In the latest installment in that ‘evolution of vampires’ thread, we learn how a chronic decline in populations of the Yellowbilled oxpecker has highlighted the pretty obvious fact that not all oxpeckers are alike. Why didn’t I mention this sooner: d’oh!
Before I got distracted by troodontids, owls, godwits, or sloths, I was talking about oxpeckers and the evolution of blood-feeding and vampirism. Here I’ll start the beginning of the end of this thread: we’ll go via blood-feeding passerines to bats, and then finish with pterosaurs. It’s likely, of course, that I’ll get distracted before I finish this (on the weekend I will be in the field again, so distraction is inevitable). Anyway… we saw in previous posts how oxpeckers turn out to be poor at tick control, but good at keeping wounds open. On some species of large mammals they may actually be truly parasitic. The adjacent photo (borrowed from here) isn’t an oxpecker: it’s a vampire bat, included to remind you that we’ll get to them eventually.
As is so often the case with neat animals these days, oxpeckers are in deep trouble. Across much of their range they have become extinct, or are in steep decline. What’s particularly interesting about this is that the two species are declining at markedly different rates: a discovery which has revealed a new and hitherto undiscovered complication as goes oxpecker ecology and lifestyle.
Oxpecker decline is partly a result of the use of arsenic-based cattle dips: the accumulation of the arsenic killing the birds (Stutterheim 1982, Stutterheim & Brooke 1981). So extensive and pervasive was the use of arsenic that it resulted in the extinction of oxpeckers within the Eastern Cape region of South Africa, leaving Shamwari Game Reserve and other wildlife areas devoid of these birds.
Since 1999 there has therefore been a major effort to reintroduce oxpeckers into these areas – there’s actually a reintroduction programme called Operation Oxpecker – and, luckily, areas like Kruger National Park appear to be over-stocked with the birds and have enough to spare. Given that oxpeckers once occurred naturally across South Africa, I personally feel that reintroduction is justifiable. But it’s ironic that the main idea behind these reintroductions is that the birds will combat the tick problems that afflict many of the large mammals in these areas. Given that, as we’ve seen in the previous posts, oxpeckers do not really help keep down their host’s parasite loads, it would be ironic if (from the point of view of the mammals with the tick problems) this was both futile and detrimental. I’m not the first person to note this, as I just discovered that an article (dated 2000) on ScienceLives! says much the same, and it quotes oxpecker expert Paul Weeks on this.
It isn’t just the use of arsenic cattle dips that have caused a decline in oxpecker numbers however. Robertson & Jarvis (2000) found that Yellowbilled oxpeckers Buphagus africanus* had declined in the East Caprivi region of Namibia to about 330 birds, compared to over 2000 in 1983/84 [adjacent image of Yellowbilled oxpecker from here]. In contrast, Redbilled oxpeckers B. erythrorhynchus remained at the same level over this time. Cattle dips haven’t been used in the area, so the decline of Yellow-billed oxpeckers must have been caused by something else, and something specific to this species. It turns out that not all oxpeckers are alike: the two species differ in bill shape and in feeding preference and behaviour. That might seem like a pretty logical discovery, but again it’s one of those obvious points that has been largely missed and obscured by the whole ‘avian cleaner-fish’ notion.
* Apparently not B. africana as I wrote in previous posts.
Yellowbilled oxpeckers, it seems, rely on the Bont tick Amblyomma variegatum as an important source of food. They seem not to be as adaptable as Redbilled oxpeckers, nor does it seem that they depend so much on ear wax, snot and blood. A serious decline in Amblyomma – caused by drought and the loss of the ground vegetation (through deliberate burning) used as cover by the ticks – therefore seems to have caused the serious decline in Yellowbilled oxpeckers (Robertson & Jarvis 2000).
I admit that I didn’t know any of this when I first became interested in the ‘parasitic oxpecker’ idea, and had assumed that the wound-feeding and blood-eating well studied in the Redbilled oxpecker (Weeks 1999, 2000) was true for both species. It isn’t. But hold on…. which oxpecker species was reintroduced to Shamwari Game Reserve and those other areas? Redbilled oxpeckers!!
In view of this information, it’s interesting to see how bill shape differs between the two oxpecker species. While I am not aware of any study that looks into this in depth, it’s obvious that Yellowbilled oxpeckers have a much heavier-looking bill that is far wider at its base that is that of the Redbilled oxpecker (Chapin 1921). The two do not really differ in how sharply pointed the bill tips are, or anything like that (adjacent pic from here). While it has been demonstrated that Redbilled oxpeckers do not function as merry little helpful tickbirds, what is now needed are studies showing whether or not the same is also true of Yellowbilled oxpeckers. Do they also exact a cost on their hosts, or are they really symbionts as so often thought? Sigh. Is it ever possible to make a generalization about animals that actually applies? That’s perhaps what makes things so fascinating.
More to come in the next post…
PS: a quote from my sloth article was featured on today’s scienceblogs homepage.
PPS: the sloth article has just become one of scienceblog’s top five most active articles! Note to self: ignore passerines, stick with xenarthrans.
Refs – –
Chapin, J. P. 1921. Notes on a new ox-pecker and other little-known birds of the Congo. American Museum Novitates 17, 1-16.
Robertson, A. & Jarvis, A. M. 2000. Oxpeckers in north-eastern Namibia: recent population trends and the possible negative impacts of drought and fire. Biological Conservation 92, 241-247.
Stutterheim, C. J. 1982. Past and present ecological distribution of the redbilled oxpecker (Buphagus erythrorhynchus) in South Africa. South African Journal of Zoology 17, 190-196.
– . & Brooke, R. K. 1981. Past and present ecological distribution of the yellowbilled oxpecker in South Africa. South African Journal of Zoology 16, 44-49.
Weeks, P. 1999. Interactions between red-billed oxpeckers, Buphagus erythrorhynchus, and domestic cattle, Bos taurus, in Zimbabwe. Animal Behaviour 58, 1253-1259.
– . 2000. Red-billed oxpeckers: vampires or tickbirds? Behavioural Ecology 11, 154-160.