The eastern bluebird (Sialia sialis) has lived in Bermuda as long as recent human memory can recall. It's considered a native species, and some people even consider the population to be a subspecies--the Bermuda bluebird (Sialia sialis bermudensis)--because it looks a bit different from its mainland counterparts: its blue is a little more purple, and its orange is a bit more "cinnamon," according to a 1901 account by zoologist (and science fiction writer) Alpheus Hyatt Verrill (Volume V, Number 6).
However, the idea that these birds are native to the island is reliant upon a rather unreliable source: human observation. A new genetic study, published this month in Molecular Ecology, shows that bluebirds are not native to Bermuda, but rather put down roots on the island after European settlers altered the landscape just 400 years ago.
Before the British settled on Bermuda in 1609, there is no evidence of any people having lived there. But the newcomers quickly left their mark. Within just a few decades, they cut down stands of the now critically endangered Bermuda cedar (Juniperus bermudiana) and other native trees, and released many non-native plants and animals, including cats, rats and pigs. The rapid habitat change and introduction of new predators led to the rapid extinction of many native animals soon thereafter.
Today, only three songbird species that are considered native live in Bermuda: the bluebird, the gray catbird, and the white-eyed vireo, all of which are common on the East Coast of the United States. But there are no fossils of bluebirds on the island going back some 400,000 years, and no breeding populations on other nearby islands. This made a group of Rutgers researchers wonder: is the bluebird truly a native species to Bermuda, or did it move in after people arrived and was improperly recorded?
To measure how closely related the Bermudan birds are to their mainland counterparts, the researchers compared 12 gene sequences among eastern bluebirds living in Bermuda, along the US East coast, and in the midwest. If the Bermuda bluebirds had been living isolated for thousands of years, they expected to see many genes unique to those birds.
However, they only found two unique genes across all the Bermudan bluebirds they studied: solid evidence that the population settled on the island quite recently. Additionally, the Bermudan birds had many fewer total gene variants in their population compared to those on the continent--in other words, if the species's DNA were a sentence, the Bermudan birds' had a smaller vocabulary. The small genetic vocabulary suggests that a small number of birds (around 50) founded the population, and that these birds aren't breeding with those on the mainland.
So instead of being a native species, as commonly thought, a smallish group of bluebirds likely settled on the island some 400 years ago and the modern population descended from those birds. It's likely that the changes human settlers made to the landscape opened up pockets of habitat that were previously unavailable.
In that case, the more brightly-hued Bermudan bluebirds must have evolved their new coloration in just a few hundred years--far more quickly than evolution typically occurs! And even more puzzling is that the physical changes are not reflected in their genes. It's likely that the evolution in coloration was not adaptive but accidental: a byproduct of what genes remained in the population when the small group was isolated. If by chance the 50 founding birds had more "dark blue" genes than found in the larger population, this isolated group would end up being darker, whether or not it helped them survive in their new habitat.
Whether the bluebirds are native or invasive isn't the real question at hand, but whether they are unique enough in the world that they deserve extra conservation protection. They are not unique from other bluebirds, according to these new findings, and if a conservation organization, for example, is trying to direct funding to preserve unique species, these bluebirds shouldn't be major priorities.
It also highlights that genetics is a useful and necessary tool in piecing together the history of organisms and ecosystems. Other subspecies, assumed to be separate based on isolation and physical differences, should be genetically analyzed to ensure that they are truly native and unique.
One of the major values of the paper in my eyes is that it forces us to challenge our assumptions. The accounts of 18th and 19th century zoologists shouldn't be held sacred as the true definition of a unique native species. People now have tools to measure what lived here before we showed up, and those tools should be used to piece together prior landscapes and ecosystems above the word of explorers using their ever-faulty human senses.
Avery J.D., Fonseca D.M., Campagne P. & Lockwood J.L. (2013). Cryptic introductions and the interpretation of island biodiversity, Molecular Ecology, 22 (8) 2313-2324. DOI: 10.1111/mec.12236