The Science of Birds - Bluebirds

Episode Date: June 3, 2024

This is Episode 97. It’s all about bluebirds: the three North American species in the genus Sialia: The Eastern Bluebird, Western Bluebird, and Mountain Bluebird.For centuries, these beloved birds have been celebrated in paintings, poems, songs, and Disney cartoons. The Eastern Bluebird is the state bird of Missouri and New York, while the Mountain Bluebird is the state bird of Idaho and Nevada.And, apparently, the Mountain Bluebird was the inspiration for the original Twitter logo.But bluebirds aren’t just cultural icons. They’ve also become valuable study subjects for scientists in the fields of ornithology and general biology. Research on these little birds has led to insights about the way the natural world works—from evolution, to ecology, behavior, diseases, and genetics. Links of InterestNorth American Bluebird Society~~ Leave me a review using Podchaser ~~Link to this episode on the Science of Birds website Support the show

I'm going to read you a quote about the eastern bluebird. It's an old-timey one from way back in 1896. Quote, the bluebird's disposition is typical of all that is sweet and amiable. His song breathes of love, even his fall call note, terwee, terwee is soft and gentle. So associated is his voice with the birth and death of the seasons that to me his song is freighted with all the gladness of springtime, while the sad notes of the birds passing southward tell me more plainly than the falling leaves that the year is dying. End quote. That poetic little paragraph was from the book Birds of Eastern North America by Frank Chapman. Here's another one, this time about the mountain bluebird. It's from the book The Birds of California written by William

Dawson, published in 1923. Quote, gentle and demure as well as brave and high-spirited is this sky-born thoroughbred of the Sierras, this bit of heaven's own blue incarnate, end quote. Sweet, amiable, gentle, high-spirited, soft, demure, these are the kinds of words that have been used to describe the personalities of bluebirds. If appearances alone could tell us all about the personality of a bird, then sure, I'd agree that the three bluebird species must all be gentle-hearted creatures. I mean, just look at their sweet little faces.

They wouldn't hurt a fly. Except that they would. Bluebirds eat flies. To a fly, a bluebird is a terrifying blue monster. We should know better than to be deceived by acute outward appearance, right? Tasmanian devils, for example, look super cuddly and pedible, but those feisty marsupials will gnaw your face off given half the chance. And remember the

Ewaks, those sentient teddy bears on the forest moon of Endor? Those little buggers decimated an entire battalion of imperial stormtroopers. Good. Yep, yep. Maybe those authors writing about birds in the late

1800s and early 1900s didn't spend enough time observing bluebirds. Because if they had, they might have realized that these small birds can be pretty violent. I mean, birds are wild animals, right? They live in a dangerous world full of challenges like predators and competitors.

Bluebirds are territorial and aggressive during the breeding season. A male defending his territory will chase an intruding male. The two birds will sometimes battle in the air, interlocking their little feet and then they fall to the ground. They flop around in the dirt, biting and battering each other. One of them might end up pinning his enemy down by standing on him and spreading his wings menacingly. The bird on top will peck savagely at the vulnerable bird under him. Female bluebirds will fight each other like this too, and they will fight with males. When pairs of mountain bluebirds nest in the same area, they have little tolerance for each other. They squabble and fight, over and over, like feuding neighbors. The bluebirds may spend so much

time clashing with each other that they actually neglect their babies waiting back at their nests. and there's even published evidence that mountain bluebirds in Montana took over a nest box by murdering the pair of tree swallows that were the original owners. A bluebird sometimes unleashes its wrath upon an enemy that cannot be defeated, the bird's own reflection. Bluebirds will scratch and peck at their reflections in car mirrors or in windows. The attack can be so relentless that the bird eventually slumps to the ground in complete exhaustion. Maybe those agro-bluebirds should take a lesson from Stuart Smalley. Instead of attacking the mirror, they could look in the mirror and practice his daily affirmations.

I deserve good things. I am entitled to my share of happiness. I refuse to beat myself up. I am an attractive person. I am fun to be with. So, yeah, it turns out, bluebirds aren't just gentle little angels filled with sweetness all the time. They're more three-dimensional when it comes to their emotions, to their personalities. The three bluebird species are really interesting, and there's more to them than meets the eye.

Hello and welcome. This is the science of birds. I am your host, Ivan Philipson. The Science of Birds podcast is a light-hearted exploration of bird biology for lifelong learners. This is episode 97. It's all about bluebirds. Not blue birds, as in birds that happen to be blue. There are a bunch of those around the world. No, today we're talking only about the three North American species in the genus Sialia,

the eastern bluebird, western bluebird, and mountain bluebird. For centuries, these beloved birds have been celebrated in paintings, poems, songs, and Disney cartoons. The eastern bluebird is the state bird of Missouri and New York, while the mountain bluebird is the state bird of Idaho and Nevada. And apparently, the mountain bluebird was the inspiration for the original Twitter logo. Remember that one? Before it got replaced by a dumb old X? May the Twitter Bluebird logo rest in peace. But bluebirds aren't just cultural icons.

They've also become valuable study subjects for scientists in the fields of ornithology and general biology. Research on these little birds has led to insights about the way the natural world works, from evolution to ecology, behavior, diseases, and genetics. One reason bluebirds make great subjects for scientific studies is their variation. Here's an interesting quote from the website, Birds of the World, and I'm paraphrasing a bit here. Quote, a hallmark of bluebird biology is a remarkable level of

individual variation in morphology and behavior, both within and between populations, as well as data indicating that bluebirds make flexible and adaptive decisions to use or not use nest boxes of different qualities or history. This shows that, that ecological and social contingencies matter to what bluebirds do. End quote. Bluebirds are so important to us, culturally, ecologically, and scientifically, that there's an entire organization devoted to their conservation, the North American Bluebird Society. I'll put a link to their website in the show notes.

Maybe after listening to this episode, you'll be inspired to become a card-carrying member of the North American Bluebird Society, or Nabs for short. Oh, and I almost forgot to mention, the bird species that I have been using for the main illustration on the Science of Birds website is an eastern bluebird. So, yeah, I like these birds a lot, too. All right, let's jump into the good stuff. Let's get to know the bluebirds. The three bluebird species are all closely related.

species belonging to the genus Cialia. And they're members of the Thrush family, Tertidae. I'll talk a little more about that later. I'll be highlighting many differences among the bluebird species today, but there are also some major similarities among them. The most obvious is probably their general body shape and size. They're also fairly similar in their habitat preferences, their diets, the tonal qualities of their songs, and in their nesting behavior. But before we into all of that, let's first look at the physical traits of bluebirds. Imagine you have a thrush, like an American Robin or a Eurasian blackbird. And yes, both of those species are actually thrushes, but that's a story for another day.

Anyway, you have one of these birds and you zap it with a shrinking ray gun. Now, don't worry, don't worry, it won't hurt the bird. the glowing blue beam from the gun just makes the birds smaller, slightly smaller. To reference a Disney movie from 1989, it's like, honey, I shrunk the thrushes. The only side effect of the ray gun is that it changes the bird's plumage color at the same time it decreases their body size. The point I'm trying to make with this silliness is that bluebirds are among the smallest members of the thrush family. All of them are between 6.5 and 8.5 inches from beak to tail. or about 16 to 21 centimeters.

That's about two-thirds the size of an American Robin or a Eurasian blackbird. Bluebirds really do have sweet expressions on their little faces, but those expressions are frozen in place, right? Regardless of how the birds are really feeling. A super angry bluebird wears the same expression as one that's filled with a radiant love for everyone and everything in the universe. Meanwhile, a bald eagle feeling similarly benevolent toward all, beings, looks like it actually wants to murder someone. The eagle can't help that it has those

supra-orbital ridges that look to us like eyebrows furrowed in anger. That's where the phrase resting eagle face came from. And I thought I just made that up, resting eagle face, but a quick Google search revealed that, nope, the internet is way ahead of me, as usual. Anyway, back to bluebirds and their gentle faces. All three species have dark eyes and a dark gray or black beak. The beak is relatively short and thick. Plumage coloration is where the three species

distinguish themselves. Of course, the dominant feather color for all three species is blue. Surprise! But the eastern and western bluebirds have some orange or reddish brown in the mix. At least superficially, the latter two species look very similar.

But it's actually pretty easy to tell them apart, at least the males. Western bluebird males have a blue throat and a rich chestnut color on the breast and shoulder. Male eastern bluebirds, on the other hand, have an orange throat breast and shoulder. Females are distinct from males in all three species, so we'd say the sexes are dimorphic or dichromatic. Females are relatively drab, with plumage that's mostly grayish. They have some blue accents in their tails and wings, however. Female eastern bluebirds have white throats and bellies, whereas western bluebird females have more of a dusky throat and belly. But what about the mountain bluebird? This species

lacks any orange or chestnut color. Now all bluebirds are lovely, but wow, the mountain bluebird is stunning. From a purely aesthetic perspective, that one is my favorite. The male mountain Bluebird is an extraordinary shade of azure or cerulean blue from head to tail. His belly and undertale coverts are white. To me, a male mountain bluebird looks like a snow cone that's been drizzled with sugary blue syrup. You know, that blue raspberry flavor that tastes just like the real thing? As long as the real thing you're talking about is actually an unholy concoction of blue food coloring and chemical extracts that was invented by people wearing white lab Coats.

Yummy! Interestingly, the mountain snow cone, I mean the mountain bluebird and the other two species don't always appear to be all that blue. The color of an individual bluebird can look different to a human observer depending on the angle of the sunlight hitting the bird. From one angle, you might think the bird looks kind of dull and almost brownish. Then at another angle, pow! That blue color just pops.

If you listened to episode 56 of this podcast, which was all about feather colors, you'll remember that the blue colors we see in bird feathers are the result of microscopic structures interacting with wavelengths of light. There's no blue pigment involved. This is called structural color. Iridescence is another form of structural color. The hue, richness, and brightness of these feathers seem to change with our viewing angle. If you, you listened to episode seven of this podcast, which was all about bird vision, you'll know that birds can see colors in the ultraviolet end of the spectrum, colors that humans can't even imagine. Yes, I know, I just referenced two previous podcast episodes. If you haven't heard those

yet, well, I guess you've got some homework to do, don't you? You might have to skip work or school tomorrow to binge on some science of birds. Right, so dig this. Research on the eastern blue bird has shown that the plumage colors of males and females are even more distinct when you consider what they look like in the ultraviolet wavelengths. These birds see each other as even more dramatically dichromatic. Researchers figured out that this distinction between the sexes results from differences at the microscopic level within the feathers. Male and female feathers have different sizes and densities of air spaces and keratin rods. Male feathers reflect UV wavelengths more strongly than those of females. So there are strong plumage differences between

adult males and females, but interestingly, the sexes of juvenile bluebirds are also distinct. And this is, as I understand it, a rare thing among birds, being able to tell males from females at the juvenile life stage by their plumage alone. Juvenile bluebirds are generally darker and more brown than the adults. They also have white eye rings, white streaks on the back, and spotted undersides. Both sexes have some blue in the wings and tail, but the males have noticeably richer blue hues. We've covered what bluebirds look like, so now let's listen to what they sound like. I track down a few recordings of their songs and calls for you. Bluebird vocalizations are relatively simple, at least when compared to the songs made by many other birds in the

Rush family. Some bluebird calls are loud, while many others are soft. Calls are simple sounds consisting of one or two syllables. Bluebird songs, on the other hand, can sound like a few calls that have been strung together in sequence. Females, at least in the eastern species, also sing and call. Here's an eastern bluebird calling and singing from a tree. It was recorded in Minnesota at the boundary between a woodland and a grassland. Researchers have discovered that these eastern bluebirds show a lot of variation in their calls and songs. Each individual bird has a large. repertoire of vocalizations, and there are recognizable differences among individuals and populations.

I'm more familiar with the Western and Mountain Bluebirds, since I live in Oregon. The most common sound I hear from the Western Bluebird is the Q or few call. Here's what that sounds like. In this recording, you can hear several Western bluebirds calling. Mountain bluebird vocalizations are pretty similar to those of the western bluebird. Here's a mountain bluebird calling from a ponderosa pine in South Dakota. Bluebird vocalizations might not sound all that complex or musical, but once you get familiar with their general tone and quality, it's possible to recognize when a bluebird is somewhere nearby, just by its voice alone.

As I've mentioned, bluebirds are in the same. the Thrush family, Tertadie. Around the world, there are about 171 species in the family turdadi, including the three bluebirds. The genus Cialia is one little evolutionary twig coming off of the branch that is the family Tertadie. Cialia comes from the Greek word Cialidos, which apparently is a kind of bird, or just a bird. How's that for some unexciting etymology. The scientific name of the eastern bluebird is Cialia Cialis. As if to say, this is one of those bird birds, you know. The western blue bird is Cialia Mexicana. Now this species certainly lives year

round across a large portion of Mexico. So you'd think that's how it got the second part of its scientific name, the specific epithet, Mexicana. But the Western Bluebird was first described scientifically in 1837 by the naturalist John Kirk Townsend. In those days, most of the region that would become the southwestern U.S., including California and Nevada, was still part of Mexico. It was the Mexican territory of Alta, California. I couldn't find out where exactly Townsend collected that first specimen of the Western Bluebird. The name scientists use for the first officially described specimen of a species is the type specimen. And the location where the type specimen was collected is the type locality.

So what I'm looking for here is the type locality of the Western Bluebird. Townsend and some of his naturalist buddies made an expedition across the Rocky Mountains in the 1830s. So I suspect the Western Bluebird type specimen was from modern-day Colorado. or something like that. If you, dear listener, happen to know the solution to this mystery, please let me know. But why do I care? I'm not really sure. I guess I just really want to know how the Western Bluebird got the scientific name,

Ciala Mexicana. See, these are the kinds of rabbit holes I go down when I'm researching my podcast episodes. This is why it takes me so dang long to write each script. But if you're a lifelong learner like me, Maybe you can relate. I'm insatiable when it comes to learning. My curiosity knows no bounds. Well, that's actually not true.

My curiosity actually hits a hard wall with topics like sports, politics, the stock market, corporate news, and celebrity gossip. Boring. But hey, that's me. Anyway, the mountain bluebird's scientific name, and I promise I won't go off on a long digression here, is Cialia Curicoides. When you see the suffix oide or oides, it means like, or in the form of.

Think of the word android. Android means like a man or like a human. The German dude who first had described the mountain bluebird in the name of science thought it looked like a European bird called the lesser white throat. The scientific name of the lesser white throat is Kuruka Kuruka. The mountain bluebird is Sialia curacoides, and that second part basically means like the lesser white throat. Got it? Good. Moving on. So, which birds out there in the world are the closest relatives of our bluebirds?

The most recent research I could find that addresses this question was published in 2013, in the journal Avian Research. A genetic study revealed that within the Thrush family Tertadie, there are several genera that are most closely related to the genus Cialia. These genera are Stizorina, neocococcusophus, and mayadestes. The first two genera, Stizorina and neocococcusis, contain four species, all of which live in the tropics of western Africa. These are birds that most of us will probably never see. But the genus Mayadestes includes some new world birds you may be familiar with. Most of them are called Solitaire. Townsend's Solitaire, Myodestes Townsendai, is the only species found in the mainland U.S. and Canada. I saw one of those little beauties

just last week while I was hiking in the Cascades Mountains. And in case you're wondering, yes, Townsend's Solitaire was named after none other than John Kirk Townsend, the guy who named the Western Bluebird. Of course, Townsend's Solitaire will evils. eventually get a new common name, right? And if you don't know what I'm talking about, just go listen to episode 87 of this podcast. There you go. More homework for you. There's some diversity at the subspecies level in bluebirds, at least for the eastern and western species. The eastern bluebird has seven subspecies, while the western has six. Ornithologists haven't identified any intraspecific variation like this in the mountain bluebird, however.

In other words, this species has no subspecies that we know of. Now, let's talk about the distribution and habitats of bluebirds. The eastern bluebird is a full-time resident in the southeastern U.S., in the highlands of Central and Southern Mexico, and southward to northern Nicaragua. In the breeding season, some eastern bluebirds migrate into the Midwest and the northeast of North America. Eastern bluebird populations probably cover a wider geographic range today than they did several hundred years ago. They expanded into the Midwest as people planted more trees where there used to be nothing but vast grasslands.

And as more artificial nest boxes have been provided, this too has allowed eastern bluebirds to expand their range. Western bluebirds have resident populations in California, parts of Oregon, and in the arid southwests. including parts of Arizona, Utah, Colorado, and New Mexico. They also live year-round in the mountains of Central Mexico. In summer, this species expands into the interior of the Pacific Northwest. The mountain bluebird is a year-round resident in only a fairly small portion of the Great Basin, in parts of Nevada and the Four Corners region. These blue raspberry snow cones move north in the summer during the breeding season,

occupying most of the interior of Western North America. some of them even get as far north as Alaska, at least occasionally. In fact, the original scientific name for the mountain bluebird was Sialia Arctica. It was collected in the 1820s in northwestern Canada. Now another note about distribution. If you're ever lying in bed in the middle of the night, stressing out because you're afraid that someday you're going to encounter a bird and you won't be able to tell if it's an eastern or western bluebird, well, let me try to put your fears to rest. The geographic ranges, or distributions, of those two species are almost completely non-overlapping. At least in the United States. In the U.S., the only place where

western and eastern bluebirds regularly overlap is in southern Arizona. Now, Mexico, that's another story. There's plenty of geographic overlap between these two species in that country. If you're a birder in Mexico, you'll have to study up on the plumage differences we talked about earlier. I touched on the topic of migration a bit already, but let's look at it more closely now. Eastern and Western bluebirds are what we call partial migrants. Not all individuals or populations migrate. Some just hang around in the same region, more or less all the time. As I mentioned, though, both eastern and western bluebirds expand their ranges northward in summer, and that's because of the individuals that do migrate. It seems that,

ornithologists don't know why some individuals migrate and others don't. This is still a knowledge gap that needs more research. The Western bluebird is the least migratory of the three Sialia species. When individual Western bluebirds do migrate, it tends to be over relatively short or medium distances. The mountain bluebird is, as far as we know, a truly migratory species, as in most or all individuals migrate. And they migrate north earlier than most other migratory birds of any kind. They return to their northern breeding habitats in late winter, when it can still be very cold when there's still snow on the ground. So these tiny blue, fairy-like creatures are heralds of spring. For thousands of years, people have welcomed the

return of the mountain bluebirds as one of the first signs of the changing seasons. Indigenous Iroquois people in North America have a traditional belief that the call of the mountain bluebird has a special power. The soft, sweet sound of this bird is, for some reason, terrifying to Sawiskerra, the spirit of winter. The bluebirds arrive, they call to each other, and Sawiskerra is like... Soykes! And he's out of there. And in his wake, the ice and snow melt away. and there is much rejoicing.

Male mountain bluebirds usually arrive first. They get to work establishing their breeding territories. A male's territory will include at least one potential nest site. Bluebirds nest in cavities, usually in trees or in artificial nest boxes. Mountain bluebirds, for example, compete with tree swallows for cavities carved out by woodpeckers. Remember from the beginning of the episode when I told you, researchers found a crime scene in one tree cavity where mountain bluebirds had killed a pair of tree swallows. Grusomely, the bluebirds just built their nest and laid their eggs right on top of the

dead bodies of their victims. Such fierce competition is what some ornithologists hypothesize has been the evolutionary force driving mountain bluebirds to arrive so early on their breeding grounds. Because why would you want to arrive in late winter when you have a serious risk of freezing to death in the snow. It seems like a stupid thing to do. But the reward is that if you arrive early enough, you can claim the best real estate, the best nest location. It's sort of like, why do I try to be the first person in line outside of Walmart on Black Friday? I risk my life every year because it's usually icy cold at 2 a.m. when I arrive. And the person behind me in line probably wouldn't think twice about murdering me if it meant they could move up one

space in the queue. But if I'm the first one through the door, I can score a new vacuum cleaner for like 40 bucks, and a deeply discounted set of towels and some DVDs for $1.99 each. I mean, I don't own a DVD player anymore. I mean, does anybody? But how could I pass up that deal? So, male mountain bluebirds are the first to arrive. They score nest cavities which are in short supply. One could argue that securing a nest cavity to raise a family is way more important than getting a discount on a vacuum. Male western bluebirds likewise arrive before the females. Male mountain and western bluebirds sing their hearts out to impress the females when they finally arrive. Eastern bluebirds, on the other hand, tend to arrive on their breeding grounds

already paired up as boyfriend girlfriend. At least that's my understanding. Now in terms of habitat, bluebirds prefer open spaces that have at least some trees for nesting and perching on. Until a couple hundred years ago, eastern bluebirds would have depended on savannas that were swept by wildfire every so often. Fires would keep the trees from getting too dense. This species would have used open pine forests, grassy balds on tops of hills, and other semi-open habitats. These days, there are also human-created environments that, eastern bluebirds occupy, orchards, patches of clear-cut forest, and even golf courses. These birds are fairly tolerant of disturbed habitats like that.

Western bluebirds zip around, flying low to the ground in park-like forests, partially burned forests, clear-cuts, and forest edges, as long as there are some trees around for nesting. The western blue bird prefers habitats that are less open than those used by eastern and mountain bluebirds. Mountain bluebirds are happy in open grassland habitats and mountain meadows, again as long as there are some trees nearby. This aptly named species, the mountain bluebird, is well adapted to life high in the mountains. Their nests have been found as high as 14,000 feet in the Rocky Mountains. That's about 4,260 meters. Such elevations are well above timber line. In other words, there are no trees growing up there. But I keep saying

these birds need trees to nest in, right? Well, mountain bluebirds in the high country make due by building their nests in rock crevices. Now it's time to talk a bit about the evolution of bluebirds. Most of what we know about this comes from a 2005 study published in the journal Molecular Philogenetics and Evolution. The researchers in this study used genetic data from DNA to work out the evolutionary tree, the phylogeny of the entire turdody family. Trees like this show us which lineages diverged in what order, and they can sometimes give us rough estimates of how long ago it was that each lineage became a unique branch. As I mentioned earlier, bluebirds, which make up the genus Cialia, are most closely related to birds in the genera, Stizzerina, neocoxifus and mayadestes. The branch of the turdadi tree that includes the ancestor of

Cialia and these other three genera originated in the new world. In North America, to be more exact. This lineage is actually one of the oldest within the family turdadi. In other words, it's the most basal lineage. It split from the rest of the family approximately 11 million years ago during the mid to late Miocene epoch. Some members of this unique group differ from the more typical thrushes in the anatomy of their vocal organs, their syrinxes. This is true for the ant thrushes in the genus neocococcusophus, but I'm not sure if it's true for bluebirds, the ones we're talking about today. So the lineage that eventually gave rise to the modern bluebirds has been around for a long time. But the three bluebird species themselves, as we know them today,

appear to have evolved much more recently. They seem to have appeared only one to three million years ago. Now, looking at the eastern and western bluebirds, you would almost certainly conclude that they are each other's closest relatives, that they're sister species. They should have a recent common ancestor that itself had split away from the ancestor of the mountain bluebird. And that's exactly what ornithologists used to think. I mean, it makes sense. But, And I bet you could hear the butt coming, couldn't you? But it turns out that the two sister species are actually the eastern bluebird and the mountain bluebird. Totally crazy, I know.

But that's what the genetic data tells us. And at least when the genetic data is high quality and robust, it don't lie. The eastern bluebird and mountain bluebird are each other's closest relatives. All three bluebird species can, apparently, hybridize, at least in captivity. But it seems the most common hybrids that occur in nature are the offspring of mountain and eastern bluebird pairs. This provides even more support for the idea that these two species are each other's closest relatives. So, in that case, what's the deal with the plumage colors among the three species? Well, there was a study in 2006 that

compared the microscopic feather structure of all three bluebird species. Mountain bluebirds don't have any melanin pigment in the outermost layer of their feathers, a layer called the cortex. The rusty or orange feathers of the other two species do have melanin in the cortex. The simplest, the most parsimonious explanation, is that the single common ancestor of all three species had both blue and rust-colored feathers. Then, one lineage split off to become the ancestor of the eastern and mountain bluebirds. Finally, most recently, the mountain bluebird went through some genetic evolutionary changes that got rid of the melanin in the cortex of its feathers. No more rust or orange color for that species. The eastern and

western bluebirds, however, still show their ancestral plumage coloration. Maybe this is like an extended family where most of the members keep to their conservative traditional ways. They dress the way their ancestors did, eat the same foods their ancestors did, and so on. But then there's that one kid in a family who discovers punk rock and decides to rebel against the old ways. They dye their hair blue, start reading Jack Kerouac, and watch a bunch of French New Wave films. They start flying around in the mountains, and they learn how to scare the bejesis out of Sawaskara, the spirit of winter. Or something like that. I don't know. I guess that analogy sort of went off the rails, but you get the idea, I think. This bluebird plumage color evolution

situation is pretty cool. Among other things, it's an example of something in nature being more complex than we once thought. Moving on to the conservation situation with bluebirds. All three species are in the least concern category on the IUCN Red List. Now, that's good news, but it doesn't mean these birds don't face challenges. Eastern bluebird populations have waxed and waned as humans have either created open habitats or destroyed them. An example of eastern bluebird habitat that has been widely destroyed is the long-leaf pine forests of the southeastern U.S.

Thankfully, the overall eastern bluebird population is increasing and has been for at least the last decade. Likewise, western and mountain bluebirds are increasing or at least stable overall. But populations of each species in certain regions have declined. It all depends on the local threats they face. The biggest problem is the loss or modification of habitat. Logging, fire suppression, grazing, and urbanization are all major threats. The availability of suitable nest sites is one of the key limiting factors. Habitat destruction removes trees with nest cavities.

And another threat is aggressive non-native cavity nesters. We're talking European starlings and house sparrows. Earlier I mentioned Nabs, the North American Bluebird Society. Nabs was started in 1978 by Dr. Lawrence Zellini. In a letter to the editor of a newspaper in Maryland, Dr. Zellini wrote this. Quote, The Eastern Bluebird, one of our finest and most useful songbirds, is in deep trouble almost everywhere.

Formerly, one of Maryland's more common birds, the bluebird has disappeared in recent years to such an extent that without help, its very survival may be at stake. The bluebirds are unable to compete with the starlings and house sparrows that were originally brought into the United States from Europe, and have now overrun the country in huge numbers. The only practical solution to this problem is to supply the bluebirds with large numbers of starling-proof nesting boxes. And that's what the North American Bluebird Society did, and what it still does. The organization has established bluebird trails across the country. These are networks of artificial nest boxes, boxes specially designed for bluebirds. Volunteers monitor the nest boxes along the bluebird trails. And these efforts have made a real positive difference for bluebirds, and that is just awesome.

It's finally time to look at what and how bluebirds eat. Diet is another thing that is shared, more or less, among all three species. In the warm months of the breeding season, bluebirds eat mostly ground-dwelling insects and other invertebrates like spiders. Invertebrates become scarce in winter, so bluebirds switch it up, adding a lot more fruit and seeds into their diet. They eat lots of small, fleshy berries. For example, a couple of plants that western bluebirds feed from in winter are mistletoe and juniper. Eastern bluebirds eat the fruits of dogwood, current, black cherry, winter berry, and yes, blueberry. Bluebirds eat blueberries, you guys. You got to love that.

Now, if I can please just see a mountain bluebird eating a blue raspberry, my life will be complete. The short, stout bills of bluebirds have a shape that's well suited for plucking insects off the ground and for plucking small berries from twigs. These birds are generally sit and wait predators. They perch on tree branches, stumps, bushes, road signs, and so on. They tend to perch low down and over bare ground or short grass. When a bluebird spots a juicy, crawling insect or spider on the ground, the bird drops from its perch and pounces on its prey. They sit, they wait, they attack. Bluebirds of all three species

will sometimes use the fly-catching foraging method. This is where they dart out from a perch to catch a flying insect in the air, then return to their perch. Mountain bluebirds are unique among their kind in that they will also hover forage. In the hover forage method, the bluebird flaps its wings rapidly to hover low over the ground. When it spots prey, it drops down and pounces. Sometimes the tables turn, though, and bluebirds themselves become the prey. The hunters become the hunted. When this happens, the predators are usually other birds. Adult bluebirds get eaten by hawks, owls, and falcons. Bluebird eggs and nestlings are often prey to critters like snakes, squirrels, chipmunks, and raccoons. Domestic cats are a major threat too, since they're

an invasive species and they kill bluebirds like it's their job. But it's definitely not their job. The job of a cat is to stay inside the house, look cute, casually knock glasses off the kitchen counter, and curl up on the laptop keyboard of its owner while said owner is trying to get some important work done. All three species in the genus Cialia are, as we discussed, doing pretty okay. Their populations are stable or even increasing. For a population to increase, you need to breed to make babies. So let's talk about the way that bluebirds do just that. These are socially monogamous birds.

A male and a female pair up for a season or for multiple seasons to raise their broods. but the social part of socially monogamous means that although the pair keeps up the outward appearance of being a devoted couple there is often some mating going on with other birds outside of the pair and now check this out the eastern bluebird was the first bird species ever in which scientists discovered that females often mate with more than one male the eggs from a single eastern bluebird brood can have an assortment of different daddies

but the resident male is apparently none the wiser, or he doesn't care, and he's the one who helps raise the chicks. So the eastern bluebird is at least partially responsible for dispelling the myth that strict monogamy is the norm in birds. We used to think that all these birds were faithful to their partners, that they practiced good old traditional family values and whatnot. But once again, birds turned out to be more complex than we gave them credit for. Some pair bonds between a female bluebird and her social mate can last for years. In a study of Western bluebirds in California, out of 117 pairs that returned to their breeding grounds for two years in a row, only seven pairs changed their mates over the course of the 12-year study.

Then we have cooperative breeding. Among bluebirds, this is mostly something that happens in the Western bluebird. This is where chicks are raised by not just their mom and dad, but also by some helpers. These can be adult males in their second year, juveniles from an earlier brood, or even adult pairs. Helpers are usually the offspring of the primary pair, or at least close relatives of some kind. Depending on the year and location, the percentage of Western Bluebird pairs that have helpers at the nest ranges from 3 to 16%. There are many nuances and interesting twists on the cooperative breeding behavior of this species. This is an active area of research, and maybe I'll talk more about these birds when I do a podcast episode on cooperative breeding.

On more rare occasions, helpers are not closely related to the pear at the nest. Sometimes they aren't even bluebirds. There are some published observations of swallows helping to raise baby bluebirds. I'm going to read you an excerpt from a study published in 1980. in the Journal of Field Ornithology. These observations were made right here in my home state of Oregon. Quote, in three independent situations, one in 1981 and two in 1982, violet green swallows were observed occupying boxes in which Western bluebirds were

currently nesting, individuals of both species often occupying the nest box simultaneously. In all three cases, swallows were first discovered in the boxes when the bluebird nestlings were 12 to 13 days old. During the period of co-occupation, which extended up to the time of bluebird fledging, none of the swallow pairs were nesting, yet they provided active care to bluebird nestlings and participated in defense of the nest. On several occasions, we observed the resident female swallow remove fecal sacks from the bluebird nest. Sacks were carried five to ten meters away before being dropped. Although direct observation, of bluebird nestlings was not possible. Indirect evidence suggests that swallows also fed nestlings.

Both male and female swallows paid frequent brief visits to the nest in a pattern that resembled that of parents bringing food to chicks. And visits were interspersed with normal aerial foraging. Cooperating male swallows actively defended the area around the bluebird nest site from other pairs of swallows, including both violet green and tree swallows. end quote. How crazy and adorable is that? Swallows helping to raise baby bluebirds. And the happy ending to the story is that somebody's house cat jumped out of the bushes and ate every last one of the birds, both swallows and bluebirds. Just kidding, that wouldn't be happy at all, would it? That would be terrible? And that is why

we need to keep our kitties indoors. What really happened is that in a couple of these cases, the violet-green swallows that had been helpers ended up claiming the nest box once the Western Bluebird family moved out. So, just how do pairs of bluebirds form in the first place? Breeding behavior is complex in these little birds, and there are many unanswered questions about it. In any case, males sing and they show off their lovely plumage for females.

But I think it's safe to say that the primary factor in pair formation is the nest cavity. Male bluebirds, at least the Western and Eastern, perform a nest demonstration display for visiting females. Here's what that looks like. The male has a nest cavity in his territory. He's very proud of his nest cavity. While a prospecting female is watching, the male holds some nest material at the entrance of the cavity. He's like, see, you put the grass in here. This is the best nest cavity.

I own it. You put the grass inside, like this. Male proceeds to look around, still holding the piece of grass or whatever. Then he pokes his head into the entrance hole. He goes in and out repeatedly, then makes a wing-waving display. Again, this whole thing is a behavior called the nest demonstration display. But it's the quality and location of the nest cavity that seem to matter most to the female. If she likes what she sees, she enters the nest cavity and the pear bond is formed.

The mountain bluebird male makes a slightly different display. He sings at or near his nest cavity. While the female watches, he flies back and forth from the cavity to a perch near her. I kicked off this episode talking about the fierce territoriality of bluebirds. It's not that bluebirds are unusually aggressive or territorial. For songbirds, they're sort of typical in that regard. The point I was trying to make is that nor are they unusual. gentle or passive, as the romantic musings of some old-timey authors would have us believe.

Okay, so maybe bluebirds are a little more aggressive than the average songbird. I mean, after all, the thing they're usually fighting over is the nest cavity, and nest cavities are highly valuable. Bluebirds are secondary cavity nesters. That means they don't excavate their own nest holes. They depend on other birds or other animals to do that work. Woodpeckers, for example, are primary cavity nesters. They excavate their own holes, which are used by secondary cavity nesters once the woodpeckers have abandoned them. Because these nest cavities are in such short supply, secondary cavity nesters like

bluebirds and swallows, sometimes have to fight over them. Once the nest site has been chosen, and perhaps fought over, nest construction begins. The female does most, if not all of the work, in building the cup-like structure. Her materials are things like dry grass, straw, conifer needles, strips of bark and moss. The inside of the nest is lined with softer things like feathers and fine grass.

A typical clutch includes about five eggs, ranging from two to seven. The eggs are a very pale blue color. Only the female incubates the eggs and broods the altricial helpless nestlings. The male delivers beakfuls of crunchy, squishy insects to the female while she's sitting on the eggs. And then both parents feed the wiggling nestlings once they've hatched.

The young birds fledge and leave the nest between 18 and 21 days after hatching. They'll depend on mom and dad for food for another one to three weeks. After that, they can feed themselves, but they may hang around and beg for food from their parents. This stage can last a month or more. It's normal for bluebirds, especially eastern and western, to raise a second and maybe even a third successful brood in a single season. Young birds from the season's first brood usually strike out on their own in the summer, leaving their parents. But youngsters from the second or third brood often stay with their parents through the following winter. Bluebirds probably live for just a couple years, on average.

But some tough and or lucky individuals will live five years or more. The oldest known eastern bluebird was ten and a half years old. The oldest western and mountain bluebirds were eight and a half and nine respectively. I wish all bluebirds, young and old, lots of luck. As they swoop around in the open forests, savannas and mountain meadows of North America, singing their soft little songs and dropping to the ground to snatch insects, they make the world a better place with their beautiful colors and their complex personalities, which are sometimes belligerent and sometimes gentle.

That does it for episode 97. I hope you leveled up your knowledge about bluebirds today. If so, go out and share some of what you learned with your friends and family, why don't you? or force them to listen to this episode. Either way, you'll be doing them a favor. And what's better than learning about bluebirds? What's better than listening to me yammer on and on about them? Going out and actually seeing bluebirds.

That's what. If possible, I encourage you to get out there and find some bluebirds in the wild. I'm going to see if I can do that next week. I'll be camping in Eastern Oregon. I want to give a gigantic thank you to all my supporters on Patreon. And the newest additions to my Patreon community are Simon and Colleen Swallow. Thank you both very much for becoming members and making this show possible.

If you, dear listener, love this podcast. If it's helping you grow your knowledge about birds, one of the best things you can do to help me make more episodes is support the show through Patreon. If you're interested in becoming a member, check out my Patreon page over at patreon.com slash science of birds. There's also a support the show. link in the show notes way down at the bottom somewhere. If you have something you'd like to share with me, please go ahead and shoot me an email.

Maybe you have a thoughtful comment about the podcast. Or you want to tell me your favorite flavor of snow cone. Or which is your favorite French New Wave film? I'd have to say that mine is probably, Honey I Shrunk the Kids. I'm pretty sure that qualifies as French New Wave, right? In any case, my address is Ivan at Science of Birds.com. This again is episode 97.

You can check out the show notes for the episode, along with some hand-selected photos of bluebirds on the Science of Birds website, scienceofbirds.com. I'm Ivan Philipson. I appreciate you being here with me today, and I'll see all you next time. Peace.