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Paridae, Remizidae, Aegithalidae (2 Viewers)

Driver, R., V. Ferretti, E.S. Burton, M.W. McCoy, K.C. Duerr, and R.L. Curry (2022)
Spatiotemporal variation in hatching success and nestling sex ratios track rapid movement of a songbird hybrid zone
The American Naturalist (advance online publication)
doi: 10.1086/720207

Hybridization often occurs at the parapatric range interface between closely related species, but fitness outcomes vary: hybrid offspring exhibit diverse rates of viability and reproduction when compared to their parental species. The mobile hybrid zone between two chickadee congeners (Poecile atricapillus x P. carolinensis) has been well studied behaviorally and genetically but the viability of hybrids, as well as the underlying mechanisms contributing to hybrid fitness, have remained unclear. To better characterize the fitness costs of hybridization in this system, we analyzed 21 years of data from four sites, including over 1,400 breeding attempts by the two species, to show that rates of hatching success changed substantially as the zone of hybridization moved across the landscape. Admixture-associated declines in hatching success correlated with reduced proportions of heterogametic (female) offspring as predicted by Haldane’s rule. Our data support an underlying mechanism implicating genetic admixture of the homogametic (male) parent as the primary determinant of offspring sex ratio, via incompatibilities on the hemizygous Z chromosome. Our long-term study is the first to directlymeasure changes in fitness costs as a vertebrate hybrid zone moves, and it shows that changes in these costs are a way to track the distribution of a hybrid zone across the landscape.
 
Grabenstein, K. C., Otter, K. A., Burg, T. M., and Taylor, S. A. (2022) Sympatry leads to reduced body condition in chickadees that occasionally hybridize. Ecology and Evolution 12: e8756.
https://doi.org/10.1002/ece3.8756

Abstract:
Both abiotic and biotic drivers influence species distributions. Abiotic drivers such as climate have received considerable attention, even though biotic drivers such as hybridization often interact with abiotic drivers. We sought to explore the (1) costs of co-occurrence for ecologically similar species that hybridize and (2) associations between ecological factors and condition to understand how abiotic and biotic factors influence species distributions. For two closely related and ecologically similar songbirds, black-capped and mountain chickadees [Poecile atricapillus and Poecile gambeli], we characterized body condition, as a proxy for fitness, using a 1358-individual range-wide dataset. We compared body condition in sympatry and allopatry with several abiotic and biotic factors using species-specific generalized linear mixed models. We generated genomic data for a subset of 217 individuals to determine the extent of hybridization-driven admixture in our dataset. Within this data subset, we found that ~11% of the chickadees had hybrid ancestry, and all hybrid individuals had typical black-capped chickadee plumage. In the full dataset, we found that birds of both species, independent of demographic and abiotic factors, had significantly lower body condition when occurring in sympatry than birds in allopatry. This could be driven by either the inclusion of cryptic, likely poor condition, hybrids in our full dataset, competitive interactions in sympatry, or range edge effects. We are currently unable to discriminate between these mechanisms. Our findings have implications for mountain chickadees in particular, which will encounter more black-capped chickadees as black-capped chickadee ranges shift upslope and could lead to local declines in mountain chickadee populations.
 
Georgy A Semenov, Claire M Curry, Michael A Patten, Jason T Weir & Scott A Taylor (2023) Geographically consistent hybridization dynamics between the Black-crested and Tufted Titmouse with evidence of hybrid zone expansion. Ornithology: posted online 8 April 2023.
Geographically consistent hybridization dynamics between the Black-crested and Tufted Titmouse with evidence of hybrid zone expansion

Abstract
We studied hybridization between the Black-crested [Baeolophus atricristatus] and Tufted [Baeolophus bicolor] titmouse across two geographically distinct transects that differ in the timing of secondary contact by hundreds to thousands of years. We found that hybridization patterns correspond to localized hybrid swarms and that the titmouse hybrid zone is likely slowly expanding over time, a product of short post-natal dispersal distances coupled with weak or absent selection against admixture. We show the southern part of the hybrid zone located in Texas is four times wider than northern region of hybridization in Oklahoma, which is likely due to geographic differences in hybrid zone age. Despite differences in width, most individuals in both transects are advanced-generation hybrids and backcrosses, suggesting geographically consistent hybridization dynamics. We documented a strong correlation between genotypes and plumage index, suggesting that hybridization has not yet resulted in the decoupling of plumage and genome-wide ancestry as observed in some other avian hybrid zones. Although our results suggest the ongoing expansion of the hybrid zone, the rate of expansion appears to be very slow, on the scale of tens of meters a year, and it will likely take hundreds of thousands to millions of years before homogenization of the parental populations. While we did not find support for partial reproductive isolation in the hybrid zone itself, there is the possibility that ecological or sexual selection limit introgression into allopatric regions. Broadly, the results of our study highlight the value of multiple, geographically distant, transects across a hybrid zone for assessing the evolutionary dynamics of hybridizing lineages.
 
Jiang, Z., G. Song, X. Luo, D. Zhang, F. Lei, and Y. Qu (2023)
Recurrent selection and reduction in recombination shape the genomic landscape of divergence across multiple population pairs of Green-backed Tit
Evolution Letters 7: 99–111
doi: 10.1093/evlett/qrad005

Speciation is fundamental for building and maintaining biodiversity. The formation of the highly differentiated genomic regions between diverging taxa has been interpreted as a result of divergence with gene flow, linked selection, and reduction in recombination. It is challenging to unravel these nonexclusive processes in shaping genomic divergence. Here, we investigate the relative roles of these processes in shaping genomic differentiation in a montane bird, the Green-backed Tit (Parus monticolus). Our genetic structure and demographic analyses identify that four genetic lineages diverge between 838 and 113 thousand years ago and there is evidence of secondary gene flow. The highly divergent genomic regions do not increase with the divergence time, as we found that the old lineages show relatively fewer numbers and smaller sizes of highly differentiated regions than the young divergent lineages (numbers, 118–138 vs. 156–289; sizes, 5.9–6.9 vs. 7.8–14.5 megabase). Across the genome, the outlier windows show a reduction in nucleotide diversity, absolute genetic divergence, and recombination rate, suggesting recurrent selection in regions with low recombination being the major driver of genomic divergence. Finally, we show that secondary gene flow tends to affect the highly differentiated genomic regions if these regions are less likely to be the targets of selection. Altogether, our study shows how common ancestry, recurrent selection, low recombination rate, and gene flow have contributed to the emergence of genomic islands at different stages of speciation.
 

A crowd-sourced genomic project to assess hybrid content in a rare avian vagrant (Azure Tit Cyanistes cyanus (Pallas, 1770))​

Martin Irestedt, Filip Thörn, Per G.P. Ericson, Hein van Grouw, Yaroslav A. Red'kin, Alexander Hellquist, Frank Johansson, Johan A.A. Nylander
www.sciencedirect.com

A crowd-sourced genomic project to assess hybrid content in a rare avian vagrant (Azure Tit Cyanistes cyanus (Pallas, 1770))

The aim of this study was to correlate plumage variation with the amount of genomic hybrid content in hybrids between Azure Tits Cyanistes cyanus (Pal…
www.sciencedirect.com www.sciencedirect.com
Avian Research, Volume 14, 2023, 100130​

The aim of this study was to correlate plumage variation with the amount of genomic hybrid content in hybrids between Azure Tits Cyanistes cyanus (Pallas, 1770) and European Blue Tit Cyanistes caeruleus (Linnaeus, 1758), by re-sequencing the genomes of museum specimens of non-hybrids and presumed hybrids with varying plumages. The project was funded by crowdsourcing and initiated when two presumed Azure Tits, observed by hundreds of Swedish birdwatchers, were rejected as hybrids based on minor plumage deviations assumed to indicate hybrid contents from the European Blue Tit. The results confirm that hybrids with intermediate plumages, so called Pleske’s Tits, are first generation hybrids (F1 hybrids). Individuals, whose plumages are similar to Azure Tits, but assessed as hybrids based on minor plumage deviations, are all backcrosses but vary in their degree of hybrid content. However, some individuals morphologically recognized as pure Azure Tits expressed similar degrees of hybrid content. The results indicate that: (1) hybrid content may be widespread in Azure Tits in the western part of its habitat distribution; (2) plumage deviation in backcrosses is not linearly correlated with the genetic degree of hybrid origin; and (3) all Azure Tits observed in Europe outside its natural distribution may have some degree of hybrid origin. We therefore suggest that it is very difficult to phenotypically single out hybrids beyond first generation backcrosses. We argue that decreased sequencing costs and improved analytical tools open the doors for museomic crowd-sourced projects that may not address outstanding biological questions but have a major interest for lay citizens such as birdwatchers.
 
Which brings up the philosophical question of when is a hybrid enough of a hybrid to be rejected? Are the white humans in western Europe hybrids because we carry a few percent of Neanderthal genes in our genome? When does a hybrid bird carry enough DNA from one parent species that one should ignore the content from the other and tick it?
Disclaimer: I have not seen the birds in this study and therefore only raise this out of curiosity.
Niels
 
njlarsen said:
... I have not seen the birds in this study and therefore only raise this out of curiosity.
Niels
Click to expand...
Niels, if even more curious, you can have a look at one of those (seriously debated) Birds, that started it all: here, here, or here, ... and elsewhere (a lot of Photos were taken of it, when it turned up at Grönhögen, Öland, Southern Sweden, in October, 2016), or just do a Google Search for Images (incl. "Cyanistes", with or without "cyanus" + "Grönhögen" + "2016", like here). Also see here.

Hybrid, or not, it's a pretty Bird!
 
njlarsen said:
Which brings up the philosophical question of when is a hybrid enough of a hybrid to be rejected? Are the white humans in western Europe hybrids because we carry a few percent of Neanderthal genes in our genome? When does a hybrid bird carry enough DNA from one parent species that one should ignore the content from the other and tick it?
Disclaimer: I have not seen the birds in this study and therefore only raise this out of curiosity.
Niels
Click to expand...
My personal opinion on this is that anything that's not an F1 hybrid should just be recorded as whatever species makes up the bulk of its ancestry. We know that hybridization plays an important role in evolution and sometimes speciation, and we know that species boundaries are not rigid, so I think that being too rigid ourselves in how we evaluate situations such as the Swedish Azure Tits described above is a misguided approach.

To me, the fact that an Azure Tit may have had one grandparent that was a Blue Tit does not make it any less of an Azure Tit than an Azure Tit whose four grandparents were all Azures. And most Azure Tits from the western part of their range will carry some degree of Blue Tit ancenstry anyway, so why stress out over it?
 
Lait, L.A., J. Enciso, T.T.M. Lekamlage, A. Veale, D.K. Abeyrama, and T.M. Burg (2024)
RADseq data reveal widespread historical introgression in four familiar North American songbirds
Evolution (advance online publication)
doi: 10.1093/evolut/qpae014

Population genetic structure is influenced by a combination of contemporary and historical events; however, this structure can be complicated by ongoing gene flow. While it is well known that contemporary hybridization occurs frequently among many closely related species, it often remains uncertain as to which populations are involved in introgression events, and this can be even more difficult to infer when introgression is historical. Here we use restriction-site associated DNA sequencing (RADseq) to look at the level of introgression among four species of songbirds in North America: the black-capped, mountain, boreal, and chestnut-backed chickadee. Samples from both sympatric and allopatric sites across the species’ ranges supported limited ongoing mixing among the four species with Bayesian clustering and PCA. In contrast, f4-statistics and admixture graphs revealed extensive historical introgression among geographically structured populations. Almost all historical admixture events were among populations west of the Rocky Mountains, and almost all populations west of the Rocky Mountains, excluding island and coastal populations, showed evidence of historical admixture. The inclusion of all four chickadee species proved crucial in differentiating which species were involved in hybridization events to avoid erroneous conclusions. Taken together, the results suggest a complex pattern of divergence with gene flow.
 
albertonykus said:
Taken together, the results suggest a complex pattern of divergence with gene flow.
Click to expand...
I have highlighted the last four words. Several years ago, this concept was considered hypothetical. What is the balance now, do most people actually accept that speciation can happen even in situations with gene flow?
Niels
 
njlarsen said:
I have highlighted the last four words. Several years ago, this concept was considered hypothetical. What is the balance now, do most people actually accept that speciation can happen even in situations with gene flow?
Niels
Click to expand...
I mean, I think there has been always acknowledgement that gene flow occurs between related populations? I think genetic analysis just allows us to trace it better.
 
Mysticete said:
I mean, I think there has been always acknowledgement that gene flow occurs between related populations? I think genetic analysis just allows us to trace it better.
Click to expand...
Most older speciation theories talked about differentiation without contact, still with possibly some hybridization and gene flow on re-contact. That sympatric populations could become species while they obviously at least early in the process will have a lot of gene flow is different, and that is what I understood from the wording that there was evidence for?
Niels
 
Semenov, G., H. Kenyon, E. Funk, W. Anderson, M. McQuillan, J. Spinelli, A. Russell, N. Martinez, A. Van Huynh, A. Alexander, R. Schweizer, E. Linck, Z. Cheviron, M. Carling, T. Roth, M. Robbins, A. Rice, and S. Taylor (2025)
Replicate geographic transects across a hybrid zone reveal parallelism and differences in the genetic architecture of reproductive isolation
Evolution Letters (advance online publication)
doi: 10.1093/evlett/qraf009

Determining the genetic architecture of traits involved in adaptation and speciation is one of the key components of understanding the evolutionary mechanisms behind biological diversification. Hybrid zones provide a unique opportunity to use genetic admixture to identify traits and loci contributing to partial reproductive barriers between taxa. Many studies have focused on the temporal dynamics of hybrid zones, but geographical variation in hybrid zones that span distinct ecological contexts has received less attention. We address this knowledge gap by analyzing hybridization and introgression between black-capped and Carolina chickadees in two geographically remote transects across their extensive hybrid zone, one located in eastern and one in central North America. Previous studies demonstrated that this hybrid zone is moving northward as a result of climate change but is staying consistently narrow due to selection against hybrids. In addition, the hybrid zone is moving ~5× slower in central North America compared to more eastern regions, reflecting continent-wide variation in the rate of climate change. We use whole genome sequencing of 259 individuals to assess whether variation in the rate of hybrid zone movement is reflected in patterns of hybridization and introgression, and which genes and genomic regions show consistently restricted introgression in distinct ecological contexts. Our results highlight substantial similarities between geographically remote transects and reveal large Z-linked chromosomal rearrangements that generate measurable differences in the degree of gene flow between transects. We further use simulations and analyses of climatic data to examine potential factors contributing to continental-scale nuances in selection pressures. We discuss our findings in the context of speciation mechanisms and the importance of sex chromosome inversions in chickadees and other species.
 

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