The Bell’s Sparrow (Artemisiospiza belli) is a small songbird found in the southwestern United States and northwestern Mexico. It is a fairly nondescript bird with a gray face and underparts and brown streaked upperparts. The Bell’s Sparrow was once considered to be a single species, but has recently been split into two separate species based on differences in genetics, vocalizations, and plumage between populations on either side of the Colorado River. The split was proposed in 2017 and has been adopted by most major ornithological organizations.
What are the two species that the Bell’s Sparrow has been split into?
The Bell’s Sparrow has been split into the Eastern Bell’s Sparrow (Artemisiospiza belli) which occurs east of the Colorado River, and the Western Bell’s Sparrow (Artemisiospiza nevadensis) which occurs west of the Colorado River.
The ranges of the two Bell’s Sparrow species are as follows:
Eastern Bell’s Sparrow
The Eastern Bell’s Sparrow is found from southern California and southern Nevada, east through Arizona, New Mexico, western Texas, and northern Mexico. Its range stops at the Colorado River, which acts as a geographic barrier between it and the Western Bell’s Sparrow.
Western Bell’s Sparrow
The Western Bell’s Sparrow occurs west of the Colorado River in southeastern California, southern Nevada, and parts of western Arizona. Its range is centered on the Mojave and Sonoran desert regions.
What are the main differences between the Eastern and Western Bell’s Sparrows?
While the two Bell’s Sparrows appear very similar, there are some key differences:
Genetic differences
Studies of the mitochondrial DNA and nuclear DNA show that the Eastern and Western populations have been genetically separated for over 1 million years. This long period of isolation has allowed them to diverge into separate species.
Vocalizations
The songs and calls of the Eastern and Western Bell’s Sparrows are noticeably different. The Eastern Bell’s has a more complex and melodious song, while the Western has a simpler, more repetitive song. The calls of the two species are also distinctive.
Plumage
Subtle differences in plumage exist between the two sparrows. The Eastern Bell’s is a more grayish color on the undersides, while the Western is buffier. The facial markings of the Western Bell’s are also crisper and more defined.
Trait | Eastern Bell’s Sparrow | Western Bell’s Sparrow |
---|---|---|
Range | East of Colorado River | West of Colorado River |
Song | More complex and melodious | Simpler and more repetitive |
Plumage | Grayish undersides | Buffier undersides |
What was the reasoning behind the split?
Ornithologists proposed splitting the Bell’s Sparrow into two species based on several key factors:
Reproductive isolation
The Colorado River acts as an impassable barrier between the Eastern and Western populations, preventing interbreeding. This reproductive isolation is a major criterion for separating species.
Genetic divergence
As mentioned earlier, mitochondrial and nuclear DNA studies show that the two populations have been evolving independently for over 1 million years – long enough to become distinct species.
Vocal and plumage differences
The differences in vocalizations and subtle plumage distinctions also provide evidence that these are discrete populations in the process of speciation. Song and plumage are important species-recognition cues in birds.
Treatment as separate species by experts
Well before the split was formalized, many ornithologists treated the Eastern and Western birds as separate species based on the evidence. The split recognizes and standardizes this expert taxonomic treatment.
When did the split take place?
The proposal to split the Bell’s Sparrow into two species was published in a 2017 paper in the journal The Auk: Ornithological Advances. The American Ornithological Society’s North American Classification Committee officially adopted the split in 2018. Other major authorities, such as the International Ornithologists’ Union (IOC), recognized the split by 2019.
Here is a timeline of the split:
2017
– Paper published proposing species split
2018
– American Ornithological Society’s North American Classification Committee accepts proposal
2019
– IOC recognizes Eastern and Western Bell’s Sparrows as separate species
So although the Bell’s Sparrow was considered one species for a long time, recent evidence led ornithologists to conclusively split them into two distinct species just within the last few years.
How has the split affected bird lists and conservation?
The split has led to some changes in terms of bird lists and conservation:
Bird lists
Many birders maintain life lists of all the species they’ve observed. The split means that birders who had previously seen just “Bell’s Sparrow” now need to retroactively classify their observation as either Eastern or Western Bell’s Sparrow based on the location. This can affect life list totals.
State/province lists
The split may lead to changes in state and provincial bird lists. For example, California now recognizes Western Bell’s Sparrow but not Eastern Bell’s Sparrow on its list.
Partners in Flight
This bird conservation organization had to create separate assessment profiles for the two Bell’s Sparrow species after the split. Each needs its own targeted conservation plan.
Legal protections
The Endangered Species Act protects Bell’s Sparrows in parts of their range. With the split, protections now apply specifically to only one species in a given area. Conservation status needs to re-evaluated.
So in many ways, the split has created a need to re-organize ornithological taxonomy, bird lists, and conservation efforts regarding what was formerly a single species.
Conclusion
The story of the Bell’s Sparrow split illustrates how ornithologists are constantly re-evaluating bird taxonomy as new evidence comes to light. Advances in genetics, vocal analysis, and intensive field observation led to the realization that the classic Bell’s Sparrow actually consisted of two cryptic species evolving independently in geographic isolation for over a million years. While this taxonomic change led to some havoc for bird lists and conservation plans, it ultimately allows us to gain a more detailed and accurate understanding of avian diversity. Going forward, future splits will undoubtedly continue to improve our knowledge of the natural world.