The Science of Birds - Crossbills

Episode Date: February 16, 2025

In this episode, which is number 111, Ivan Phillipsen explores the fascinating world of crossbills, a unique group of finches with an extraordinary beak adaptation. These birds, found across North America, Europe, and Asia, use their specialized crossed bills to extract seeds from conifer cones—a skill few other birds possess. Ivan discusses the six recognized crossbill species, their distinct feeding strategies, and how their beaks evolved for this specialized diet.The episode highlights the complex classification of crossbills, particularly the Red Crossbill, which has multiple "call types" that may represent different evolutionary paths. Ivan explains how these birds are nomadic, moving vast distances in search of conifer seed crops, and how some species, like the Cassia Crossbill, have adapted to specific tree species. The episode also covers the recent debate in the ornithological community about whether certain crossbill populations should be considered full species. Additionally, Ivan discusses how crossbills’ courtship behavior, nesting habits, and vocalizations vary among species.Links of Interest Crossbills of North America: Species and Red Crossbill Call TypesWhite-winged Crossbill feeding [VIDEO] ~~ Leave me a review using Podchaser ~~Link to this episode on the Science of Birds website Support the show

Hello and welcome. This is the Science of Birds. I am your host, Ivan Philipson. The Science of Birds podcast is a lighthearted exploration of bird biology for lifelong learners. This is episode 111. And it's all about crossbills. This is a special group of finches. Crossbills are all the birds belonging to the genus Loxia,

and most ornithologists agree that there are six species in that genus. Finches in general, members of the avian family fringility, are specialized seed eaters. They're granivores. The typical bill of a finch is stout and cone-shaped. It's an excellent tool for cracking open small seeds. Crossbills are among the most specialized members of the Finch family. They eat seeds, yes, but their focus is on the seeds hidden inside the cones of coniferous trees.

We're talking pine cones, spruce cones, and so on. From a distance, you might mistake a crossbill for something like a house finch. But if you get a closer look, you'll see that there's something really weird about its beak. like it's deformed or something. The top and bottom halves of the bill are indeed crossed. In this episode, we'll talk about why crossbills are so unusual, what's going on with that strange beak, and why scientists have been fascinated by these birds for a long time.

First, let's get familiar with the birds. with the basic traits of crossbills, what they look and sound like. If we ignore the bill for just a moment, if we just look at the body, we see that the body and wing shape of a crossbill are pretty typical for a small songbird, nothing unusual there. But I suppose the head might be on the large-ish side, proportionally. In terms of overall size, the six species are roughly similar, somewhere between 5.5 to 7.5 inches long,

or 14 to 20 centimeters. But within the red cross-bill species, there are different populations, or types, and these show a range of body and beak sizes. But yeah, how about that bill? That's far and away the most interesting feature of these birds. At least I think. Instead of having the typical cone-shaped bill of a generic finch, a cross-bill's bill is longer, and the top and bottom halves end in pointy little hooks.

The upper mandible curves down and the lower mandible curves up. The two halves criss-cross each other, making a sort of X shape. This is sort of like when you cross your index and middle fingers. It makes a similar shape. You know, like when you make a promise to your best friend and you cross your fingers behind your back to make it sort of okay that you're lying and have no intention to honor the promise? Like when I crossed my fingers when I said to my friend,

Of course I'll take care of your pet cats if you get hit by a truck and die. You can count on me. A key thing to point out here is that the beak shape is asymmetrical in the left to right dimension. So we describe it as being laterally asymmetrical. That's a super rare trait in the beaks of birds. There are only a few other species I can think of with this kind of lateral asymmetry in the beak. First, there's the Rye Bill of New Zealand, W-R-Y-B-I-L. That's a member of the Plover family.

The entire tip of the Rye-Bills bill curves to the right. And then there are two Hawaiian honey creepers. One is the Akeke'e from the island of Kauai, and I hope I pronounce that right, and the other is the Akepa on the big island. They're both in the genus Locksops, and they too have bills that crisscross. but not quite as conspicuously as what we see in the crossbills. In some individual crossbills, the lower mandible crosses the upper mandible on the right side. In other individuals, it crosses to the left.

In the red crossbill, it seems the ratio of crossing directions is one to one. In other words, you're just as likely to find a red crossbill with a lower bill that twists to the right as one whose lower bill twists to the left. But interestingly, this ratio is different for some populations of another species, the white-winged crossbill. In that species, the ratio can be more like three to one, with three times as many individuals having a lower bill that crosses to the right. So how did the crossbill get its unique beak? Well, one legend from medieval Europe explains that the red crossbill, being a bird of pure heart and honorable intentions, tried to help Jesus when he was crucified on the cross. And since I just said cross, you might be thinking,

oh, I get it. Jesus is on the cross, cross bill. I totally get it. Nice try, but nope, that's not it. Because there's more. The legend is that the brave little bird tried desperately to pry out the nails from Jesus's hands. Not only did the cross bill fail miserably, but in the process of working on those nails, the bird's beak got all bent out. of shape, permanently, and somehow that acquired trait got passed down to the bird's offspring. So if this legend were true, it would be an example of Lamarckian evolution. You know, Lamarckian evolution, the very much disproven idea that organisms can pass on traits they acquire during their lifetime to their offspring? As you might guess, the shape of a crossbills beak

is actually related to what and how these birds eat. We'll go into more detail later on how these birds eat, but basically, that weird asymmetric bill is great for prying open the sealed scales on a pine cone or a spruce cone or whatever. So a crossbill is equipped with a specialized tool that gives it access to conifer seeds, a food source that few other animals can reach.

There are six crossbill species. and each tends to specialize in feeding from one or a few conifer tree species. The cones of these trees come in different shapes and sizes. Depending on the tree species, the scales on the cones might be thin and papery or thick and woody, and the seeds themselves have different sizes. So beak sizes and shapes vary among the crossbill species because they are adapted to specific cone types. For example, the parrot crossbill, true to its name, has the biggest, chunkiest beak of all,

and it specializes on eating the seeds of Scots pine. And even within species, there are populations that specialize on different cones, and therefore those birds have slightly different beaks. Pretty cool, right? But we'll talk more about beaks in a moment. In terms of their feathers and plumage, male crossbills are red. Most are brick red or an orange red. Male white-winged crossbills, however, are more of a scarlet or vermilion red.

And they have a couple white wing bars. The old medieval legend that I mentioned earlier also explained that crossbills are red because when that proverbial bird tried to pry out the nails, it got soaked with the blood of Jesus. But how about that other legend? The one that says crossbills look the way they do because long ago, a sneaky crossbill was in a parking lot one night, trying to use its beak to break into someone's Hyundai Alantra. But then the car's owner walks up, sees what's going on, and they're like, hey, what are you doing? Get out of here, you no good bird. And they scare off the crossbill by dousing it with a bottle of Mountain Dew Code Red.

And that children is how the crossbill got its twisted beak and its red. color. Yeah, so anyway, males are red or they're kind of an orangish color. But females, structurally females look very similar to the males. But instead of being red, they're yellowish or sometimes an olive gray color. As in many other finches, the extent of red in a male crossbills plumage might be used as a signal to females which males are the best males. Because that red color comes, not from Mountain Dew Code Red, but from carotenoid pigments in the diet. Males with the most red in their feathers might be the ones who are the best at finding food, and therefore the best at feeding a female and her chicks during the breeding season.

Now it's time to get into the vocal sounds of crossbills. Like most other finches, male crossbills have songs, and both sexes make a variety of calls. For our first example, we have the song of a male white-winged crossbill, Laxia Lucoptera. This species is also known as the two-barred crossbill. That's what they typically call it in Europe. But this particular male is singing from the top of a balsam fir tree in Newfoundland, Canada. Again, that was a white-winged or two-barred crossbill. Next we have that species' closest relative, the Hispaniolan crossbill, Loxia Megaplaga. We'll hear some simple calls being made by Hispaniolan crossbills in the diminutive.

and Republic. Over in Europe, one of the species is the parrot crossbill that I mentioned, Laxia Pitaeopsetakis. This is the sound of a flock of parrot crossbills in Sweden. Then we have the Mountain Dew Code Red Crossbill, Loxia Curva Rostra. But that common name is way too much of a mouthful, so we usually shorten it to just Red Crossbill. This is probably the most familiar crossbill species to most of us, and it's been extensively studied by ornithologists. And now that we're talking about the Red Crossbill, this is as good a time as any to introduce you to the idea. of call types. This species lives in forests all across the northern latitudes, in North America, Europe, and Asia.

Traditionally, Red Crossbills were categorized into subspecies based on variations in bill and body size, and these subspecies were thought to have non-overlapping breeding ranges. However, ornithologists eventually figured out that many of the subspecies actually have pretty widely overlapping breeding distributions, and that goes against the way we expect subspecies to operate. So ornithologists went back to the lab, did some research and hard thinking, and they decided that variation in bill and body size in the red cross bill is better categorized by vocalizations, especially the contact calls. Remember that contact calls are short, simple sounds that birds use to stay in touch with each other,

often when they're flying around in a flock. So now these days, we often talk about the distinct vocalizations of red crossbills as defining different call types or egot types. Okay, so each call type represents a somewhat distinct population of red crossbills. We're not really thinking of them as subspecies, but something a little different. And hey, let's make things more confusing.

there are still some subspecies of red crossbills, especially in Eurasia. So there's a lot going on here. It's a complex situation. But this complexity is one of the big reasons crossbills are so interesting to scientists. Anyway, each call type appears to be associated with, and quite possibly adapted to a particular conifer species. This specialization is reflected in bill shape and the anatomy of the palate, inside the mouth. I'll come back to this topic when we talk about crossbill evolution, but for the moment let's get back to listening to some vocalizations. Here are some nice calls from red crossbills in West Virginia. Now I don't know what call type of these are, so let's just

appreciate the typical sound of a red crossbill flock. Next we have a red crossbill calling from the branches of a ponderosa pine tree. This was recorded in South Dakota. The call type of that bird is a type 2. And what kinds of cones do you think type 2 red crossbills speak? specialize in? That's right, the cones of ponderosa pine. I have another red crossbill recording to play for you. This one is from call type N-11, also known as the Carpathian crossbill. This type was originally identified in the Carpathian Mountains of Central Europe. Okay, so this is a

European individual. And finally, we have the vocal sounds of the Scottish crossbill, Loxia Skodica. This one is easy to identify by its thick accent. We're standing here down on the borders. We're just outside Melrose, which is a famous town. Over the back of us here, we've got the Eogne, one of the most famous landmarks on the borders. It's three hills. And the myth is that Merlin, the magicians split one hill into three.

All right, now let's get into the diversity. distribution, and habitats of crossbills. As I mentioned, these birds are members of the Finch family, Fringility. Episode 22 of the podcast was all about that family. There are three sub-families within the family fringility, and the crossbills belong to the Cardwellini sub-family. That's the one that includes most of the world's finch species. All six cross-bill species belong to the genus Loxia,

spelled L-O-X-I-A. This comes from the Greek word loxosos, L-O-X-O-S, which means crooked, oblique, or twisted. And remember I mentioned those two Hawaiian honey creepers that also have slightly crossed bills? Well, they're also in the Finch family, and their genus name Loxops, L-O-X-P-S,

as the same root word. So the six cross-bill species are, the red crossbill, Cassia crossbill, Scottish crossbill. That's right, there really is a Scottish crossbill, and then there's the parrot crossbill, the white-winged or two-barred crossbill, and finally we have the Hispaniolan crossbill. Before 2017, there were only five crossbill species. That's because the Cassia crossbill, Loxia Sinicayurus, was only recently recognized as a species. We'll talk more about that in a second. Now, in case you're wondering, the closest relatives of crossbills are the red poles, or I guess I should say the red pole. Because until recently, ornithologists

recognized three species of red poles, all in the genus Acanthus. But in 2024, the three species were lumped into one species, the red pole, acanthus flamia. So yeah, anyway, that is the finch species most closely related to our crossbills. In terms of their geographic distribution across the planet, crossbills are found only in the northern hemisphere, mostly at the higher latitudes where there are vast expanses of coniferous forest. Crossbills of one species or another are found across North America, Europe, and Asia. The red crossbill is the most widespread.

I've seen this species in many places in Western North America, but also in Spain, Iceland, Vietnam, China, and Bhutan. Some red crossbill populations are found as far south as Central America, Southeast Asia, and Northern Africa. Birds in these southern populations still depend on coniferous trees, though, so they live only high in the mountains where conifers grow. The white-winged crossbill, too, has a large distribution that spans both hemispheres. But the other species have relatively small distributions. The Parrot Crossbill lives only in Northern Europe.

The Scottish Crossbill lives in Northern Scotland. The Hispaniolan Crossbill lives in just a few patches of forest on the Caribbean Island of Hispaniola. And the Cassia Crossbill has a teeny tiny distribution in just one small part of Idaho. It's just a little dot on the map of North America. Now it goes without saying that the key component of habitat for crossbills is the presence of coniferousal. forest. Each species and each call type prefers forest with the tree species it's best adapted to. Conifer species used by these birds include Douglas fir, many species of pine, as well as spruces, furs, larches, and hemlocks. Where the birds actually live and breed depends on the availability

of conifer seeds. It's not enough just for the trees to be there. There have to be productive cones. A key behavior of crossbills is that they live in flocks all year long, and flocks require large crops of cones. But this food resource varies a lot, both seasonally, so through time, and across the landscape, spatially. So most crossbill populations are nomadic. Flocks wander around, sometimes over thousands of miles, to find patches of forest where cones are plentiful and full of seeds. In some years, in some places, large cone crops allow crossbills to hang around for months and months, filling their tiny bellies with tasty seeds and maybe cranking out some babies. When seeds are scarce, however, the crossbills move to greener pastures, so to speak. They

search the land far and wide for food. This can lead to the phenomenon of eruption that's spelled I-R-R-U-P-T-I-O-N. Eruptions are the irregular, large-scale movements of birds, including crossbills, outside their typical range. And the key word there is irregular, because unlike annual migrations, eruptions are more random in time and space. In crossbills, eruptions occur when cone crops in their typical forest habitats are scarce, forcing the birds to move southward or to lower elevations in search of food.

For us birders, a crossbill eruption can be an exciting event. Maybe you live somewhere outside the typical zone of occurrence for crossbills. You basically never see them. But then one winter day, you're sitting on your back porch, sipping some tea or maybe some mountain dew code red, when the sun is suddenly blotted out. No, it's not just a whiff of cloud. it's a flock of red crossbills. The red and yellow birds descend on your neighborhood pine trees, attacking the cones with great gusto.

You fall to your knees and weep at the beauty of it all. Your life is now complete. Let's jump in our time machine now and travel back to the ancient coniferous forests of about four. million years ago. We're going to look at the evolution of crossbills. So here we are, four million years ago, walking around in a beautiful forest in the far northern latitudes. And we see a little finch hopping around among the needles of a spruce tree. This is the ancestor of modern crossbills. It likes to feed on the seeds hidden within cones, among other things, but maybe its bill still looks like that of a typical finch. Cone crops went through periods of boom and bust four million years ago, just like they do today.

We can imagine that during a particularly bad year, some flocks of our ancestral finch flew out of the coniferous forests, desperate to find food elsewhere. They ended up settling in forests where they could eat the seeds of trees like alder and birch. This wayward group of finches ended up evolving into the Red Pole. The red pole has a small bill that isn't crossed. It eats many kinds of seeds. Those of conifers, yes, but also other kinds of trees and all sorts of shrubs, herbaceous plants, and grasses. The ancestral finches that stayed behind among the pines and spruces, well, they survived and evolved. They grew larger and developed their trademark beaks to become the crossbills. Now that's cool and all, but if you

think like a scientist, you might be feeling skeptical here. I mean, how did a finch with a standard issue cone-shaped bill evolve that wacky asymmetrical bill? What possible advantages could have existed in the early and intermediate stages when the two halves of the bill crossed over each other only slightly? How could natural selection have favored that situation? Well, there's actually a really cool study that suggests how this might have happened in the evolution of crossbills. The study was published in 1991 in the journal Nature. The researchers, Craig Benkman and Anna Lindholm, ran a clever experiment. In their controlled study, Benkman and Lindholm trimmed the crossed portion of the beaks on four red crossbills, while another group of

three crossbills was left unaltered as the control group. The trimming wasn't harmful, to the birds. This is basically like you trimming your fingernails. The beak tips don't have any nerve endings. The birds with these artificially straightened beaks were just as good at extracting seeds from open cones as the control group. Right? So open cones with the scales spread apart. But these birds had a hard time with closed cones. And it's really closed cones that are the specialty of crossbills. However, as their beaks re-grew, the ability of these birds to access seeds in closed cones got better and better. Interestingly, this regrowth mirrors the natural beak growth of young birds after fledging. Because a crossbill actually hatches out of the egg with a straight, symmetrical beak.

Within a month, the crossbills in the experimental group had regrown their bills. Their seed extraction skills returned to normal. And this showed a clear correlation between beak shape and function. The cool idea here is that beak regrowth in this experiment simulates the steps of evolution. It's like watching a couple million years of evolution in the beak of crossbills on fast forward, so that it all happens in just one month. And you're seeing a gradual increase in foraging efficiency as the mandibles go from uncrossed to crossed. Pretty neat, huh?

So that was the Benkman and Lynn Holm study of 1991. Now, let's return to looking at the evolution of crossbills. Over the last few million years, the common ancestor of the crossbill diversified into the handful of species we have today. So these birds went through a little adaptive radiation. In other words, one species split into multiple species in a relatively short amount of geological time. The primary mechanism behind this adaptive radiation is most likely divergent selection for using

different food resources, such that each species becomes well adapted for extracting the seeds of a specific type of conifer cone. The last major split in the crossbill lineage happened roughly a million years ago during the Pleistocene Epic. That's when the white-winged cross-bill split from the Red Crossbill. Each of these lineages has gone through further diversification in the last million years. And since the last glacial period of the Ice Age ended, about 10,000 years ago, the Red Cross Bill has diversified into the many call types and into some subspecies. In North America, ornithologists recognize 10 call types. The differences in anatomy, genetics, and ecology among these call types, might reflect the early stages of speciation.

So given another million years or so, some of the call types might evolve into full-fledged species. Who knows? There's one more thing I'd like to talk about with regard to evolution. Recall that the Cassia crossbill, Laxia Sinusayurus,

wasn't recognized as a separate species before 2017. It used to be treated as just a call type of the Red Cross Bill. It was Type 9, the South Hills Red Cross Bill. Just looking at a Cassia Crossbill in the wild, you can't tell it apart from the Red Cross Bill. It's really only the voice of this species that we can use to tell it apart in the field. And even that is not easy for most of us. So we'd say that the Cassia Crossbill is a cryptic species, kind of a hidden species. These birds live only in the lodgepole pine forests of the South Hills in Idaho,

and the name Cassia comes from Cassia County, where most of these birds are found. We can thank Craig Benkman, a professor at the University of Wyoming, for discovering that the Cassia Crossbill is a distinct species. If that name is familiar, that's because Dr. Benkman was also the lead author on that study where they clipped the beaks of crossbills. So, Bankman and his colleagues studied the Type 9 Red Cross Bill for years before making the case that it should be treated as a species. Their reasoning was that Cassia Crossbills, unlike most Red Crossbills, don't wander from forest to forest.

So they aren't nomadic. They stay in their safe space in southern Idaho. They use their relatively large bills to open the rather large cones of the local lodgepole pines. Some red crossbills like type 5 and type 2 will sometimes stray into the South Hills of Idaho, but they rarely stick around to breed. The lodgepole pine cones are too large for their bills, and even when those birds do breed there, they tend to choose partners with their same call type, rather than with the resident Cassia or aka Type 9 crossbills. So this kind of mating, only between members of the same call type,

suggests that Cassia and Red Crossbills have been and will continue to be on separate evolutionary paths. One of the interesting parts of the Cassia Crossbills story is that in the South Hills of Idaho, there are no squirrels. In other similar North American forests, squirrels are competitors of crossbills

for conifer seeds. But the South Hills are surrounded by dry, high desert habitat and have been isolated like this for thousands of years since the end of the last ice age, with no squirrels acting as a force of natural selection on the cones of lodgepole pine. And that is why the specific epithet, the scientific species name for the Cassia Crossbill is Sinicayurus. Sinicayurus, spelled S-I-N-E-S-C-I-U-R-I-S, is Latin and means without squirrels. So in the absence of pesky squirrels,

the pines have ended up with these extra large cones. And those pines have been co-evolving with crossbills instead of squirrels in an evolutionary arms race. The escalating defenses of the pine cones against the cross bill led to the bird developing a more efficient bill shape for opening the cones. And what's more, the lodgepole pine cones

in the south hills are extra difficult to open up because the scales are sealed shut by sticky sap. It's like they're glued shut. The cones tend to persist on the tree branches rather than fall to the ground, and they can stay shut for decades before the heat of a fire or an extreme heat wave causes them to finally burst open. Otherwise, it takes years and years for even a tiny crack to appear between the scales. But once that happens, the Cassia Crossbills swoop in and start going to work. So this whole situation causes there to be a long-term, somewhat reliable supply of cones in the forest. There's an enormous seed bank there. But trouble may be brewing for the Cassia Crossbill.

There's some controversy in the bird world with dark rumors swirling around the species status of the Cassia Crossbill. Because birders and ornithologists have been documenting these birds in forests beyond the South Hills of Idaho. Cassia crossbills have turned up as far away as Colorado and Wyoming, and I think there was even one recorded in California. Some people have pointed out that if a key piece of evidence for treating the Cassia Crossbill as a species is its geographic isolation and sedentary behavior, well, maybe that doesn't hold up after all, you know, if these birds are flying to and fro across the western states. In response, some ornithologists have proposed that we strip the Cassia Crossbill of its species status,

that we should downgrade it back to being just a call type again. But that debate rages on in the scientific community. Some ornithologists have published their arguments for why, not just the Cassia crossbill, but also the Scottish and parrot crossbills, shouldn't be treated as species. But Bankman and several other researchers published a response, defending the Cassia and the other crossbills as being real species. Based upon what I've read, what I've seen, I personally feel pretty good about the Cassia crossbill being treated as a species.

But it doesn't matter what I think, we'll just, I guess, have to see what happens in the next few years. Meanwhile, I guess I'll have my fingers crossed. Not because I'm lying or whatever. No, this time it's because I'm hoping for good luck. Cassia crossbills forever! Let's look now at the conservation status of crossbills around the world. Most of the species are listed in the least concern category on the IUCN red list. The Cassia Crossbill, however, isn't listed in any of the

categories. My guess is that because this species is so new, it was only classified as a thing that exists in 2017, scientists haven't yet assessed it to determine its conservation status. That said, there is definitely some concern about the Cassia Cross bill. It has such a small, geographic distribution that it wouldn't take much to wipe out the entire species. Probably the biggest threat is climate change. Some climate models predict that things will get too warm and dry in southern Idaho for lodgepole pines to persist there. And Cassia crossbills depend almost entirely on those pines.

So if the trees disappear, so might the birds. This is why species status can be really important in terms of conservation. If the Cassia Crossbill continues to be classified as a species, we will hopefully pay more attention to its protection and conservation. But if we decide that, nah, it's just one red crossbill call type among 10 or so other types in the new world, well, in that case, some might say it's not a big deal if that one call type goes extinct. The most threatened crossbill species is probably the Hispaniolan crossbill. It's listed in the endangered category by the IUCN.

It has a small overall population size, somewhere between 1,000 and 3,500. It also has a highly restricted distribution. These birds are found in only a few patches of habitat on just one island, and that habitat is threatened by logging, fires, and conversion to agriculture. Some researchers have predicted,

that these threats, not to mention climate change, could cause the forests of Hispaniola pine to disappear in the next 150 years. And that would most likely spell doom for the crossbills there. Now it's time to look a little more closely at the diets and foraging behaviors of crossbills. We've established how much these birds love and depend on the seeds of coniferous trees, especially those found inside the closed, sealed scales of a cone. But I should point out that crossbills sometimes eat other things too when their preferred seeds are in short supply. They'll eat the seeds and leaf buds of various other plants,

and they'll even eat some invertebrates like caterpillars, ants, spiders, and so on. But as for the conifer seeds, that's the real specialty of crossbills. And then there are deeper levels of specialization for each crossbill species and specializations among the call types and subspecies. For example, remember the type 2 red crossbill? That type prefers the hard, woody cones of pines. This type is most efficient at extracting seeds from ponderosa pine, but these birds will also feed from other pines like Lodgepole,

Jeffrey, Red, Jack, and Virginia pine. And it's not uncommon for them to eat the seeds of soft-coned conifers like spruces. As another example, we have the Type 3 Red Crossbill. This one specializes in feeding from the diminutive cones of Western Hemlock. This is, I believe, the smallest Red Crossbill call type in North America, in terms of body size, and it has the smallest bill, a bill that corresponds well to the western hemlock cones. As for the diets of the other crossbill species,

parrot and Scottish crossbills both prefer Scots pine. The Hispaniolan crossbill eats mostly the seeds of Hispaniolan pine, and white-winged crossbills prefer to forage from spruce and larch cones. Amazingly, white-winged crossbills are known to eat as much, much as 3,000 conifer seeds in a day. And that's probably similar to what the other species do. But hey, I've been dancing around the actual question of how a crossed bill works as a tool. How do these birds actually feed from cones? Here's the basic process. The cross bill perches on top of a cone, or even hangs upside down from it, like a little tiny parrot. The bird wedges

the tip of its beak between two cone scales to reach the seed tucked deep inside. But to do that, the cross bill twists its head and lower mandibles sideways to pry the scales apart. So it uses its bill sort of like a crowbar to get leverage. Once the seed is exposed, the bird scoops it out with its tongue, or sometimes it uses the hooked tip of the upper mandible to pry out the seed. The tongue, which can be extended well beyond the beak, is held in a groove in the upper mandible, in the palate. And this groove perfectly matches the size of the seed.

So finally, the bird cracks the seed open by applying upward pressure with a ridge on its lower mandible. So it squeezes and cracks the seed between the upper and lower mandible using these grooves and ridges. And it swallows the good stuff and tosses the seed husk aside. Crossbills will sometimes break off a cone first and then carry it to a perch. The bird then works over the cone, scale by scale, rotating it while pinning it down with one foot. So this is sort of like eating corn on the cob.

The process of extracting a single seed might take just two or three seconds, and that's how these birds are able to eat, like 3,000 seeds in a day. Crossbills are certainly social birds, but they often fight each. other over cones or over perching sites or mates. They fight on tree branches in the air or tumbling around on the ground. It can be pretty dramatic. Anyway, if you'd like to see this, get a visual. I'll put a link in the show notes to a YouTube video by the Cornell Lab of Ornithology that shows how feeding works in the white-winged crossbills. We come now to the discussion of breeding in crossbills.

One of the interesting things about these birds is that they're able to breed at just about any time of the year. For example, white-winged crossbills have been documented breeding in each of the 12 months. Crossbills in general are nomadic little buggers that will settle down to breed whenever and wherever they find a large supply of conifer seeds. Sometimes it turns out that that's in the middle of the winter. It's most common for them to breed in late winter or in spring. A crossbill might breed a couple times in a given year when conditions are good. In other years, however, it might not breed at all if seeds are in short supply. These birds are monogamous, one male pairs with one female.

And some research suggests that mating outside of a bonded pair is rare, or non-nonage. When two crossbills initially find each other and fall in love, there's some courtship behavior involved. First, the male sings to attractive mate. Females may judge males on their songs and on their red carotenoid-rich plumage. Then males chase females as they fly through the forest, and this too is a form of courtship behavior. As pairs form, they begin to engage in courtship feeding. At first, they just touch bills or nibble at each other's bills. But this progresses and eventually the male feeds the female by hacking up wads of conifer seeds and offering them to her as a tasty snack. These pairs are thought to form mostly between members of the same feeding flock and therefore

the same call type. So after courtship and mating, now it's time to make a nest. The nest is a cup made of twigs, grass, and other plant fibers. It's lined with moss, lichen, hair, and fine plant material. There are usually three eggs in a clutch, more or less. And the incubation period is typically two weeks, or maybe a little less than two weeks. And it's only the female that incubates the eggs. She'll stay on top of them non-stop for days in very cold winter weather. So she might even get covered up by snow as she works to keep her eggs warm. Meanwhile, her her male partner keeps foraging for seeds and regurgitating them for his lady. After the eggs hatch, it's still just mom that broods the chicks.

The chicks hatch out blind and mostly naked, and as I mentioned, they're born with straight, normal-looking bills. It takes about a month for the upper and lower mandibles to finally cross. Both parents feed the chicks by regurgitating a dark slurry of conifer seeds, moistened with a liquid that might be saliva and maybe a few insects in the mix. At least in the case of red crossbills, the youngsters fledge at about 18 to 22 days old. They slowly learn to manipulate cones and extract seeds. But they suck at this at first. They drop cones and they're all awkward as they try to pry the scales apart.

But after a couple months, they've honed their skills well enough. Young crossbills have a high degree of dispersal flexibility, while many stay with their natal flocks initially, so they stay in the flock they were born in, others will integrate into new groups of the same call type as they seek out food. Red crossbills have been recorded breeding as far as 1,800 miles or 3,000 kilometers away from where they were born. And that's pretty remarkable for a songbird. That's why the red crossbill has been described as being one of the most dispersive songbirds in the world. Immature crossbills often have a hard time surviving their first winter, because they're just not all that efficient at finding cones

and getting seeds out of them, compared to seasoned adults, that is. So all the more reason to stick with a flock, so that your chances of finding food and surviving are much better. If the harsh winters and starvation don't kill a young crossbill, there's always bloodthirsty predators to worry about. Crossbills in North America are prey for raptors like sharp-shinned hawk, Cooper's hawk, Merlin, Peregrine falcon, northern pygmy owl, and northern shrike. In Europe, there are other similar species like the Eurasian Sparrowhawk that terrorize crossbills. If a crossbill can survive the elements and evade all those predators, it might live for four or five years in the wild. Some of the oldest on record,

the really lucky ones, were about eight years old. I am always excited to see crossbills. I might be out birding and I hear their characteristic flight calls and I look up and I'm just searching the skies for a flock to fly over or to land in some nearby conifers. I just love these. guys. I double-checked my e-bird records, and the only species I've seen so far is the red crossbill. I hope to see the others at some point. I might be visiting the island of Hispaniola in the Dominican Republic next year, so you know I'll be looking for the Hispaniolan crossbill there. Crossbills, as a group, are interesting because they have those wonderfully weird beaks and their

unique feeding strategy. And by studying crossbills, ornithologists can help us learn a lot about the way new species form. There's still a lot we don't know about the degrees of reproductive isolation in crossbills, or to what extent the different call types and subspecies are on different evolutionary paths. But who knows? Maybe we'll decide someday that there are even more crossbill species out there. Hey, thanks for learning about crossbills with me today. I hope you picked up some new facts about these awesome little birds. As I mentioned, I've seen Red Crossbills in several countries on opposite sides of the planet.

And that's always a strange thing, seeing a bird that you have in your backyard in some far away exotic place. And not because it's an invasive species there, but just because it has such a vast distribution. You know, like the barn swallow and the peregrine falcon. Anyway, I want to say that I'm able to keep offering pie. episodes like this one, and sharing bird learnings for free to all the children of the world because of the wonderful support I get from my community on Patreon. Each patron ships in a little cash every month to keep me going and to make this show possible. So we all owe them a huge amount of gratitude.

Thank you. My newest patrons are ML, Kalila Ann, Linda LaFawn, Sam Nichols, and Steve McCartner. Welcome and much gracias to you all. Seriously, you're amazing. If you, my friend, are thinking about becoming a patron, you can check out my Patreon page over at patreon.com slash science of birds. And there's a support the show link in the show notes in your podcast app. Now, if you do plan to become a patron, that's wonderful, but I suggest you avoid using the Patreon app on any iPhone. Because you'll end up paying more. more and that extra money doesn't help me at all. So please just use your desktop or mobile browser

instead of the Patreon iOS iPhone app. If you have a few words you'd like to share with me, please go ahead and shoot me an email. Maybe you have a comment about the podcasts, or you want to weigh in on the question of species status in the Cassia Crossbill. Or maybe you want to offer your own legend for how Crossbills got their twisted beaks and red color. In any case, my address is Ivan at Scienceofbirds.com. This is and has been episode 111.

You can check out the show notes for the episode along with some curated photos of crossbills on the Science of Birds website, science of birds.com. I'm Ivan Philipson and true fact about me, I do not presently own a car. I get

around in my city of Portland, Oregon, mostly by walking. I love walking. But we also have a great public transportation system, and I've got a bike, and there's Uber, and all of that jazz too. Anyway, thanks for listening today, and I'll catch you next time. Peace.