Occasional paper: On the curious diet of the Speckled Mousebird

So my wife took this picture in our garden yesterday, here in Kigali, Rwanda:



Take a close look.  This little bird — about the size of an American cardinal, or a European robin — is facing us.  It’s also facing the sun, though you can’t see that.  It is holding two twigs with its little claws, and… it’s puffing out its breast feathers in a very weird way.  It looks like a breeze is ruffling them.  But there is no breeze.

So we did a quick look-up and found: this is Colius Striatus, the Speckled Mousebird.  Long tail, “scruffy” crest, check.  Thin, rather hairlike breast feathers, check. Very common across tropical Africa, okay.  And then this:  

“Speckled mousebirds… can often be spotted roosting in groups where they’ll buff up their feathers. They do this to allow more sunlight to hit their bodies which helps speed up the fermentation process.”

Wait, what?


Okay so let’s talk for a moment about “folivory”, which is the fancy term for “eating leaves”.

If you want to eat leaves as your major food source, well, there’s good news and there’s bad news.  The good news is, leaves are everywhere!  And you don’t have to be stealthy or clever to sneak up on them, and they’re not going to run away.

The bad news is, leaves don’t have a lot of energy.  Your typical leaf is mostly water and cellulose with a bit of starch thrown in.  In terms of calories per kilogram, it’s one of the least energetic foods you can get.  So, an entire kilogram of lettuce only has about 150 calories.  That compares to about 550 calories for a kilogram of apples, or about 2,700 for a kilogram of beef. This is why we eat salads to lose weight, yes?

Now leaves as an *occasional* food source are fine.  They do tend to contain useful vitamins and minerals, which makes them very useful as part of a varied diet for an omnivorous animal.  But if leaves are all of your diet, or a lot of your diet, then you’re going to encounter some issues.  Leaves are such a low-energy food that either you have to eat a lot of leaves, or you have to just digest the heck out of the leaves that you do eat, so that you extract every possible calorie of nutrition.  Or both. 

This is why cows have four stomachs, yes?  They’re eating grass — grass is leaves — and then they’re sending it through a digestive system that is much more complex than yours or mine.  One thing they’ll do along the way is ferment the grass, which lets them crack energy out of all that cellulose, getting more calories per kilogram.  But fermentation is complicated:  it needs either a big complex digestive system filled with symbiotic bacteria, or heat and lots of time — preferably both.

Okay so if you’re an animal that’s going to eat a lot of leaves, that generally gives you three options.

— Be big.  Cows, elephants, gorillas.  You have lots of room for a massive digestive system that can extract all your caloric needs.
— Be cold-blooded.  Insects, slugs, iguanas.  Cold-blooded animals need a lot less energy!  So, it’s easier to get your energy needs from leaves.
— Be slow.  Sloths, koalas.  

There are some interesting edge cases and exceptions (ask me about rabbits, or better don’t) but broadly speaking those are the options.

Okay so if you think a moment you’ll realize that all of these make it challenging for a bird to survive on leaves.  Birds generally have blowtorch metabolisms, with base body temperatures quite a bit higher than most mammals’.  Birds are generally small.  And birds can’t usually afford to be slow.

There’s exactly one (1) bird that lives on leaves and nothing but leaves:  the hoatzin.



Briefly, the hoatzin is nature’s attempt to evolve a sloth again, starting with a bird.  They live in South American tropical forests and they spend most of their time crawling slowly through the tree canopy.  And they can fly, but just barely — it’s an awkward flapping scramble, just enough to escape a predator.  

Okay, so the hoatzin is the only bird that lives entirely on leaves.  What about birds that eat a lot of leaves, but other stuff too?

Well, that gets you geese.  Geese typically get between a quarter and half their calories from grass and other leaves.  But geese are also perfectly good fliers and they don’t lack energy; as anyone who’s ever confronted one can tell you, geese are horribly strong and quick.  They can make this work because they are (for a bird) quite big, so they have room for a relatively large digestive system.  That said, geese can’t live on grass alone, and will aggressively seek out other food sources — as anyone foolish enough to offer one bread will quickly realize.




All right then, some big birds can eat a lot of leaves.  But a small bird couldn’t manage it.  Right?

Until yesterday, I would have agreed.  But I would have been wrong.  The mousebird gets about half of its calories from leaves.  

How?

It cheats.  The mousebird uses a trick called “heterothermy” — the ability to change its resting body temperature.  A few warm-blooded animals can do this, usually for purposes of hibernation or torpor.  So for instance, when bears hibernate, their core body temperature drops from about 99 degrees F (38 Celsius) to around 88 degrees f (31 Celsius).  And it stays that way for months at a time.  Bears have a metabolic “low gear” that most mammals lack.

And so does the mousebird.  But the mousebird can drop its body temperature /much/ lower.  They can go all the way down to around 75 degrees F (24 C) — room temperature, more or less.  That is crazy low for a warm-blooded animal!  That’s reptile territory.  But the mousebird makes it work.  And that means the mousebird no longer has that hot, fast, high-energy bird metabolism that needs vast amounts of calories.  It can live on the limited energy it gets from leaves.

TBC, the mousebird doesn’t eat leaves alone.  They’ll eat all sorts of plant material — seeds, fruit.  In our garden, we see them drinking nectar from the flowers of the big yucca plant by the garden gate.  But the mousebird’s ability to drop its body temperature means that when high-quality plant food is scarce — for instance, during Rwanda’s dry season, which is right now — it can switch to eating low-energy leaves, slow down its metabolism, and survive just fine.

Okay, but there’s still one problem left: the mousebird wants to get maximum energy from its leafy diet.  To do that, it needs to ferment its food.  That requires either a big cow-like digestive system, or — at a minimum — heat and time.  But the mousebird has lowered its body temperature!  So, where will the heat for fermentation come from?

From the hot tropical sun, thank you very much.  The mousebird will eat a belly full of leaves, and then it will fly up to a sunny perch, and then it will just… bask.  It lowers its metabolism, conserving energy, but its body temperature stays high because of the sun.  Its belly is covered with long, hairy feathers that help it absorb solar heat.  So its digestive system stays hot and can ferment the leaves aggressively.  If you go back to the top of the post, that’s exactly what the mousebird in our garden is doing in the photo. 

Meanwhile the mousebird is torpid, but awake. Presumably it’s just alert enough to drop off its perch and flutter into cover if danger should approach.

“Basking in the sun to digest a meal” is a behavior that we associate with reptiles: a turtle on a log, a snake by the side of the road.  You really don’t expect to find it in a bird.  But the mousebird is pretty successful.  It’s not a weird one-off like the hoatzin.  There are half a dozen species of mousebird, spread all across Africa.

— That whole “cold blooded / warm blooded” thing we learned back in grade school?  Basically correct, but the details get infinitely complicated.  There are some reptiles that are kinda warm blooded.  There are birds and mammals that vary their body temperature.  There are insects that can warm themselves up.  Heck, there are a couple of plants that produce internal heat. 

The world is wide and full of wonders, you know?  And sometimes they’ll come and perch in your garden.