Mother’s milk... and Dad’s too

This post is not about elephants. Or lions.

I am NOT going to talk about yesterday's dawn-chorus of roaring, or being woken this morning by the excited, come-hither calls of spotted hyenas.

In fact, today's post is about the very epitome of peace: the dove (well actually a pigeon, but who's splitting hairs).

You see, I've been spending the last few days with Ecthelion, whose favourite termite mound is shaded by a fruiting rock fig.

The roots of the large-leaved rock fig (Ficus abutiliflora) can split rocks and plummet down 60 metres/yards to tap underground water sources.

I arrive each morning to a festival of birds: hornbills, starlings, louries and pigeons, all hopping about among the leaves, gorging themselves on the tiny figs. With squawks of protest, most of the revellers flee as I approach, but the African green pigeons (Treron calva) are made of sterner stuff. They simply shuffle sideways or crane their necks to peer down at me from between the leaves. Dressed smoothly in soft greens, and decorated with epaulettes of lavender, they're designed to blend into the foliage. To aid the disguise, they've developed a preternatural stillness. I find it disconcerting. Their persona is not improved by their unnerving, ice-blue eyes, or a bill – blood red and sharply hooked – that hints of deeds far more sinister than fruit-munching. And when viewed from below, they reveal a hitherto unsuspected brazenness: brilliant yellow leggings and scarlet toes.

Would you trust this bird? Photo by Johann du Preez.

After a while the pigeons tire of glowering down at me and go back to clambering silently about the branches. When I'm not looking, they'll even hang upside down, parrot-like, to pluck the small spherical figs from the branch tips. If I move suddenly, they burst from the foliage in a clamour of wing clapping, and fly - swift and straight - to a nearby dead tree. Here they line up on a sun-drenched branch to soak up the warmth and delicately preen one another's feathers.

Like all pigeons, African green pigeons have some weird habits. Not the least of which is feeding their chicks on milk. No they don't steal it, they brew the stuff themselves.

Midway through incubating their eggs, something strange begins to happen to both Mum and Dad. A rush of the pituitary hormone prolactin (the same hormone that prompts milk production in mammals) makes the walls of their crop thicken up and blood vessels invade the area. By time the first chick scrambles from its shell, the adults' crops have trebled in weight and are sloughing off cells to form a cottage-cheese-like goo, called 'crop milk' or 'pigeon milk'. The newly hatched squabs (the in-house term for pigeon chicks; personally I'd go back into my egg if someone called me a squab) feast exclusively on this for the first few days, thrusting their heads right inside the parents' beaks to gobble down the goodies.

Pigeon milk compares well with mammal milk; it's very rich in protein, moisture and fat, but contains no carbs. And it's more effective at promoting rapid growth than the milk in your refrigerator. Parent pigeons, however, only produce the stuff for the first week or so, and then they wean their little ones on to solids.

Enjoying the morning sun. I try not to identify with this species' plump shape... I'm sure I'll lose weight once the summer comes... Photo by Arno and Louise Meintjes.

But why do pigeons go to all this trouble? The only other birds known to feed their kids on throat secretions are flamingos, who cough up a mixture of blood and fat. But flamingo chicks need to grow fast if they're to avoid being left high and dry when their shallow breeding pools dry out. Baby pigeons face no such deadlines.

The answer seems to be two-fold. Firstly, pigeon eggs are unusually small (relative to the size of Mum) and secondly, little pigeons are very well developed when they eventually hatch. As a result, pigeon chicks have squandered their entire food store (the egg's yolk) by the time they hatch. This is not normal. Most baby birds emerge with up to a third of their yolk intact. With this rich source of fat and protein embedded in their tummies, the chicks of other species don't eat for their first few days, giving their gut a chance to adapt to solid food. It's thought that pigeon parents cover this 'down-time' for their hungry chicks by coughing up pigeon milk.

 Figs are the favourite food of African green pigeons (Treron calva) who dine almost exclusively on fruit. I was going to call them fruitarians but apparently strict fruitarians (and some vegans) boycott figs because they contain insects! NB: figs are pollinated internally by a symbiotic wasp who ultimately dies inside the fruit.
Photo by J. Muchaxo.