In preparation for my trip to South Africa, I purchased a few bird books of the region and attempted to familiarize myself with families of birds of which I had no previous knowledge. The exception: Red-billed Oxpeckers. I can’t recall the first nature documentary that introduced me to one of these birds pecking away on a mammal of some sort, but they have been synonymous with Africa in my mind for quite some time.
Red-billed Oxpeckers are fairly common in savannah grassland and bushveld regions of Sub-Saharan Africa, but they only occur where there are tick-infested mammals. Their hosts include impala, kudu, zebra, giraffe, buffalo, rhino, as well as domesticated cattle (but not elephants! They will not tolerate oxpeckers!). Even in the absence of their mammal symbionts, Red-billed Oxpeckers are easily recognized. They are starling-sized, plain, brown birds with a diagnostic all red bill and distinctive yellow rings around their bright red eyes. Their bills are pointed and laterally compressed which helps the birds comb through theirs host’s fur and pry out well embedded parasites. They have strong legs and long, particularly sharp claws which enable them to cling onto the sides and backs of their hosts at precarious angles. They also use their short, stiff tails as props.
I have long been intrigued with symbioses of all kinds. As an undergrad and graduate student, my research focused on a range of interactions – beneficial fungal endophytes in leaves, harmful plant pathogens, and mycorrhizal symbionts that nourish plant growth, but sometimes cheat. In recent years, many of the “classic” tales of mutualism have been upended, or at least shown to be far more complicated than originally thought.
The relationship between Red-billed Oxpeckers and the mammals on which they peck are one of these classic tales. Oxpeckers perch on large mammals and eat ticks, lots of them – an adult bird will eat over 100 engorged ticks and thousands of larvae in a day, as well as other ectoparasites, dead skin, earwax, etc. This interaction is usually painted as quite a lovely picture – the mammal benefits from a serious reduction in parasite load, and the bird gets a meal! Well, the preferred food of the Red-billed Oxpecker is blood, and in addition to removing ticks the bird will peck at the mammal’s wounds to keep them open as a source of fresh blood. Gareth brought this to my attention when he noted that open sores on giraffes often appear to be aggravated by oxpeckers. But, they are also known to help clean-up open wounds, targeting the rotting wound tissue and ultimately cleaning up a lesion. So, are these Oxpeckers vampires or tick birds?
Well, this interaction certainly seems to be more complicated than originally thought. Similar to the cleaner fish of coral reefs, the system consists of a “guest” cleaning or grooming a larger host “client”. But how does the client fare? In one study, the parasite load of cattle was NOT reduced in an experiment that excluded Red-billed Oxpeckers from enclosures. Other studies have documented that RBOX spend 85% of their foraging time feeding on blood, not removing ticks. Excluding oxpeckers also resulted in fewer wounds and much faster wound healing time. On the other hand, in the same study, excluding oxpeckers caused a marked increase in earwax levels! But still, more of a foe than a friend, it seems. Perhaps an Oxpecker symbiont does come in handy in environments with heavier parasite loads? It is also quite possible that the nature of this relationship varies from host to host – briefly and anecdotally, I noted wounds or open sores on giraffes in Kruger, but I did not observe any on impala. Some authors suggest they are parasitic on hippos, commensalists on impala, and mutualists on rhinos. It is now quite clear that mutualisms are not always the perfect happy-happy relationship that they are often reported to be, but rather lie on a continuum that is influenced by many other ecological factors and this appears to be true for oxpeckers and their hosts! Regardless, it was fascinating to have the opportunity to observe these interactions, between oxpeckers and a variety of hosts, in the wild!