Crossbill Finches, Evolution & Bisexual Monkeys

Parts 1-4: It’s not really about the birds…

Male South Hills crossbill. Photo by CRAIG BENKMAN

…it’s never about the birds... It’s always about something more important.

Part 1. How’d that happen?

Odd looking bird, isn’t it? It’s a South Hills crossbill finch, one of about six crossbill species. This one lives in Idaho, USA. You’ve probably guessed that they’re named for that unusual beak. It starts out okay but keeps growing until they’re about 45 days old. The result is that the bottom half gets pushed to the right or left and ends up crossing over the top half.

According to medieval legend, this began on Good Friday when some crossbills twisted their beaks trying to pull the crucifixion nails from Jesus’ hands. Thereafter, by some divine intervention, all crossbills displayed the trait.

From “The Legend of the Crossbill” by Longfellow:

And by all the world forsaken,

Sees he how with zealous care

At the ruthless nail of iron

A little bird is striving there.

Stained with blood and never tiring,

With its beak it doth not cease,

From the cross it would free the Saviour,

Its Creator's son release.

It’s a moving poem, even if you’re not religious. But, to be honest, I’m not sure the more “scientific” explanation is any more believable.

Part 2. So, What's a Finch Beak Supposed to Look Like, anyway?

Generally, birds have beaks that align, like the jaws on a pair of pliers. If you’ve ever taken a biology course you’ll remember “Darwin’s finches,” a group of about 13 species, all but one of which lives on the Galapagos Islands (west of Ecuador). The other one lives on Cocos Island (southwest of Costa Rica).

Interestingly, Darwin’s finches aren’t real finches. They’re tanagers. Nevertheless, they make an appearance as finches in every Introductory Biology course that I know of. They’re touted as a quintessential example of evolution by natural selection.

Darwin’s finches gained notoriety for two reasons. First, of course, is that they were collected and studied by Darwin. The second is that they fit a popular narrative about evolution which, by the way, is generally mis-taught and misunderstood.

Here’s the backstory: Darwin’s finches apparently evolved from a single species that got stranded on the Galapagos Islands around 2 million years ago. Over time, their descendants evolved into about 14 species that differ in size, song, color, and — most famously — in beak shape and feeding behavior. It’s the beak & feeding thing that’s misunderstood.

Darwin’s finches have beaks that range from small, thin and pointed to large, thick and stout. And, as you might expect, different species tend to eat different foods. That’s led most teachers, science writers, and textbook authors to claim that these different beaks evolved — or adapted — to the particular foods that the birds found on the islands: “…the finches had to evolve beaks which could take advantage of the food supplies available to them… [So,] different finch populations evolved to eat different food sources (emphasis added).”

The outcome is usually described like this: The cactus finch has a long sharp beak adapted for biting off pieces of cactus pulp, little ground finches have small broad beaks just right for eating small seeds and flowers, insect-eating finches have tweezer-like beaks perfect for grabbing insects, large ground finches have stout beaks adapted to crack open the largest seeds, and the so-called, “vampire finch” has a needle-sharp beak “perfectly adapted” for stabbing other birds and slurping up their oozing blood, and so on. In other words, each species has “…distinctive beaks adapted for their specific diet.”

Pretty cool story, huh? With few exceptions (even among professors), this is how everybody talks about evolution. Consequently, it’s how virtually everyone thinks about evolution: Whatever you see in nature has some purpose, everything has evolved to do whatever it’s currently doing, and organisms are, therefore, perfectly adapted to their environments.

Yes, it is a cool story, but it’s not just wrong… it’s impossible. First, organisms — either individually or as a group — can’t evolve in order to do something, or adapt to something. Consequently, there really isn’t any such thing as being “perfectly adapted.”

In the first case, it’s pretty obvious that individuals can’t adapt to an environmental need. Here’s my classroom explanation: Imagine you went out to a club and saw a really attractive person at the bar that you wanted to meet. All of a sudden, you can’t get better looking, can you? You’ve got to play the cards that God dealt you, as my grandmother would say. Look in the mirror. That’s your A-Game.  You can only work with what you have. If it doesn’t work, you’re out of the gene pool. Capisci?

But, how about the group? Don’t groups evolve? Well, yes, but not in the teleological way that most people think. Let’s do the thought experiment. Work with me here…

Let’s say a flock of finches (all the same species) get trapped in a Whole Foods grocery store. They start flying around looking for something to eat. (That’s what happened on the Galapagos, right?)

The first thing that you gotta’ realize is that no two of the finches are exactly alike. Just like people, some will be stronger or more aggressive, some fatter, some slower, some will have bigger beaks, some will have better eyesight, some will be smarter, and some will have different food and color preferences. Also, some won’t have a clue.

After a bit of random exploring, let’s say a small group finds its way to the cheese display and they’re persistent enough to peck through the heavy wax wrapper around the Gouda. They like the cheese, stay congregated around the display, and chase off any potential interlopers.

The more skittish birds might flit around the fresh produce nipping off bits of broccoli and leafy greens. Others, attracted by bright colors, settle on the fruit shelves and are content with their choice. And, so on… The birds that can’t figure out what to do will starve to death.

After a while, there’ll be several bird communities spending most of their time living, loving and singing in each of these foodie habitats. Occasionally, some birds will visit other food isles, eating a bit here and there, but all will have their preferred foods and favorite roosting sites.

So, let me ask you, did any of these birds adapt to the foods they ended up eating? Did the broccoli eateries adapt to the broccoli? Did the cheese eaters adapt to the Gouda? Or did they all just play the cards that nature dealt them and make the best of an awkward situation?

Here’s another question. Let’s say there’s a mutation among the cheese eaters. You know, an anomaly like we talked about in the last post. What if one of the cheese eaters laid an egg with a mutated gene that caused the hatchling’s beak to keep growing longer than usual. In fact, its beak grew so long that the bottom half twisted to the side and grew over the top half. Despite the facial deformity, however, this young bird learned to tilt its head sideways, tear open the Gouda wrapper and scrape out some cheese. After a while, it became pretty efficient, but still had a heck of a time drinking. That required a sort of sideways scooping movement through a puddle followed by a quick backwards head toss. It worked fairly well but spilled a lot.

Part 3. Did you hear this one?

A couple of young biology professors walk into a grocery store that’s been infested with finches. One says, “Look! I think I’ve discovered a new species of bird that evolved to eat cheese!”

Her friend replies breathlessly, “OMG, there’s more…  Look, a new cross-billed sub-species that’s perfectly adapted to rip open Gouda wrappers! Look at the way it turns its head at just the right angle to catch the tip of its bill in the wax before yanking it backwards. It’s the miracle of evolution. Animals adapting to their environment... Amazing!”

“Eww!” says the first, “What about all those dead birds on the floor? They look like they starved to death.”

“I don’t know,” her friend says, “Just ignore them. Let’s write a grant proposal to study that extraordinarily well adapted, cross-billed-cheese-eater bird.”

“I’m in…,” her friend replies, “it looks like tenure for me!”

Part 4. Was It the Crucifixion Nails, Pine cones, or Just a Fluke of Nature?

I’ve got a problem. In a few weeks, I have to teach a couple of lab classes about crossbill finches. It’s part of a team-taught Introductory Biology course, so I can’t change the lesson too much.

The students have to read a research paper about crossbills written by a couple of scientists who think they’ve figured out the adaptive advantage of that odd beak.

Here’s their story: Like some other finches, crossbills eat pine cone seeds. If you've ever seen a pine cone up close, you’ll know that the tab-like scales that contain the seeds can be closed tightly against the cone (in summer), or spread wide apart (as they are in autumn).

When scientists tested birds in the lab, they found that crossbills can get seeds out of both open and closed pine cones. Straight-billed finches are only good at getting seeds out of open cones; they have a difficult time with the closed cones. The scientists also discovered that if they clipped the crossbill’s beaks short, the birds couldn’t get to the seeds until their beaks grew back. So, they concluded that crossbills have a hypothetical “enhanced foraging” advantage over the straight-billed finches. And, they claim, that’s why there are crossbill finches.

I guess that’s what I’m supposed to teach.

Couple problems here: First, rather than being an advantage, crossed beaks actually make the crossbill’s life more difficult. For one, they’re slower at getting the seeds out of open cones than their straight-billed relatives. Second, crossbills don’t live on pine cone seeds alone. If you’ve ever watched them try to catch insects, or scrape up dirt (which they eat for minerals), or drink water, you can see they’re not as efficient as they could be. Besides, the straight-billed finches are still doing fine. They don’t seem to be at any disadvantage.

Here’s another thing, something that everyone ignores. The genetic mutation that causes crossed bills occurs in some other birds that don’t even eat pine cone seeds… including the domestic Swiss chicken Appenzeller Barthuhn where it’s referred to as a beak deformity because, well, it is. It’s certainly no adaptive advantage when you’re pecking at chicken feed.

So, what am I supposed to teach? Is a crossed beak the result of divine intervention, an evolutionary adaptation to the environment, or did crossbills just get dealt a bad hand… and they’re struggling to do the best that they can with what they have?

Inevitably, we all take one of these perspectives from time to time. The one we choose determines how we see the problem and judge the actors. Should we condemn them, see them as the product of evolutionary determinism, or treat them with compassion?

That’s why I always think carefully about the finches when I read an article that tries to use evolutionary theory to explain things like human personality traits, or our voting preferences, or our parenting behaviors, or why we like to play the victim, or why we enjoy BDSM fiction.

Mostly, I think about that cheese-eating crossbill trapped in the grocery store… it’s only a crossbilll after all…

(don’t miss that last link)

Parts 5-7…. Bisexual Monkeys coming next