The Science of Birds - New World Warblers

Episode Date: May 12, 2021

New World Warblers are all the birds in the family Parulidae. You’ll sometimes hear these birds referred to as ‘Wood-Warblers.’New World Warblers are among the most celebrated and sought after birds in North America. We love them here. They’re colorful, they’re energetic, and many of them have pleasant little songs.These birds have also been the subjects of many scientific studies over the years. Which is great, because it means there’s a lot that ornithologists know about the biology of these magnificent little creatures.{Special thanks to Sue Riffe for permission to use her Kirtland's Warbler recording}~~ Leave me a review using Podchaser ~~Links of InterestHow to Avoid Getting Warbler NeckLink to this episode on the Science of Birds websiteSupport the show

There's a special place called Magee Marsh in Ohio where warblers and the humans who love them converge every spring. This site is a 2,000-acre complex of wetlands and forest on the southern shore of Lake Erie. In late April and early May, up to 37 New World Warbler species pass through McGee Marsh as they migrate north into the boreal forests of Canada. These birds have come all the way from Mexico, Central America, and South America. Crossing Lake Erie is no small feat for a tiny bird, so the warblers often stop to rest and refuel at McGee Marsh. Some will stick around to nest here, but most of them will continue northward over the lake.

Every year, hundreds, and even thousands of birders come from all over North America and all over the world, actually, to visit McGee Marsh during peak. Warbler awesomeness. Birders and photographers crowd onto the wooden boardwalks to get close-up looks at colorful birds like blackthroated blue warbler, bay-breasted warbler, prothonatory warbler, Canada warbler, chestnut-sided warbler, and a whole mess of other species. Sometimes the crowd of birders packs in so tightly on the creaking boardwalk that people are elbowing each other and jostling around to get a good look or a photo when a warbler

hops on to the scene. It's often possible here to see these little birds at eye level and close up, which is a rare experience elsewhere. The gathering at Magee Marsh is basically like Woodstock or Lollapalooza for birders. People actually call it warbler stock. So when a gorgeous warbler lands near the boardwalk, everyone gets worked into a frenzy. A mosh pit starts up and basically all hell breaks loose. Crowdsurfing, fist fights, beer bottles, and giant camera lenses getting smashed on people's heads, lots of yelling and screaming. Well, that's what I imagine happens anyway. I don't actually know since I haven't been to McGee Marsh myself.

I really, really want to go, though. It sounds like a rockin' good time. Hello and welcome. This is the Science of Birds. I am your host, Ivan Philipson. The Science of Birds podcast is a lighthearted, guided exploration of bird biology for lifelong learners. This episode is all about New World Warblers. These are all the birds in the family Paroolady.

You'll sometimes hear these birds referred to as wood warblers. Like, for instance, in the field guide I use from my region, Sibley Birds West. New World Warblers are among the most celebrated and sought-after birds in North America. We love them here. They're colorful, they're energetic, and many of them have pleasant little songs. These birds have also been the subjects of many scientific studies over the years, which is great because it means there's a lot that ornithologists know about the biology of these magnificent little creatures. And this means there's a lot for us to say about them, so let's dig in and get up to our elbows in warbler lore.

Let's start by looking at the general features of New World Warblers. The body plan, the shape and size of birds in this group, is remarkably uniform across all the species. They're small and compact. Their various body parts tend to fall in, to the medium category, medium-length beaks and tails, medium wingspans, medium leg length, medium head size, and so on. Among these small birds, the smallest is probably Lucy's warbler of the desert southwest. This species is only 11 centimeters, or 4.33 inches from

beak to tail. As for the biggest New World Warbler, well, not that long ago, the undisputed giant in this family would have been the yellow-breasted chat. That species is 18 centimeters or a bit over seven inches long. In other words, friggin' huge! But the yellow-breasted chat was always an oddball among New World Warblers, showing several features atypical for the family. And, sure enough, genetic data eventually helped ornithologists figure out that the yellow-breasted chat is not at all a member of the family, Parulity. In 2017, this bird was given its own family, Ictereidae. It's a family of one, just a single species.

As for the New World Warblers that aren't impostors, the largest species in the family Paruladi is the oven bird, at 16.5 centimeters long. That's about 6.5 inches. The bills of all these warblers are typically slender, straight, pointy, and of short or medium length. This bill shape works well for snatching up small insect prey. The plumage colors and patterns of New World Warblers are pretty awesome.

If you quickly flip through illustrations of these species in a field guide or a bird app, you should be left with an impression of lots of vibrant yellow hues, as well as bluish-gray, white, and black. And some species sport patches of bright red or orange feathers. The plumages of these warblers are so different among species, but at the same time, their body architecture is very similar. It's like when a greedy toy company decides to make an easy buck by producing a new plastic action figure using the exact same mold as a figure already on the market. They just slap some different paint colors on the new figure and

give it a catchy name and there you go. Why, that's not Captain America. That's his new pal, Lieutenant Freedom. Collect them all, kids. Sort of like with action figures, the desire to collect them all is strong among birders with respect to warblers. Plumages in New World Warblers are similar between the sexes, but males of many species are more brightly colored. That's especially true for species that migrate north to breed. Species that are residents of the tropics, on the other hand, tend to have very similar-looking males and females. So, northern migrants show sexual dimorphism, whereas tropical residents, not so much. You know, warblers aside, the avian fauna of North America isn't really all that colorful,

as a group, at least not when compared to the birds of, say, South America, Australia, or Southeast Asia. So many people here in North America sort of go gaga over any of our bird species that show some flashy plumage. I'm not ashamed to admit. that I am one such person. If New World Warblers weren't so colorful, birders and amateur naturalists probably wouldn't get as excited about them. That's sad, but probably true, don't you think?

Luckily, our warblers are colorful. This wonderful group of birds brightens up our forests, shrublands, and wetlands in spring and summer. When you're lucky enough to have several warbler species flitting around in your view, it's easy to feel like a kid in a candy store. It's hard to decide which is your favorite, and you just want all of them.

But then we have the orange-crowned warbler, Leothlipis Salada. This is one of the most commonly observed warbler species in the U.S. Don't let the name fool you, though. This is a dull olive-grayish, or sort of dusky yellow bird, and you'll almost never see that orange crown. Those orange feathers are rarely visible. The orange-crowned warbler is kind of like the nose. no-nonsense generic brand version of a New World Warbler. It's an excellent little bird,

don't get me wrong. I'm always happy to see or hear an orange-crown warbler. But it's just about the least colorful member of the family Perulity. I suppose somebody had to have that humble distinction. However, the song of this little bird is pretty groovy. Here's the pleasant, descending trill of an orange-crowned warbler. There's a lot of variation in the song from one male orange-crowned warbler to another, so these guys can often be identified individually by their songs. Now that we're talking about warbler vocalizations, let's keep talking about them, and let's listen to a few. Warbler songs are generally high-pitched, simple, and pleasant sounding.

Some species sing a song that's mostly buzzy, while others use clear-toned notes. So I wouldn't say that warblers really warble all that much. Here's an example of a buzzy warbler song. This is the Northern Perula, Setefaga Americana, singing in the woods of New Hampshire. The Northern Perula is, like so many members of its family, a gorgeous little bird.

The male has a deep blue-gray head and wings. A partial white eye ring highlights his eyes. His throat and breast are bright yellow, separated by a band of black and reddish-brown. As another example of a species with a more buzzy song, I give you Townsend's warbler, Cetaphaga Townsendye. This is a bird I hear in my backyard.

It's a handsome species with bold, black and yellow markings on its head and breast. Here's another species. This is a blue-winged warbler in Massachusetts. The oven bird, Seyurus Aurocapilla, provides a great example of a bird with a clear-toned song. people often describe that song as sounding like teacher teacher teacher the oven bird is the largest new world warbler species as you may recall it looks to me almost like a thrush it's brownish olive on the head and back and has a white underside which is decorated with bold stripes and spots it also has white eye rings and a pair of distinct dark stripes on its crown. This next one is one of my favorite warbler songs. The song of the common yellow throat, geothlipis trichus, is often described as wichity, wichity, wichity. Here it is.

Witchety, wichity, wichity indeed. Now, I know what you're thinking. Wichity, that's the name of those giant wood-eating moth larvae in the deserts of Ossetty. Australia. Witchety grubs, am I right? Indeed, witchety grubs are the hot dog-sized white caterpillars that some Aboriginal Australians snack on while in the outback. I hear the grubs taste like almonds. Delicious, I'm sure, but why in the world would the common yellow throat sing the praises of a massive larval insect that lives on the other side of the planet? Who knows? Who knows? In terms of the general features of New World Warblers, we've covered their size and shape, their bills and plumage colors, and their voices.

In the behavior department, these birds are notoriously hyperactive. They seem to be in continual motion, bouncing from twig to twig and flitting around all over the place. This means it's hard to get a good, long look at a warbler. By the time you focus your binoculars on one, it'll probably zip off in a random direction, and you might not find it again. Warblers can be especially hard to zero in on when they're high up in trees, and that's often where they are, since most of them forage for insects in trees. They're arboreal insectivores.

The last outstanding feature of this family I want to mention here is their migratory behavior. About half of all new world warbler species are long-distance. migrants. One of the most joyful natural events during spring in North America is the return of our migrant warblers. That's why events like the festival at McGee Marsh are so popular. Wave after wave of neotropical migrant warblers arrive in the U.S. and Canada starting in April. Some distances these small birds fly every year are astounding. We'll come back to talk a bit more about warbler migration later. Now let's have a look at the diversity, distribution, and habitats of New World Warblers.

The biological family, Paruleti, is nested within the order Paceriformes. You may recall that Paceriformes is the order that includes all of the so-called songbirds. This is a wildly successful and diverse group. More than half of the world's almost 11,000 species are songbirds, or what we call passerines. So Paceriformis is by far the largest order of birds. So New World Warblers are passerines of the family Parulity. This family includes 108 species, according to the Clements checklist, which is what

eBird is based on. The origin of the family's name comes from a case of mistaken identity. Carl Linnaeus, the Swedish dude who started this whole binomial nomenclature thing, our system of standardized scientific names, well, in 1758, old Carl got his hands on a specimen of the northern Perula, one of our New World Warblers. Linnaeus thought this bird was a kind of tit, so he assigned it the name Paris Americanus.

Paris, P-A-R-U-S, is the genus for the great tit of Europe. and a few other species. The family name for our warblers, Perulidi, was derived from this erroneously applied genus name, erroneous because the northern Perula is not a tit, not even close. This species was reclassified and now goes by Setaferga Americana. So we've got 108 warbler species in this family,

and the species are sorted into 18 genera. There are also two species that are quite possibly extinct. We'll circle back to them when we talk about conservation. The most diverse genus is setaphaega, which has 34 species. The next most species-rich genus is Mayothlipus, with 15. The latter group of birds are found only in South America, so we don't see them up here in the north. Geographically, species diversity in this family is

concentrated in eastern North America, the West Indies, Mexico, Central America, and the Andes of South America. The United States has 55 warbler species. Canada has 45 and Mexico has 61. Obviously, there is a fair amount of species overlap between these countries. Not all of these species have the word warbler in their common name. We've already talked a bit about the oven bird, the common yellow throat, and the northern perula. We also have red starts and water thrushes, but they're all perulids, members of the family perulity. So we can safely call these birds new world warblers without having to worry that someone is going to call the cops on us. This whole time I've been saying new world warblers, right? We have to make

this distinction because there are birds in the old world that people also call warblers. None of those birds, however, are closely related to parolids. A bunch of old-world families include birds with the word warbler in their name. There are at least seven such families, including the African warbler family and the Australian warbler family. So just keep in mind that birds belonging to different families are as different from each other as a cat is from a dog. African warblers, for example, aren't closely related to living.

leaf warblers or to bush warblers, and none of these is closely related to our New World Warblers. Such is the eternal challenge of making sense of common names, so it goes. As for who is closely related to New World Warblers, genetic data and analyses point to birds in the family Icturidae. These are the New World Blackbirds, the metal larks, orioles, cowbirds, and so on. The top three warbler species observed most frequently in the U.S. are the yellow-rumped warbler, the yellow warbler, and the common yellow throat. Apparently, having the word yellow in your name is a prerequisite for getting into the top three slots. So the next time someone asks you,

Hey, what species in the family, Perulidae is most frequently encountered in the United States? You can fake your way to the answer by just mumbling, I don't know. Yellow, yellow, something, yellow, something, yellow something, warbler? For the record, the top three most commonly observed species in South America are Yellow Warbler, Blackburnian Warbler, and Tropical Perula. Here's the sound of a tropical perula singing in a Brazilian forest. I believe the tropical perula, Satafaga Pitya Yumae, gets further south than any other species in this family,

down to a latitude of about 40 degrees south in Argentina. The other end of its distribution reaches just north of Mexico. The only place you're likely to see a tropical perula in the U.S. is in southern Texas. More than half of the 108 perulid warbler species are migrants that breed way up in the boreal forests of Canada and Alaska. summer in the boreal forest brings an explosion of insects that the birds feast on. And the days in those northern regions are super long in summer, allowing extra time foraging. Boreal forests provide such important breeding habitats for migrant warblers and other birds

that we refer to them as North America's bird nursery. Besides all these migrant warblers, there are other species that are relatively sedentary. Most of these live year-round as residents in either Mexico, Central America, or South America. Wherever they happen to live or breed, New World Warblers occupy a range of habitats, mostly of the forest or woodland variety. A handful of warbler species forage on the ground, but most are creatures of the trees. Some species have limited ranges because of their very specialized habitats. Kurtland's Warbler, for example, nests only in young,

jack pine forests. The yellow rumped warbler is at the other end of the spectrum. It's a habitat generalist that thrives in multiple forest types across North America. The penchant of warblers for frenetic foraging high in the treetops, combined with the ravenous desire of birders to see these adorable birds, has led to the widespread occurrence of a spinal affliction known as warbler neck. Hello, Mrs. Johnson. I'm Dr. Smith. I've had a look at your x-rays. Tell me, Mrs. Johnson, would you say that you like birds? Do you spend long hours peering through binoculars aim straight up into the canopy overhead, with your head craned backward at a severe

angle? Mm-hmm. Mm-hmm. Well, that's just as I suspected. Mrs. Johnson, I'm afraid I have some bad news. There's no easy way to tell you this, so I'm just going to be blunt. You have warbler neck. That's right, warbler neck. The discs between your cervical vertebrae have all but turned to guacamole because of the extreme pressures they endured while you were enjoying the antics of small, colorful songbirds. I hope it was worth it, Mrs. Johnson, because there's nothing we can do to help you. You might want to put your affairs in order. But seriously, warbler neck is real, people. I'll put an article in the show notes on how to avoid the chronic pain that can result from

bad posture while birding. Don't let your love of warblers make you end up like poor Mrs. Johnson. Going way back in time, roughly 10 million years, we might find the common ancestor of all Perulid warblers. That's about the time warblers were diverse. emerging from their closest relatives, those blackbirds and other members of the family Icturidi. Since that time, New World Warblers have gone through a spectacular adaptive radiation. In other words, from a single common ancestor, this lineage split into more and more species

to eventually fill a bunch of ecological niches. So one species evolved into the 108 species we see today over the course of the last 10 million years or so. Biologists have been interested in the evolution and ecology of New World Warblers for a long time. That means there are lots of published studies on these little buggers. Research over the last decade or so resulted in some interesting discoveries about the evolution of our parolid darlings. The foundation for a lot of this research is a well-supported evolutionary tree that describes the genetic relationships among species in the family perulity.

That evolutionary tree, what we call a phylogeny, was published by Erbie Levet and his colleagues in 2010, in the journal Molecular Philogenetics and Evolution. One of the big discoveries is that some large-scale evolutionary studies show that the ancient geographic origin of these birds was in the north, likely in North America. and there's good evidence that the earliest warblers were migratory. So migratory behavior is primitive in this group, rather than something that evolved more recently. These are some important findings,

because there have been two competing hypotheses about the origin of migration in birds that breed in North America but spend the winter in the tropics. Did they originate in the tropics and then gradually migrate further and further north in the summer to breed? Returning to the balmy tropics in winter? That's the scenario suggested by the Southern Home Hypothesis. Alternatively, the Northern Home Hypothesis

suggests these birds originated in the north, closer to where they breed in the present. Then they began to migrate further and further south to escape the harsh conditions of winter. Winters during the ice ages would have been especially challenging. The tropics would have looked even more inviting to northern birds, in those chilly years. The northern home hypothesis seems to offer the best explanation for how migration evolved in Perulids,

because remember there's evidence that these birds originated in the north and they were migratory from the get-go. Interestingly, all of this also points to the idea that the many resident warblers in the tropics actually lost their migration behavior. They don't migrate, but they evolved from northern ancestors who were migratory. who were migratory. Pretty cool, huh? Briefly, I want to give you an example of a fun study on the evolution of New World Warblers. This one was authored by researchers at the University of Windsor in Ontario, Canada.

It was published in 2020 in the Biological Journal of the Linnaean Society. Specifically, this study was looking at the evolution of plumage colors in 15 warblers that breed in the boreal forests of Ontario. The idea was to test for evidence of both natural selection and sexual selection in shaping plumage patterns among these birds, both males and females. The researchers collected data on feather color and brightness from museum specimens. They calculated values of color, brightness, and contrast for nine different body regions for each bird, and they got data from multiple birds within each species.

The other data they gathered was from the background, substrates against which male warblers tend to sing in the wild. The pine warbler, for example, sings most often with leaves or bark in the background, rather than flowers or, ironically, pine needles. The scientists collected samples of the corresponding bark, leaves, etc., and analyzed the colors and other visual qualities of these substrates. So they had some hard, objective data for the warbler's plumage colors, plumage, brightness, and so on. And they had some objective data from the background substrates.

All of this should allow for a quantitative analysis of how conspicuous, or alternatively, how camouflaged a warbler is in its natural environment. In their analyses, the researchers also included information about the height at which males of each species sings, and the ambient light environment around his perch, and a number of other variables. As you can imagine, there's a lot of more to it than this, but to keep a long story short, there were some interesting findings in this study. The researchers found evidence of both natural and sexual selection operating simultaneously. Because for some species, a bird can have certain body regions that are colorful and highly contrasting. These would be attractive to the opposite sex and therefore the result

of sexual selection. But that same bird can have other body parts that provide camouflage, making it cryptic against certain backgrounds. So there's your natural selection. The clearest example given by the researchers is for warbler species that nest on the ground. The oven bird, for example, and the common yellow throat. Males in such species tend to be drab and camouflaged on their upper bodies. That makes sense because predators for ground nesting birds

are more likely to strike from above. But these same male warblers are more brightly patterned, and or colorful underneath, on their throats and breasts, so they can sort of have their cake and eat it too. They can be all sexy for the ladies, but disappear into the background like miniature ninjas when predators are lurking about. The last thing I want to mention about evolution

in the New World Warblers has to do with hybridization. In bird evolution, we tend to think of a single genetic lineage like a species, splitting into two species, and those are then forever isolated from each. other. But sometimes birds of two different species pair up, mate, and produce offspring that not only survive but can produce viable babies themselves. This sort of thing feels kind of like speciation in reverse. Two species are melding together through time rather than becoming more and more different. In North America we have four Warbler hybrid zones. These zones are

narrow geographic regions where the ranges of two species overlap and there is some interbreeding. They persist from year to year, but they may expand, contract, or move. You'll find hybrid warblers flapping around in these zones, alongside quote-unquote pure bloods of the two parental species. The species pairs in the four hybrid zones are blue-winged and golden-winged warbler, McGillivray's and Morning Warbler, Townsend's and Hermit Warbler, and Audubon's and Myrtle Warbler. In each case, the two species sharing a hybrid zone are each other's closest relative. They are what we call sister species. And yes, I do know that Audubons and Myrtle warblers are actually considered subspecies of the yellow-rumped warbler. Once upon a time,

however, they were treated as full species, and there's a chance they will be again someday. Anyway, these hybrid zones are interesting. With the power of modern genetic data and statistical methods, we can identify hybrids and map hybrid zones better than ever. In the Cascade Mountains of the Pacific Northwest, close to where I live, we have the hybrid zone of the Hermit Warbler and Townsend's Warbler. This Warbler hybrid zone is almost like a straight-up war zone, because Townsend's warblers have been steadily encroaching. southward into the range of the hermit warbler. The two species have very similar habitats and ecological requirements. Both have bold black and yellow markings on their heads. But Townsend's

warbler males are all hopped up on testosterone and they're way more aggressive than hermit males. So Townsend's warbler males win more matings with females of both species. Townsend's warblers may be literally breeding hermit warblers out of existence. If we can't get them out, we'll feed them out. Yes, like what you said, evil king in the movie Braveheart. The hybrid zone between these species has already moved thousands of miles. Will it keep moving? Will hermit warblers keep losing ground to their belligerent cousins?

Or will the hybrid zone stabilize? It's hard to say what the future will bring for these two species. I love them both, but I really hope that the Hermit Warbler can stand its ground. All right, it's time for a little more coffee. It's also time for me to thank the newest patrons of the Science of Birds podcast. I now have a Patreon page, which gives you, my dear listener, a great way to support the show. By making a small donation every month, you too can become a patron and help me to keep creating this content for you.

So I want to give a warm shout out to my new patrons, Tom, Tracy, Eric, Haley, Diane, Val, Jennifer, Jerry, and Marilyn. Thank you guys so much. I'm honored to have your support. If you are interested in becoming a patron, you'll find a link to my Patreon page in the show notes. And you can also just go to patreon.com forward slash science of birds. We have arrived at the buzzkill segment of the episode. It's always important to discuss the conservation of whatever bird family or species we're focusing on, even though this subject can be a downer, gloom and doom and whatnot. But there are sometimes glimmers of hope.

And I'm always touched and uplifted by stories of people or organizations or governments going to great lengths to protect wild birds and their habitats. I do have a little bit of good news regarding warbler conservation. I'll get to that, but first, let's look at the state of birds in the family perulity. Of the 108 species, six are classified as endangered by the International Union for the Conservation of Nature, the IUCN. The golden-cheeked warbler, setafaga Chrysopperia, is one of these endangered species. This species is an endemic breeder in the state of Texas. In fact, it's the only bird that nests exclusively in the Lone Star State.

The golden-cheeked warbler has a small breeding range, and the juniper oak woodlands it depends on are threatened by development and agriculture. Another six perulid species are in the vulnerable category, and nine are considered near-threatened. Vulnerable is worse than near-threatened, in case you were wondering. And there are two species that are technically classified as, critically endangered. But sadly, the last confirmed sightings of these birds were in the early 1960s. Backman's warbler is or was found in the southeastern U.S. during its breeding season.

And Semper's Warbler is known from only the Caribbean island of St. Lucia. Habitat destruction is the primary culprit in the demise of Backman's Warbler, while the ravages of introduced mongooses are implicated in the loss of Semper's warbler. Now let's consider Kirtland's Warbler, Setaferga-Kirtlandii. This species breeds in scattered locations around the Great Lakes area in North America. It spends its winters in the Bahamian archipelago in the Caribbean. Kurtland's warbler is large, for a Perulid anyway. Males have dark gray upper sides and bright yellow undersides.

Their eyes are set off by a partial white eye ring, like the northern Perula we talked about earlier. Female Curtland's warblers are similar, but their colors are more muted. Today, this species is one of the nine New World Warblers in the near threatened category. But not that long ago, it was on the endangered species list in the U.S. In fact, this bird was among the first species put on that list when the endangered species list when the Endangered Species Act was passed in 1973. Curtland's warbler was probably never super-abundant. It has a very specific requirement for breeding habitat,

young jack-pine forest that's less than about 15 years old. When the forest matures beyond that, these warblers aren't interested. Kurtland's warblers declined dramatically through the 20th century as humans destroyed and altered jack-pine forests. Adding to the threat of habitat loss was the threat of brood parasitism by brown-headed cowbirds. The cowbird population had increased because of human activities. If you want to learn more about brood parasitism, you can check out episode 12 of this podcast, which is all about that topic.

In the late 1970s and 1980s, there were less than 200 breeding pairs of Kurtland's Warbler. But protection under the Endangered Species Act led to some of the United States. dramatic efforts to save this bird. Most importantly, there has been and continues to be a massive effort to clear and replant jack pine forests on an annual basis. Thousands of acres of this warbler's breeding habitat are maintained in Michigan and Wisconsin. Brown-headed cowbirds have also been trapped extensively. These brood parasites now seem to pose a minimal threat to the warbler. So from a rock-bottom population of 167 singing males in 1987, there are now at least 4,000 mature Kirtland's warblers flitting around in the jack-pine forests of the Midwest.

In 2019, the species was delisted, removed from the endangered species list. The U.S. Fish and Wildlife Service had concluded that the Kirtland's warbler population had recovered. Hooray! See, there's your good news, your glimmer of hope. This singing male, Kurtland's Warbler, sounds like he's feeling pretty good about how things are going. But, and there's always a but, right? The long-term fate of Kurtland's Warbler is probably going to depend on the never-ending, large-scale management of its jack-pine habitat. So, just because this bird was delisted, conservationists can't just high-five each other, pack up and go home to binge watch Netflix or eat some Hot Pockets.

Hot Pockets! Just for the record, Hot Pockets is not a sponsor of this show. Besides breeding habitat, we also have to remember that Kurtland's Warblers actually spend most of their year in the Caribbean Islands. So just protecting and managing their breeding habitat isn't necessarily enough. to keep this species from going extinct. For migrating species like Kirtland's Warbler, and there are a lot of them, we have to conserve their breeding habitats and winter habitats.

Threats to the winter habitat of Kirtland's Warbler in the Bahamas and other islands include hurricanes and the impacts of climate change. Winter droughts are likely to become more severe with climate change. Rising sea levels threaten to destroy the bird's habitat and reduce the fresh water needed by plants that the birds feed on. Climate change threatens New World Warblers in general. One potential problem for migrant species is that the timing of the seasons is getting out of whack in some regions.

As the climate warms up, spring is arriving earlier in parts of North America. Plants and insects grow and reproduce in spring in response to rising temperatures. Buds are breaking earlier and caterpillars are fattening up early. earlier than they used to. But migrating warblers don't use temperature as their cue to depart the tropics and head north. Instead, they rely on the changing length of daylight, what we call photo period. Well, climate change doesn't affect photo period. Day length across the seasons is the same as it ever was. So, a black-throated green warbler, for example, might take off from Honduras in March and head north. Its destination is eastern Canada. Countless generations of

its ancestors have departed the tropics at this same time of year. Meanwhile, spring came two weeks early in the Appalachian Mountains. Our little black-throated green warbler makes a stopover in those mountains, in desperate need of some food to refuel. But when the bird arrives, the caterpillars have already come and gone. It may not get enough food to survive the rest of its long journey. This is just a hypothetical scenario, and lots more research is needed to investigate this potential problem, but some evidence suggests that this is already happening to some birds, including New World Warblers. What do Paroolid Warblers eat, and how do they forage?

They eat mostly insects and other small invertebrates, as I mentioned earlier. They're insectivores that eat things like flies, wasps, leafhoppers, beetles, moths, caterpillars, and spiders. Setefaga, the name of the most species-rich genus in this family, comes from ancient Greek words meaning to eat insects. How a warbler forages depends on the species. Some glean their prey from leaves, twigs, or flowers, while, hopping around in a tree or shrub. Some do the same while hovering for brief bouts. And some warblers sally out from a perch to catch flying insects mid-air. A smaller number of warbler species prefer to forage on the forest floor or along the banks of streams or rivers.

Louisiana and northern water thrushes, for example, specialize in foraging for aquatic insects on the margins of flowing water. There isn't a ton of various. ability in the shapes of warbler bills. They're narrow, pointed, and shortish, as we discussed. We see this bill shape in insectivorous birds around the world. It's ideal for snatching small prey from the surface of a plant or from among litter on the ground. New World warblers are not well known as birds who visit backyard feeders. This makes sense since they aren't seed eaters. They prefer their prey alive and wriggling, apparently. That said, many warblers will gobble up berries in late summer or fall

as they prepare for migration or once they're underway. And some species eat fruit or nectar in the non-breeding season. The yellow-rumped warbler, America's most common species, has perhaps the most varied diet and widest array of foraging strategies. Birders affectionately call these warblers butter butts. Or in Mexico, you might say, Rabadilla de Montequia Rump of butter

Our little butter butts glean insects in trees and they do a lot of fly-catching They poke around in seaweed on the beach To look for invertebrates And they snatch them from spider webs Off of piles of cow poop And so on

The yellow-rumped warbler switches to eating berries in fall It's the only warbler species That can digest the wax in wax Mertle berries Mmm tastes like candles I suppose that's why the eastern subspecies is called the Myrtle Warbler, because it eats myrtle berries.

This wax-digesting superpower allows yellow-rumped warblers to survive the winter in northern latitudes. This species winters further north than any other member of its family. You ever wonder how biologists figure out exactly what a small bird species eats? I mean, rarely is it possible to get good data by just watching a warbler as it forages for bugs or whatever. There are better approaches. The brute force approach, and my least favorite by far, is where a person blasts the bird with a shotgun, then cuts open its stomach to examine the contents. That's a pretty old-school method, and I'm definitely not a fan. But it came across a

recent study that used a less murderous, less ballistic approach. The researchers wanted to determine the diets of five migratory warbler species in their winter habitat of coffee plantations in Jamaica. The researchers were interested in how differences or similarities in these diets might reflect competition or resource partitioning among the five warbler species. So what did the researchers do to get a look at what these warblers were eating? First, they used mist nets to catch American red starts, northern perula's, and the other three species of interest. Then, each bird was given a healthy dose of antimony potassium tartrate, right down the gullet. What is antimony potassium tartrate, you ask?

It's an emetic, of course. It induces barfing. In short order, the little warbler hacks up the contents of its stomach. After a brief rest, the bird curses the researchers and flies off among the coffee plants to refill its tiny belly with tasty bugs. The researchers collect the stomach samples to examine carefully in the lab. Maybe not a fun experience for the birds, but at least no shotguns were involved. There's another study I have to tell you about.

It's the famous warbler study conducted by Robert MacArthur, published in 1958 in the journal Ecology. MacArthur looked at the foraging behaviors of five warbler species in coniferous forests in the northeastern U.S. These five species are often seen hopping around and feeding in the same tree. This presented a problem to biologists because all five species seemed to be occupying the exact same niche. That wasn't supposed to be possible, theoretically. MacArthur wanted to figure out what was going on, so he recorded detailed observations of

where and how each warbler species feeds in a conifer. What he figured out is that these warblers aren't all behaving the same way. They actually use food resources in the tree differently. So the Cape May warbler forages among fresh needles at the top of the tree. Black-Burnian warblers prefer to search the upper branches. Yellow-rumped warblers, our Rabadilla de Montequia buddies, those little guys tend to look for insects on bare or lichen-encrusted branches at the bottom of the tree, and so on.

MacArthur's study was one of the first to describe the phenomenon of resource partitioning in birds. This study has since become a textbook example. We've talked about resource partitioning before, like in episode two, which was all about competition between species. So, contrary to what it had seemed, these five warbler species do not all occupy the same niche. They avoid direct competition with each other by foraging in. different parts of the same tree. Weirdo alert. Weirdo alert. Now is as good a time as any to alert you to one of the weirdos in the family

Parulity. The bird in question is the black and white warbler, Nyotilta varia. That scientific name means variegated moss plucker, or something like that. Some say this refers to the way the black and white warbler forages, and others say it has to do with the bird using moss in its nest. In any case, this is the only species in the genus Nyotilta. Many warblers can be tricky to tell a part in the field, but this species is pretty unmistakable.

It's, well, black and white all over. It has bold black stripes running all up and down its body. It looks like something from a Tim Burton, movie, like a miniature avian beetle juice. That's moderately weird, but the way the black and white warbler forages is also highly unusual for this family. It scoches and clambers around on tree trunks and big limbs, probing for invertebrates in bark crevices. Its bill curves downward a bit, which makes it well suited for this foraging strategy. This warbler behaves more like a nut hatch or tree creeper.

The Black and White Warbler has several nicknames, but my favorite is Zebra Creeper. I'm going to put that on my long list of possible band names. Zebra Creeper. You know, for whenever I get around to forming a rock band that becomes super famous. Everybody put your hands up and make some noise for Zebra Creeper! Black and white warblers breed across most of eastern North America well up into the boreal forests of Canada. The species has a large winter range from southern Texas and Florida in the U.S. to Mexico and on down to northern South America.

I saw one most recently when I was birding in southern Texas in early March. Here's a male black and white warbler singing in a New England forest. It's pretty high-pitched, so you might have a little trouble hearing it. Okay, okay, so as weirdos go, the black and white warbler isn't that weird. But still, it's a fun species that I wanted to highlight. I want to talk some more about migration in New World Warblers. We touched on this earlier with the McGee Marsh Mosh Pit and all that. As I write this episode, it's early May.

This is peak season for the return of Mawksh. migrant warblers in North America. Our neotropical migrants don't all arrive at once, however. Each species has its own biological schedule. Some arrive earlier than others. The result is that our warblers come in waves through April and May. In the McGee Marsh area, on the south shore of Lake Erie in Ohio, the earliest warblers show up in mid-April. That first wave includes palm warbler, Nashville warbler, and the zebra creeper. Species that arrive later in May include magnolia, blackpole, and bay-breasted warblers. Warblers come in waves because each species times its migration based on factors like migration distance, food availability,

when certain plants leaf out, weather patterns, etc. Timings can also vary within a single species. Males and females and different age classes will often migrate at different times. For example, adult males usually arrive on the breeding grounds first because they need to get busy establishing their territories. The champion of migration among New World Warblers is the Black Pole Warbler, Setaephaga striata. This species breeds way up in northern Canada and Alaska. In the fall, Black Pole Warblers make an epic journey south to northern South America. Some individuals fly almost 5,000 miles or 8,000 kilometers. No other warbler makes such a long migration.

And get this, Black Poles flying south head out over the Atlantic Ocean, leaving land in the northeastern U.S. They end up island hopping down to South America. but almost 1,800 miles of that route is over open water. During one long stretch, these tiny birds fly over the ocean nonstop for three days. We know this because researchers have attached miniature geolocator devices to the backs of blackpole warblers. This is an absolutely amazing display of endurance. It's one of the most spectacular migratory feats among birds. Long-distance migration is perilous for any migrating warbler.

Every spring and fall, a high percentage of warblers never make it to their destinations. These little birds face more than just an endurance test. They face storms, aerial and terrestrial predators, and man-made hazards like tall buildings and wind turbines. They migrate at night and can get disoriented by the lights on our tall structures. It's amazing, really, that any migrating warbler completes its journey, and many of them do it repeatedly over their lifetimes. New World Warblers are, as a rule, socially monogamous breeders. Recall that the social part means that a male and female form a pair and the male sticks around

to play an active role in raising his young. Either member of the pair may or may not mate with other birds on the side. Social monogamy does not equal sexual monogamy. So our warblers, they're socially monogamous, and they show biparental care of their young. Mom and dad share in the duties of rearing the brood. The female is usually the one who builds the nest. Warbler nests are cup-shaped in general. Some species like the familiar Wilson's warbler,

build them on the ground. Others place their nests way up in the forest canopy. The female also does all the incubating, but the male brings food to her while she's on the nest. Both parents help feed the insatiable chicks and continue to feed them for a month or so after fledging. Then the youngsters get kicked to the curb. Species that breed in the northern temperate latitudes, lay more eggs, spend less time incubating their eggs, and less time. caring for their fledgling chicks. That is, compared to warbler species that breed in the tropics.

Northern migrants have to rush, you see, because winter is coming. Residents of the tropics, on the other hand, have more time to take it easy. Those birds are all like, there is no need to worry, my friends.

Life is beautiful in a tropical paradise, no? Just relax. Ha, ha, ha. Some young birds in tropical species will stay with their parents through their first winter until the next breeding season. With luck, a New World Warbler will successfully navigate the challenges of life to breed and survive up to 10 years. The family perulity is diverse and fascinating. New World Warblers give us a lovely example of a large adaptive radiation.

in songbirds. And they've taught us many other things about ecology and evolution, such as resource partitioning, hybridization, and migration. But there is still so much more to learn about these birds. I'm sure future studies will continue to reveal amazing things about warblers. Those of us who live in North America are so lucky to have these little sprites flitting around in our backyards and in our wild places. As I was writing this episode, there were some moments.

where I could hear the songs of orange-crowned warblers and Wilson's warblers just out of my window. That made me really happy. And hey, if you don't live in the Western Hemisphere, you better get over here and get a look at our warblers. They won't disappoint you. Well, the orange-crowned warbler might disappoint you, if you were expecting to see an orange crown anyway. But this little bird has other, more subtle charms, like its upbeat attitude and its song. Well, this was a big episode, and I appreciate that you listened to the whole thing. Did you learn some fun facts to share with your friends at the country club?

Or perhaps your improv group or your Pokemon club? In any case, go out and spread the warbler love by telling a friend or two some things you learned today. In case you didn't know, it's easy to get on the Science of Birds newsletter email list. Just get yourself over to Scienceofbirds.com and you'll find a place on the home page to enter your email address and then you're pretty much all set.

If you have something you'd like to tell me about the show or maybe what you're going to call your future rock band, please shoot me an email. The address is Ivan at Scienceofbirds.com. The show notes for this episode, which is number 28, are on the Science of Birds website,

which is, again, scienceofbirds.com. I'm Ivan Philipson. Thanks for being here, and I'll catch you next time. Cheers.