Barn swallow, Hirundo rustica
Barn swallows (Hirundo rustica) are one of only a handful of species that have co-evolved with humans but are not parasitic on us. This commensal evolution depended on the ecological niches we have provided during our own evolution and, in this case, as with House sparrows, particularly the social evolution of agriculture and settled dwellings on and in which to live. Barn swallows nest almost exclusively in man-made structures, hence their popular name.
Now researchers have tentatively established when the species took up residence with humans and when they radiated relatively rapidly into the six regional subspecies now recognised.
The study, reported in Molecular Ecology, suggests that the original divergence from other members of the Hirundo genus occurred about a million years ago but this was followed by a population bottleneck about 7,700 years ago, near the time that humans began building substantial structures, followed soon by radiation into localised subspecies. This contradicts earlier findings, based on a smaller data set and using a different technique that suggested swallows had already subspeciated prior to human settlements.
How have barn swallows, Hirundo rustica, diversified over the last 7,700 years or so, alongside human expansion? Barn swallows (Hirundo rustica) have undergone significant diversification over the last 7,700 years, influenced largely by their close association with human expansion and activity. Here are the key aspects of their diversification:This study by researchers from the Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado used Bayesian statistical techniques on the whole genomes of 168 individuals from the two subspecies that are farthest apart, H. r. savignii, a non-migratory species that lives along the Nile in Egypt, and H. r. erythrogaster from North America which over-winters in Central and South America.
In summary, barn swallows have diversified significantly over the last 7,700 years, largely due to their close association with humans. This relationship has facilitated their global spread, leading to genetic, behavioral, and morphological diversification as they adapted to various human-influenced environments.
- Habitat Adaptation:
- Shift to Human Structures: Barn swallows originally nested in natural sites such as caves and cliffs. However, with the advent of human-made structures, they adapted to nesting on buildings, bridges, and other constructions. This shift has allowed them to expand their range significantly.
- Global Expansion: The adaptation to nesting on human structures facilitated their spread across the globe, following human settlements and expanding into areas they might not have reached otherwise.
- Genetic Divergence:
- Population Structure: Studies have shown genetic differentiation among barn swallow populations from different geographical regions. This differentiation is partly due to the isolation of populations that adapted to specific local environments and human cultural practices.
- Subspecies Formation: There are several recognized subspecies of barn swallows, differentiated by physical characteristics and genetic markers. These subspecies have adapted to local conditions and may exhibit differences in plumage, size, and migratory behaviors.
- Behavioral Changes:
- Migratory Patterns: As barn swallows spread to new regions, their migratory patterns diversified. Populations in different parts of the world developed distinct migratory routes and timing, reflecting the varied climatic and environmental conditions they encountered.
- Feeding and Breeding: The abundance of insects around human habitations has likely influenced their feeding habits. Moreover, the availability of nesting sites on human structures has impacted their breeding cycles and success rates.
- Evolutionary Pressures:
- Natural Selection: Barn swallows face different selective pressures in varied environments, leading to evolutionary changes. For example, variations in climate, predation, and food availability have driven adaptations in their morphology and behavior.
- Sexual Selection: There is evidence of sexual selection influencing traits such as tail length and plumage color. These traits can vary between populations, contributing to their diversification.
- Human Influences:
- Agricultural Practices: Changes in agricultural practices have affected barn swallow populations by altering their food supply and nesting sites. For example, modern intensive farming can reduce insect populations, impacting swallow food resources.
- Urbanization: Urban environments offer new challenges and opportunities, influencing barn swallow behavior and adaptation. Some populations have thrived in urban areas, while others have struggled.
Barn swallows may indeed have evolved alongside barns, humansRegrettably, the copyright holder, John Wiley and Sons, will not allow reprinting the abstract to the paper, but it can be read here.
As humans evolved and expanded, so too did barn swallows, new research from CU Boulder suggests
The evolution of barn swallows, a bird ubiquitous to bridges and sheds around the world, might be even more closely tied to humans than previously thought, according to new study from the University of Colorado Boulder.
The research, published this week in Molecular Ecology, offers preliminary insight suggesting that the barn swallow and its subspecies evolved alongside—but independently from—humans. These new results make it one of the only known species, in addition to microscopic organisms like bacteria or viruses, to have developed in such a way, upending previous assumptions that barn swallows evolved prior to human settlement.
Humans could be a really big part of the story. There's very few studies that can point to the exact influence of humans, and so here, this coincidence of human expansion and permanent settlement and the expansion of a group that relies really, really heavily on humans is compelling.
Associate Professor Rebecca J. Safran, co-author
Associate professor of ecology and evolutionary biology
University of Colorado
Barn swallows are found across the northern hemisphere and are characterized by their mud-cup nests that are built nearly exclusively on human-made structures. Despite their prevalence, however, not much is known about their evolutionary history, the timing of their expansion from northern Africa (where they originated) or how the six subspecies evolved so physically and behaviorally different yet remain almost genetically identical.
Previous studies published in the Proceedings of the Royal Society of London and Molecular Phylogenetics and Evolution looked into these questions and found that the different subspecies split early, well before human settlement.
This new study, however, gave the topic a fresh look by examining the whole genome of 168 barn swallows from the two sub-species farthest apart on an evolutionary scale: H. r. savignii in Egypt (a non-migratory species that lives along the Nile) and H. r. erythrogaster in North America (a species found throughout North America that migrates seasonally to South America).
These data—which are of the order of 100,000 times bigger than the previous dataset used—were then analyzed with more sophisticated computational resources and methods than previously available. This allowed researchers to get a more complete picture that places the timing of barn swallow differentiation or speciation (i.e., when the barn swallow subspecies separated) closer to that of when humans began to build structures and settlements.
The previous studies were playing with the idea of potential impact on population sizes due to humans. Our results suggest a much more substantial link with humans.
Chris C.R. Smith, lead author
Department of Ecology and Evolutionary Biology
University of Colorado, Boulder, Colorado.
Barn swallow subspecies are found throughout the northern hemisphere.Illustrations courtesy of Hilary Burn, and map courtesy of the Safran lab
These new preliminary findings also suggest that this evolutionary link may have been forged through a “founder event,” which is when a small number of individuals in a species take over a new environment and are able to expand their new population there thanks to an availability of resources and an absence of competitors. For barn swallows, this event may have occurred rapidly when they moved into a new, relatively empty environment: alongside humans.
Everyone is always wondering how do you study speciation? It’s been viewed as this long-term, million-year (process), but in barn swallows, we are not talking about differentiation within several thousands of years. Things are really unfolding rather rapidly.
Associate Professor Rebecca J. SafranIt’s interesting to study speciation in the beginning steps.
Chris C.R. Smith
The likely explanation for the population bottleneck seen some 7,700 years ago is that this was a small founder population that adapted to living alongside early human settlements. This founder population then increased and diversified as settled agriculture with permanent structures spread geographically, giving relatively isolated breeding populations subject to local environmental pressures and/or genetic drift within those populations.
Here then we have a genetic evolutionary explanation for the current diversity within the species Hirundo rustica with genetic changes mapping onto known historical changes in their environment, once they adopted a commensal existence with humans, and evidence of a small founder population making that initial adaptive change. In fact, in just 7,700 years following this founder event, the species had diversified part way to full speciation. Some of the subspecies interbreed naturally when they come into contact, but others do not, due to already established behavioural and geographical barriers to hybridization, yet the genetic diversity across the whole species is still relatively small.
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