Welcome to Tetrapod Zoology ver 2: and we start with blood-eating birds….
To everyone who has come over from the blog’s former home on blogspot, thanks for coming on over, and to new readers: welcome. This is a blog devoted entirely to discussion of the evolution, history, diversity, biology and behaviour of tetrapods, and for the two or three of you that don’t know, a tetrapod is any vertebrate animal that possesses four limbs, or descends from an ancestor that had four limbs. Essentially it’s amphibians, mammals and reptiles (the latter including birds), and the close fossil relatives of these groups. The adjacent image, incidentally, has no relevance whatsoever with regard to this post, I just thought it was a cool photo. No prizes for working out what it is.
The invitation to join scienceblogs came in December 2006 and was officially finalized on 22nd January: the day after Tetrapod Zoology’s first birthday. Wow. So from nothing to a scienceblogger is about a year – that’s not bad. So, what’s going to be different now that I’m blogging on a new site? To be honest, not much. I’m going to continue to do the same sort of thing I’ve been doing. However, there is now an incentive to produce more posts, so not only will I try and be more productive, but I will also try and break up long posts (viz, those of over 1000 words) into several smaller ones. This all means that there will be lots more stuff to look at so, if you like what I do, my advice is to visit, visit, and then visit again.
I’m not yet happy with the overall look of this site. I went to the effort of producing some nice banners but I can’t yet work out how to them get up. I also have yet to add various stuff to the sidebar and so on. But I’ll get there eventually. Evolution rules, after all.
Anyway, that’s the preamble… on to the work. The last thing I was talking about – before Tetrapod Zoology’s first birthday – was that evolution of vampires post. And I can’t stand the incessant requests any longer, so let’s get this out of the way. To begin with I will make it clear that I’m not just going to be talking about the sort of vampires you’re familiar with (the desmodontine microbats), but, in fact, about passerines. Perching birds. Yes, those cute little African passerines that cling to the sides of giraffes and impalas – the oxpeckers – might not be the oh-so-helpful tick-birds that most people have assumed they are. They may, in fact, be avian vampires. But don’t get too excited, as a lot of work remains to be done on the ecology of these birds, and controversy remains.
Though sometimes given their own ‘family’, Buphagidae, it’s generally agreed that the oxpeckers are highly unusual starlings, perhaps related to the Asian starling Scissirostrum dubium* or the glossy starlings. There are two species, the Red-billed oxpecker Buphagus erythrorhynchus of eastern and southern Africa, and the more widespread Yellow-billed oxpecker B. africana. A few subspecies have been named for both species but it remains to be seen whether they are valid or not; a third species, the small, dark B. langi of the Congo Basin, was named in 1921 but is today regarded as synonymous with B. africanus [the adjacent image of a group of B. erythrorhynchus is borrowed from naturalia.org].
* Remind me one time to relate the curious tale of ‘Gibsone’s nondescript’.
Oxpeckers have stiff, almost woodpecker-like, tail feathers, and particularly large and strongly curved foot claws. They adopt a number of peculiar postures, including an erect penguin-like one when they alight next to another oxpecker and an open-winged sunbathing posture, seen sometimes as they cling to the side of a mammal. Many African mammals respond to oxpecker alarm calls and even domestic horses learn to recognise it. Oxpeckers will sometimes return to the body of a host mammal even after it’s been shot dead. Best known for feeding on ticks, their diet is in fact far more diverse: they also eat flies, lice and mites, and very occasionally they may pick at carrion. There is one report of a Red-billed oxpecker eating fruit, but this is now regarded as erroneous. Significantly, they eat snot, ear-wax, tissue and blood. Lots of blood.
The conventional interpretation of oxpeckers is that they live in symbiosis with large hoofed mammals, providing a grooming service and in turn obtaining meals. In other words, they are avian ‘cleaner fish’. However, when we look at the feeding behaviour and food preferences of oxpeckers, we see that things might not be so rosy. It’s true that oxpeckers eat ticks – obviously a beneficial thing for the mammal that has the ticks – but, do they really do such a good job of tick removal, and is it really ticks that they’re most interested in? Apparently not, as some studies strongly indicate that blood is their preferred food (Bezuidenhout & Stutterheim 1980, Weeks 1999). In fact oxpeckers spend about 85% of their foraging time feeding – not on ticks – but on blood, ear wax, and on material that they ‘scissor’ out of the fur. Weeks (2000) argued that this ‘scissoring’ is not for small ticks, as previously assumed, but for flakes of dead skin.
While they ingest blood by eating engorged ticks, they also obtain it by feeding on wounds, and because they keep wounds open, they may have a serious and detrimental affect on the health of the mammals they associate with.
Wound-feeding in oxpeckers has long been known about; R. E. Moreau reported this in 1933 in the first ever proper study of oxpecker diet. Even better, Shelley’s 1906 The Birds of Africa explains how Baker, the explorer, employed boys to chase oxpeckers away from pack animals, while two reports from the 1930s described how oxpeckers kept open the sores of pack mules and donkeys to the extent that the animals could no longer be used (Feare & Craig 1998, pp. 256-257). Goodwin (1963) allowed a captive Red-billed oxpecker to take blood from a wound on his hand and reported that, while the bird did not make any effort to enlarge the wound, it used pressure from its bill to increase the flow of blood. Olivier & Laurie (1974) described the interactions observed between oxpeckers and hippos; the oxpeckers ‘fed entirely on tissue gleaned from wounds, often to the extreme discomfort of the hippo’ (p. 169). Weeks (2000) also reported cases where oxpeckers were observed pecking at the place where a tick was attached to its host, yet making no attempt to dislodge the tick: the oxpecker was instead more interested in enlarging the wound [image above borrowed from african-safari-pictures.com].
Despite all this it was mostly assumed that the wound-feeding and blood-eating was an incidental consequence of the foraging for, and ingestion of, ticks. But in view of all of the new data it is tempting to wonder if oxpeckers are on their way to specialized vampirism. Do oxpeckers actually make wounds in order to feed on blood?
I have to stop there. More to come, not only on oxpeckers but on vampirism in other passerines, in bats and (supposedly) in pterosaurs.
Refs – –
Bezuidenhout, J. D. & Stutterheim, C. J. 1980. A critical evaluation of the role played by the red-billed oxpecker Buphagus erythrorhynchus in the biological control of ticks. Onderstepoort Journal of Veterinary Research 47, 51-75.
Feare, C. & Craig, A. 1998. Starlings and Mynas. Christopher Helm, London.
Goodwin, D. 1963. Some behaviour of a captive red-billed oxpecker. Avicultural Magazine 69, 113-117.
Olivier, R. C. D. & Laurie, W. A. 1974. Birds associating with hippopotamuses. Auk 91, 169-170.
Stutterheim, I. M., Bezuidenhout, J. D. & Elliott, E. G. R. 1988. Comparative feeding behaviour and food preferences of oxpeckers (Buphagus erythrorhynchus and B. africanus) in captivity. Onderstepoort Journal of Veterinary Research 55, 173-179.
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.