Generally, a species is considered the largest group of organisms, where if they reproduce, they produce fertile offspring. An often used example is the breeding of a horse and a donkey, produce a mule, which is non-fertile, proving the two parents are different species. That rule doesn’t apply to all though, which we can see if we look at the Genus of warbler Vermivora, containing the Blue-winged Warbler and the Golden-winged Warbler. This will focus on those two species and their hybrids.
Both of these species can be found in the eastern half of North America, down into Central and South America during the non-breeding season. These two species are actually very closely related, studies from the Cornell Lab of Ornithology’s Fuller Evolutionary Biology Program actually found that the Blue-winged Warbler and the Golden-winged Warbler actually are 99.97 percent genetically alike. And as Cornell states “The research also shows that golden-wings and blue-wings have even less genetic differentiation than two subspecies of the Swainson’s Thrush, the Olive-back and Russet-backed forms.” Gustave Alexson wrote a paper on the topic and states that the difference in the species “could be akin to the difference between humans with and without freckles” Which I find absolutely fascinating.
While these two species are genetically similar, they are very different in appearance and have slightly different songs.
The Blue-winged warbler is mostly yellow, with a black line through their eye, blueish-gray wings, and two white wing bars. They can be found in scrubby habitats and field edges. Their song can be said to sound like Bee Buzzzz and is very buzzy sounding.
The Golden-winged Warbler is overall grayish, with it being a darker gray on its back and wings, and more white on the belly, a bold black facial mask outlined in white, black bib under its beak, a yellow crown and bold yellow wing patches. It can be found in shrubby habitats near the edge of taller forests, preferably close to water. Their song is very similar to the Blue-winged Warbler with a few extra notes, a buzzy sounding Bee Buzz Buzz Buzz. And is one of the fastest declining songbird species in North America, this species is also even a candidate for a federal Endangered Species listing.
There was even a study done by Cornell Lab postdoctoral researchers Scott Taylor and David Toews who collaborated with partners from Cornell Universities Department of Biological Statistics and Computational Biology, the University of California at Riverside, and Environment and Climate Change Canada that proved that the difference in these two species was really only affecting their outward appearance. After analyzing the entire genome of both the species, only six regions, which consisted of the .03 different found genetically diferent between the species, of their genomes really showed a major difference. Four of these genetic differences are displayed in their plumage.
Since these two species are so genetically similar, can share the same habitat and have very similar songs, it’s not too surprising to hear that these two species can hybridize.
These two species do produce some interesting hybrids, and depending on the parentage and backcrossing, can lead to a wide variety of different plumages.
These two hybrids are called either a Brewster’s Warbler or a Lawrence’s Warbler.
The Brewster’s warbler was first identified in 1874, thinking it was a new species, and was given the scientific name of Vermivora leucobronchialis. It wasn’t until later that is was correctly identified as a hybrid from the Blue-winged Warbler and the Golden-winged Warbler. This is the more common of the hybrids. It has the gray body and yellow crown of the Golden-winged Warbler. It also has a thin black eyeline, inherited from the Blue-winged warbler. They also have two wing bars that will typically be a light yellow, an in between of the two parent species. The Brewster’s warbler may also have a yellowish wash in its breast. Like many warbler species, the females tend to be paler.
The other hybrid of the Blue-winged Warbler and the Golden-winged Warbler is the Lawrence’s warbler. Like the Brewster’s Warbler, it was once thought to be its own species with the scientific name of Vermivora lawrencei, but around 1880, it was concluded it was a hybrid. This hybrid is much rarer than the Brewster. They have the same yellow body and gray wings of the Blue-winged Warbler, with the black mask and bib of the Golden-winged Warbler.
There can also be a wide range of traits, with some being more or less prominent as they continue hybridizing or back crossing with their parent species, leading to some very interesting looking birds.
An interesting thing to also note about these hybrids is that they can sing the song of either parent species, so it is always recommended if you hear either the song of the Blue-wing or Golden-Winged warbler to find the bird and confirm its species.
A proposal by John T. Nichols from the American Museum of Natural History was that the Brewster’s Warbler was displaying the dominant traits of the two species, and the Lawrence’s Warbler was a display of the recessive traits. So when the pure Golden-winged Warbler and a pure Blue-winged Warbler reproduce, they will produce offspring that contain both species dominant and recessive traits but will only display the dominant traits, resulting in a Brewster’s Warbler. But if these hybrids again reproduce, there is a chance that an offspring inherits just the recessive traits from the parents, resulting in a Lawrence’s Warbler. This explains why the Brewster’s warbler is much more common than the Lawrence’s, as it doesn’t require multiple instances of hybridization.
One of these different genetic regions that were studied in the previously mentioned study was the gene that controlled the throat coloration. The black throat back on the Golden-winged Warbler is recessive, so it will only be seen in individuals that possess a pair of the recessive alleles for that trait. Leading it to only being seen on Lawrence’s Warbler.
Another of these studied genetic regions controls the color of the body. Here, the Blue-winged Warbler’s yellow body is the recessive trait. Again, resulting in this coloration only being seen on the Lawrence’s Warbler.
While is is believed that these two species have been hybridizing occasionally in the past, hybridization is believed to have greatly increased since the 1700’s when the European settlers started clearing forests, this change in the environment is believed to remove the habitat barrier and caused these two species to begin to cohabitat the same areas more often.
Though this hybridization and how close these species are genetically is fascinating, it leads to some problems in trying to preserve these species. As said earlier, the Blue-winged Warbler is a pretty common warbler in its range, though on the other hand, the Golden-wing is pretty rare, being a candidate for federal endangered listing. This hybridization can potentially muddle the species genetically together. I do wonder if the rate of hybrids being produced will increase as Golden-wings, whose population is decreasing, find it easier to find a mate of a Blue-winged Warbler, whose range is expanding, rather than a Golden-winged. As where I am currently, hybrids are pretty rare.
While hybridization isn’t helping the lower population of Golden-winged Warblers, other threats are the main cause of their decreasing population, primarily the loss of young shrubby forests.
It seems like a general consensus by many experts that this hybridization is completely normal, these two species have been hybridizing for possibly thousands of years and have formed a version of an evolving species complex. These hybrids and the process of this hybridization may even have evolutionary benefits, being able to adapt to environmental changes better. Only time will tell how this progresses and what more is discovered about these two species and their hybrids.
References/Further Reading