman, L., Dale, J., & Medina, I. (2021). Male‐biased sexual selection, but not sexual dichromatism, predicts speciation in birds. Evolution, 75(4), 931-944.

Featured image: Superb Fairywren (Malurus cyaneus) © Benjamint444 | Wikimedia Commons

Hybridization of the habitat: More admixed chickadees in urban areas

Hybrids are more common in heterogenous landscapes.

In 1948, the botanist Edgar Anderson published a paper in the journal Evolution, entitled “Hybridization of the Habitat.” Using his studies on several plant genera, such as Tradescantia and Iris, he explores why hybrids are often rare in nature. Apart from the difficulty of detecting hybrids (which has greatly improved since we have genetic tools), he noted that a suitable habitat for the resulting hybrids is generally missing. Because hybrids often show intermediate characteristics they cannot compete with the parental species and will thus not establish themselves. An important exception concerns heterogeneous landscapes, which tend to be scarce. He stated that “Such heterogeneous habitats are seldom or never met with, the only approach to them being found in places where man has greatly altered natural conditions.” These hybridized habitats might thus be the best places to find hybrids. In the decades following Anderson’s paper, this phenomenon has been observed in several animals and plants (as reviewed by Grabenstein and Taylor in 2018, and by me in 2021). A recent study in the Journal of Avian Biology adds another example to the growing list hybrids in heterogeneous, hybridized habitats.

Admixed Birds

According to a recent estimate of avian hybrids in North America, species in the family Paridae produce the most commonly observed passerine hybrids. It is thus no surprise that Brendan Graham and his colleagues focused on four members of this family: the Black-capped Chickadee (Poecile atricapillus), the Mountain Chickadee (P. gambeli), the Chestnut-backed Chickadee (P. rufescens), and the Boreal Chickadee (P. hudsonicus). Using microsatellites, the researchers determined the number of hybrid individuals between these species across North America. The genetic analyses revealed admixture among five of the six species pairs examined. There was no apparent admixture between Mountain and Chestnut-backed Chickadees. Interestingly, almost all of the admixed birds were phenotypically classified to one species, confirming Edgar Anderson’s claim that hybrids are often hard to find (without genetic tools).

Urban Areas

Next, the researchers focused on the locations where these admixed birds were observed. They found that “over 70% (81 of 112) of individuals identified as admixed were found in urban parkland or mixed forest habitat.” This finding is in line with the concept of “hybridization of the habitat” in which hybrids are more common in human-mediated or heterogenous landscapes. The exact mechanisms underlying increased hybridization rates in these areas remain to be determined, but could be related to higher food availability in urban settings (such as in hummingbirds). Nonetheless, this study is a nice example of avian hybridization in an increasingly anthropogenic world.

Top figure shows the distribution of 122 admixed individuals across the four habitat types. Bottom figure indicates the occurrence of admixed individuals within each species pair across the four habitat types. From: Graham et al. (2021) Journal of Avian Biology.

References

Graham, B. A., Gazeley, I., Otter, K. A., & Burg, T. (2021). Do phylogeny and habitat influence admixture among four North American chickadee (family: Paridae) species? Journal of Avian Biology, 52(5).

Featured image: Chestnut-backed Chickadee (Poecile rufescens) © V.J. Anderson | Wikimedia Commons

From the Amazon to the Atlantic Forest: the evolutionary story of the Blue-backed Manakin

What geological and ecological processes explain the distribution of this species?

At the moment, I am enjoying a well-deserved holiday with my family in Belgium. Apart from writing blog posts for the Avian Hybrids Project, I fill my free time with reading books and watching some Netflix-series (for those interested: I am currently watching The Walking Dead and Titans). Nothing more relaxing than immersing yourself in good story. But you don’t need to switch in your television or dive into the writings of Stephen King or George R. R. Martin to find these stories. Nature is full of amazing stories on a huge variety of species. For example, a recent study in the Journal of Ornithology used genetic analyses and ancestral area reconstructions to tell the evolutionary story of the Blue-backed Manakin (Chiroxiphia pareola) across South America. Sit back, relax and enjoy this epic journey from the Amazon to the Atlantic Forest.

Changing Rivers

Our story starts about 3 million years ago in the Amazon region where an ancestral population of the Blue-backed Manakin resides. At the end of the Pliocene (ca. 2.8 million years ago), massive erosion events changed the river dynamics in this area and split the ancestral population in two. Later on, the resulting western population gave rise to the subspecies regina and napensis, which became isolated on opposite sides of the Marañon River and Ucayali River. The eastern population, on the other hand, moved into the Guiana Shield (a mountain range in the northeast of South America), evolving into the subspecies pareola. These events nicely show how different processes – vicariance and dispersal – come into play during the evolutionary history of a bird species.

Fast forward to about 0.5 million years ago and we can see that the eastern population has colonized the Atlantic Forest. In previous blog posts – on the Variable Antshrike (Thamnophilus caerulescens) and Buff-browed Foliage Gleaner (Syndactyla rufosuperciliata) – I described several routes into the Atlantic Forest. But how did the Blue-backed Manakin get there? Based on ecological niche modelling, the authors argue that this species followed “a scenario in which a connection between the Amazonian and northeastern Atlantic Forest occurred in the early/middle Pleistocene via routes that extended through the interior of the Brazilian northeast.” This connection is supported by studies on other bird species and analyses of plant fossils.

Ancestral area reconstruction and diversification history of the Blue-backed Manakin. From: do Nascimento et al. (2021) Journal of Ornithology.

Taxonomy

And there you have it, the evolutionary story of the Blue-backed Manakin. Currently, this species is classified into four subspecies: napensis, regina, atlantica and pareola. However, the genetic analyses in this study uncovered five distinct lineages, not four. It seems that the subspecies atlantica and pareola are composed of more genetic groups than currently thought. A taxonomic revision might thus be warranted. And if these changes in classification are published, you can read the entire taxonomic story at the Avian Hybrids Project.

References

do Nascimento, N. F. F., Agne, C. E. Q., Batalha-Filho, H., & de Araujo, H. F. P. (2021). Population history of the Blue-backed Manakin (Chiroxiphia pareola) supports Plio-Pleistocene diversification in the Amazon and shows a recent connection with the Atlantic Forest. Journal of Ornithology, 162(2), 549-563.

Featured image: Blue-backed Manakin (Chiroxiphia pareola) © Steve Garvie | Wikimedia Commons