Mutualistic birds roost in the armpits and bellies of giraffes

This is a short but sweet report on a new 3-page paper documenting the nocturnal activities of the yellow-billed oxpecker (Buphagus africana). It shows that, at night, these birds actually roost on the bodies of large mammals in Africa.

There are actually two species of oxpecker in Africa: the yellow-billed and the red-billed (Buphagus erythrorhynus). Their niches are somewhat different, with the red-billed found on 15 host species ranging in size from impala to elephants, and the yellow-billed specializing in larger animals that have big surface areas, like buffalo, eland, rhinos, and giraffes.

Both are presumably mutualists, for they land on the mammalian hosts and consume arthropods like ticks, and the mammal benefits by getting cleaned (they therefore tolerate the birds). A single yellow-bill, for instance, can eat 100 engorged ticks per day or 13,000 tick larvae. They also will take some of the mammals’ blood, sipping at the wounds. But on the whole this looks as if both animals benefit from the association, so it’s probably a mutualism.

Here’s a yellow-billed oxpecker cleaning a mammal (species unknown). They scissor their bills through the fur, picking up parasites:

And some red-billed oxpeckers cleaning an impala:

A new article in the African Journal of Ecology by Meredith Palmer and Craig Packer (reference below, free access, pdf here) used a combination of camera traps and citizen science volunteers (who identified the birds) to reveal that the oxpeckers don’t all go back to the trees at night, but some actually spend the night roosting on the mammals. Studies at Serengeti National Park in Tanzania revealed 25 instances of yellow-billed oxpeckers resting on mammals. Most were on giraffes (20), but some were on eland and buffalo. The birds tend to roost in the “armpits” and bellies of the animals (see photos below), with about 3.3 birds roosting on each giraffe examined and 1.6 on eland and buffalo.

(From paper): F IGURE 1 Camera trap images depicting the nocturnal roosting habits of yellow-billed oxpeckers on (a) giraffe, (b) eland and (c) buffalo. Yellow-billed oxpeckers can be identified by their distinctive coloured bills. [Colour figure can be viewed at wileyonlinelibrary.com]

The birds also changed where they hang out from day to night. During the day, yellow-bills hang out on buffalo (seven times more frequently than on giraffe or eland), while at night they roost on giraffes far more often (7 times more often on giraffe than on other species). Despite the much greater abundance of buffalo than giraffe in the Serengeti, the birds clearly like to spend the night clinging to a giraffe.

Why is that? Look at the pictures. The authors suggest (just a hypothesis, of course), that “the tall stature and long legs of giraffe may present warmer, safer nooks in which to spend the night, supported by our observations of oxpeckers congregating on to the undersides, rather than the more exposed flanks).

Red-billed oxpeckers didn’t seem to roost on any species at night, though they forage on giraffe and buffalo during the day. The authors further speculate that since the red-billed oxpeckers can exploit a wider range of hosts, they don’t need to stay with a single host at night.

That’s the story; as I said, short and sweet, but nice. I’m not sure if this is the first report of a bird roosting regularly on a large mammal; readers can enlighten us here. And it must be a restless night hanging on to the armpits of a giraffe. The birds in Figure 1a look like their perch is a bit precarious!

Oh, and congrats to the volunteer citizen-scientists who pored through the photos identifying birds (their IDs were checked, of course). If you get an opportunity to participate in a project like this, try it—it’s fun!

UPDATE: Reader John called my attention to a 2000 paper suggesting that the oxpeckers aren’t mutualists but perhaps VAMPIRES! Here’s the abstract (pdf here). The claim that the tick load doesn’t go up when oxpeckers are excluded does suggest that maybe they’re in it for the blood (that is, they’re parasites), and the mammals get no benefit. But this article uses cattle, not wild mammals, and I don’t know where scientific opinion stands these days.

h/t: Tom

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Packer, C. and M. S. Palmer. 2018.  Giraffe bed and breakfast: Camera traps reveal Tanzanian yellow-billed oxpeckers roosting on their large mammalian hosts. African J. Ecology, early view, doi 10.1111/aje.12505