Avian chimera?

When I was a child, one of my most treasured possessions - second only to my collection of plastic model animals - was an encyclopaedia of vertebrate animals (OK, I was a pathetic kid). I'd spend hours studying the entries, from aardwolf to zorilla, and hungering for all the wonderful creatures that lived where I didn't.
My enthusiasm was largely restricted to fur-covered beasts, but there was one exception. One image, of a bird, kept catching my attention. It didn't look like a bird. It was a small fluffy creature clothed in soft greys, with a fuzzy crest and long tail. It was dangling underneath a tree branch and flaunting a tummy as smooth and round as a well-fed kitten's. My book said it was a mousebird (a name that seemed as strange as sphinx or griffin), and it belonged to an ancient order found only in Africa.

Haunted by this image, I was determined, when I finally made it to Africa (the Mecca of mammal-devotees), to find a mousebird and see whether the photo in my childhood 'bible' had lied.
What I found surpassed my expectations: mousebirds don't just look strange, they're delightfully weird.

Unlike other birds, a mousebird's feathers don't grow in orderly tracts (lines), they sprout out recklessly all over the body, giving the bird a furry look. This photo, of a local speckled mousebird (Colius striatus), was borrowed from here.

Even as I write this, a small flock of red-faced mousebirds is whistling from the bushes outside my window. I still feel a thrill on seeing them. Although demure in cinnamon and grey, this species sports a scarlet Zorro mask and a pencil tail. But what's really cool about mousebirds is the way they scurry through the branches. When feeding, they dangle beneath the branch with legs splayed sideways, hang upside down from grappling-hook toes or use their bill to haul themselves up to higher branches. They can do these weird, un-birdlike things because their legs are set unusually far forward and they've got two reversible toes (that is, they can swivel to point either forward or backward; not that they can be worn inside-out).

Among their other strange habits, mousebirds seduce their lovers with a bouncing courtship dance in which they bob up and down so vigorously their feet leave the perch. Photo by Martin Heigan.

But if you look closely at what these little birds are doing, as they scamper about the bushes, you're in for a shock. They're harvesting leaves! Their beaks have a sharp cutting edge (called a tomium) specially designed for cutting up veggies. Although they also like to feast on fruit, flowers and buds, at suppertime, young leaves is their dish of choice.
Not shocked?
Then tell me, how many small birds have you seen munching leaves??
There's a good reason for this (no, not poor observation skills). Leaves are dismally low in calories (ask any dieter) and folivores (leaf-eaters) must eat loads if they're to get the energy they need; a problem when it comes to getting airborne. What's worse, leaves are mighty difficult to digest. The only way to break down the cellulose is by fermentation with the help of gut-dwelling microorganisms; and these little guys work sloooowly.

Of the world's six mousebird species (all of whom call Africa home), two hang out around here. This is the red-faced mousebird (Urocolius indicus). Photo borrowed from here.

So how do vegan mousebirds cope?
Firstly, they're seriously into energy-saving. At night the flock huddles tightly in a rugby scrum, which lets each bird save 40-50% on its overnight heating bills. And on chilly nights, or when food is scarce, mousebirds don't even try to stay warm; they cold-bloodedly sink into a torpor, letting their body temperature fall by 8-10 degrees C. Once the sun rises, they hang about basking, feathers fluffed out, until they're feeling their usual hot-blooded selves (41.5 C). This helps them get by on fewer calories.

And how does a small flighty bird cope with a huge bellyful of rotting leaves? By restricting their leaf-munching to the late afternoon. In this way, they've got all night to digest their burdensome meal, freed from the need to find their wings.

Speckled mousebirds stoking up their leaf-fermentation vats (i.e. warming their tummies in the sun). Photo by Martin Heigan.