Black-and-White Warbler

 

Black-and-White Warbler (1)

Spring is here and so are the warblers! Most of these tiny, colorful birds spent the winter far to the south, in the Carribean, Mexico, Central, or South America and are now flying back to their breeding grounds in North America. Some will just be passing through on their way to their final destination farther north, while others will stay for the whole summer, defending a territory, pairing off, and raising young. Whatever their final destination, there are dozens of species passing through Connecticut right now and I am doing my best to enjoy every minute of it!

Of all the warbler species that return north each spring, the one that I am always most excited about is the black-and-white warbler (Mniotilta varia). As their name implies, black-and-white warblers are not quite as colorful as some of their relatives, but their streaks of black, blue-grey, and white feathers are visually striking nonetheless. They have a fairly distinctive song that sounds a little like somebody rubbing a wet finger on a glass and they're usually pretty easy to spot compared to other warblers that spend more time in the canopy. As a result, black-and-white warblers are often some of the first warbler species that new birders learn. 

All these features contribute to my love of the black-and-white warbler (especially the last one - craning my neck and squinting into the sun can get a little old!), but the main reason I find them so interesting has more to do with their ecology than their appearance. Black-and-white warblers are an excellent example of something called convergent evolution. Convergent evolution occurs when two organisms that are not closely related but live under similar environmental conditions develop similar characteristics to deal with those conditions. A classic example of this phenomenon is the similarity between sharks and dolphins. Both sharks and dolphins are large, aquatic predators and they have developed similar body shapes, fins, and color patterns that allow them to be affective in that niche. And yet, despite their similarities, these two groups are not closely related - dolphins are mammals and sharks are fish, two groups separated by millions of years of evolution. Their similarities are the result of adaptive responses to similar conditions, not being related to each other.

White-Breasted Nuthatch (2)

You don't need to visit the ocean to see convergent evolution in action, however. If you live in the eastern United States, you can just go for a little walk in the woods on a nice spring day and keep your eyes open for two common birds. The first of these, of course, is the black-and-white warbler. Like many birds, black-and-white warblers feed mostly on insects. Unlike other birds, however, they do not forage for them in the canopy, on the forest floor, or in the open air; rather, they look for insects under the bark of trees. To do this, they crawl vertically up and down tree trunks, using their beaks to probe crevices under the bark for grubs and caterpillars. This behavior is very much like that of the white-breasted nuthatch (Sitta carolinensis), another common songbird which has developed some similar traits to the black-and-white warbler, including a relatively flat, streamline body, a short neck and tail, and long claws on their hind toes, which help both species to perch and move vertically on the trunks of trees. 

And yet, despite their behavioral and physiological similarities, we know from genetic testing that black-and-white warblers and white-breasted nuthatches are not very closely related - a 2019 analysis of the evolutionary relationships between songbirds by Oliveros et al. estimated that the last common ancestor between wood warblers (Family: Parulidae) and nuthatches (Family: Sittidae) lived in the Oligocene, a little over 25 million years ago! Indeed, a closer look at the two species will reveal a few important differences in addition to their superficial similarities, such the nuthatch's distinct, upturned bill and the fact that the black-and-white warbler nests on the ground, while the nuthatch nests in tree cavities. Thus, it is clear that this is a case of convergent evolution, one easily observable by anyone who has ever had to do a double take in order to figure out whether they are looking at a nuthatch or a warbler. Such moments provide a rare, if indirect glimpse into the unfathomable expanse of evolutionary time, a scale at which two groups of birds can diverge and travel along very different evolutionary paths for hundreds of thousands of generations before meeting again, if only for a momentary millennium, on the trunk of tree. 

Other Warblers and Convergent Evolution:

Ovenbird (3)

The wood warblers are a very diverse group of birds that have adapted to occupy many different habitats and exploit many different niches. As a result, convergent evolution between warblers and other birds is actually pretty common. In addition to the black-and-white warbler, another good example of this pattern is the ovenbird (Seiurus aurocapilla). Ovenbirds look and behave very similarly to the spotted thrushes in the genus Catharus. Like the thrushes, ovenbirds live in interior forests and spend most of their time on the ground or in low branches. In order to blend into the leave litter and speckled light of the forest floor, they have olive-brown feathers on their backs and white bellies with dark spotted bellies, making them look a whole like thrushes and leading to frequent misidentifications. 

Hermit Thrush (4)

The ovenbird is a warbler, however, and according again to Oliveros et al., their last common ancestor with the thrushes (Family: Turdidae) lived millions of years ago in the Oligocene. The ovenbird's is another case of convergent evolution, this time playing out in the shadows of the forest floor, where solid brown, rufus, or olive back feathers and streaked or speckled, white stomachs provide excellent camouflage for thrushes and ovenbirds alike. Adding even more to the confusion are the water thrushes in the genus Parkesia which, despite their common name, are also warblers. They were called water thrushes because they too look somewhat similar to the spotted thrushes, but instead of foraging on the forest floor, they hunt for fish, amphibians, and aquatic invertebrates near and in bodies of water. I guess this is why my botanist friends always just call things by their scientific names, huh?

Sources:

Oliveros, C. H., Field, D. J., Ksepka, D. T., Barker, F. K., Aleixo, A., Andersen, M. J., ... & Faircloth, B. C. (2019). Earth history and the passerine superradiation. Proceedings of the National Academy of Sciences, 116(16), 7916-7925.

Peterson, R. T. (2020). Peterson field guide to birds of eastern and central North America (7th edition). Houghton Mifflin Harcourt.

The Cornell Lab of Ornithology (2023). Black-and-white warbler. All About Birds. https://www.allaboutbirds.org/guide/Black-and-white_Warbler  

The Cornell Lab of Ornithology (2023) White-breasted nuthatch. All About Birds. https://www.allaboutbirds.org/guide/White-breasted_Nuthatch

The Cornell Lab of Ornithology (2023) Ovenbird. All About Birds. https://www.allaboutbirds.org/guide/Ovenbird

Photo Credits:

(1) "Black-and-white-warbler" by Dennis Church CC BY-NC-ND 2.0

(2) "White-breasted nuthatch" by Kelly Colgan Azar CC BY-ND 2.0

(3) "Ovenbird (90497)" by Rhododendrites CC BY-SA 4.0

(4) "Hermit thrush in GWC" by Rhododendrites CC BY-SA 4.0