Yet more musings on the evolution of blood-eating in passerine birds...
If I were to indulge in one of those credibility-destroying rampant speculations about a possible future course of evolution, I might speculate that Redbilled oxpeckers are on their way to specialized and dedicated vampirism. At the moment they seem not to be in the habit of making their own wounds in order to feed, but it's not so far-fetched to imagine that they might learn this ability in the future. If they did do that, then we would at least know how sanguivory, or haematophagy, or vampirism, or whatever, did evolve within this group: they would have started by wound-feeding, and later switched to wound making. But how did the wound-feeding start? Presumably the birds first started crawling around on big mammals to eat the associated insects, then later started to take advantage of snot, or earwax, and then learnt that blood and skin were even more nutritious.
The first stages in this process are perhaps illustrated by the various bird species that, on occasion, remove ectoparasites from the bodies of large mammals. This behaviour is pretty widespread: Fan-tailed ravens Corvus rhipidurus may groom camels, some drongos glean ectoparasites from giraffes, Black-billed magpies Pica pica have been observed grooming moose, some bulbuls groom klipspringers, some caracaras clean tapirs and sloths (mmm.... slooooths), and trumpeters may pick parasites from the fur of brocket deer (Peres 1996). Oxpeckers are apparently highly specialized starlings, and we know that other starling species, namely the Pale-winged starling Onychognathus nabouroup, Tristram's red-winged starling O. tristramii [adjacent image of this species from here] and Somali red-winged starling O. blythii, are facultative removers of ectoparasites on such hoofed mammals as zebras, donkeys, camels, ibex, klipspringers, domestic cattle and giraffes (Feare & Craig 1998, Fennessy 2003).
This raises the question that some of you have already been asking: was a similar path followed by the ancestors of vampire bats? Well that's a very good question, and we'll come back to it shortly-ish.
The finches strike
As I've hinted a few times - and as you probably know (well done you) - Redbilled oxpeckers are not alone among passerines in being part-time parasites: another species that sometimes picks ectoparasites from larger animals also practices vampirism, and even without this behaviour it's already a well-studied and highly interesting bird. It's one of the Darwin's finches of the Galapagos, the Sharp-beaked ground finch Geospiza difficilis. Here I'm going to try really hard to not veer off at a tangent and talk about any one of the many, many interesting things that could be said about these finches and their discovery, diversity, evolution and ecology. I will however repeat that oft-uttered complaint that the birds would perhaps be better named Gould's finches, given that John Gould (1804-1881) is the one who first described them and thereby brought them to scientific attention.
G. difficilis occurs in the highlands of the large central islands of the Galapagos as well as the low islands Genovesa, Darwin and Wolf. Interestingly, the only low islands it occurs on are those where the highly similar Small ground finch G. fuliginosa is absent (Schluter et al. 1991). By far the most variable of all Darwin's finches in both morphology and diet, G. difficilis appears to be the oldest of the ground finches (Petren et al. 1999a, b, Grant et al. 2000), and three subspecies are recognized: G. d. debilirostris of Fernandina and Santiago, G. d. difficilis of Genovesa and Pinta, and G. d. septentrionalis of Darwin and Wolf. Among these subspecies, it is G. d. septentrionalis alone that feeds on blood, and it does this by pecking at the feather bases of boobies. Unsurprisingly it is therefore sometimes known as the Vampire finch. It is also unusual in feeding on nectar and on breaking into booby eggs to drink their contents. The blood-feeding behaviour was first observed (so far as we know) in 1964, and published in 1965 (Bowman & Billeb 1965) [adjacent image shows G. d. difficilis - it is actually very difficult to get hold of images of G. d. septentrionalis - borrowed from here].
In keeping with their ecological opportunism, it follows that the finches don't only take advantage of the blood of seabirds. In their 1995 film Islands of the Vampire Birds, David Parer and Elizabeth Parer-Cook filmed a sequence where a bird fed on David Parer's blood after he cut himself on rocks. It seems that, wherever there are blood-feeding tetrapods, people somehow manage to 'accidentally' get the animals to drink their blood, though bad luck Percy Fawcett: he apparently slept with his feet hanging out the end of his tent in order to lure in a hungry vampire bat, and was unsuccessful.
Compared to the other G. difficilis subspecies, G. d. septentrionalis has proportionally longer wings and a longer, more tapered bill (Lack 1969). It is also different from the other subspecies in that owls and hawks do not occur on the islands where it lives, and it is perhaps as a consequence of this that it is (even by Galapagos standards) unusually tame, and not afraid to approach much larger animals. It is these morphological and behavioural characteristics combined which led Bowman & Billeb (1965) to propose that vampire finches first took to feeding on the black hippoboscid flies that congregate on the white plumage of local boobies, later learning by accident that their bills were well suited to pierce skin and cause blood to flow. This is a fascinating hypothesis, and obviously highly similar to what I discussed above for starlings. However, the finches also remove ticks and other parasites from giant tortoises, thereby raising the possibility that they first learnt to get blood from wounds made by parasites, and then later switched to making the wounds themselves. This latter idea is perhaps not as good given that the finches have not been reported to drink tortoise blood... yet.
Might these scenarios also explain how vampirism arose in bats? Stay tuned....
Refs - -
Bowman, R. I. & Billeb, S. I. 1965. Blood-eating in a Galapagos finch. Living Bird 4, 29-44.
Feare, C. & Craig, A. 1998. Starlings and Mynas. Christopher Helm, London.
Fennessy, J. 2003. Palewinged starling gleaning on desert-dwelling giraffe, northwestern Namibia. Birds Numbers 12 (1), 20-21.
Grant, P. R., Grant, B. R. & Petren, K. 2000. The allopatric phase of speciation: the sharp-beaked ground finch (Geospiza difficilis) on the Galapagos islands. Biological Journal of the Linnean Society 69, 287-317
Lack, D. 1969. Subspecies and sympatry in Darwin's finches. Evolution 23, 252-263.
Peres, C. A. 1996. Ungulate ectoparasite removal by Black caracaras and Pale-winged trumpeters in Amazonian forests. Wilson Bulletin 108, 170-175.
Petren, K., Grant, B. R. & Grant, P. R. 1999a. A phylogeny of Darwin's finches based on microsatellite DNA length variation. Proceedings of the Royal Society of London B 266, 321-329.
- ., Grant, B. R. & Grant, P. R. 1999b. Patterns of evolution in Darwin's finches: microsatellites provide a new perspective. Proceedings of the American Association for the Advancement of Science, Pacific Division 18 (1), 76.
Sato, A., O'Huigin, C., Gigueroa, F., Grant, P. R., Grant, B. R., Tichy, H. & Klein, J. 1999. Phylogeny of Darwin's finches as revealed by mtDNA sequences. Proceedings of the National Academy of Sciences 96, 5101-5106.
Schluter, D., Ratcliffe, L. M. & Grant, P. R. 1991. The taxonomic status of the Small Genovesa ground-finch in the Galapagos. The Auk 108, 201-204.
Hey, Darren. You keep mentioning G. dificilis subspecies with names like G. f. debilirostris>. Are those f.s typos for d.?
Nice post! I always like to see stories about the Geospiza.
In the original post looking at oxpeckers specifically, what is the level of prevalence of the blood-feeding trait in the populations of oxpeckers studied? are there some that do whenever presented with the opportunity, and some that don't?
or are they all opportunistic blood feeders?
Great blogs. A photo of a Sharp-beaked Ground-Finch 'in action' can be seen here:
Errm? Should those subspecies be G. d. etc rather than G. f. ?
I think I have some sort of dyslexia. This problem is now corrected (the G. d. one, not the dyslexia).
Some years ago I saw in a documentation such a vampire finch feeding from a large wound at the auricular region of a young booby. The wound looked really horrible as it was about 2 or 3cm in diameter and you could nearly look into the skull of the bird. Strangely the booby did not seem to have had much pain, and it was said that the wound was despite is shocking appearance not very harmfull. The finch did not only feed on the blood, it was also very eager to keep it open. But I dont know if there was already a wound which was enlarged by the finch, of if the finch itself was responsible for it.
I'm glad that finches have come up as I have a slightly ignorant question to ask; it isn't entirely related to you post, but hopefully you can help.
How do we know that the morphological changes in Darwin's finches are the result of Natural Selection (operating on changes induced by BMP4 and making use of plasticity) and not solely phenotypic plasticity? In other words, did the Grants actually objectively demonstrate selection, not just PP?
This is a subject that is interesting me a bit at the moment. The following paper motivated this:
Losos, J.B. et al (2000) Evolutionary implications of phenotypic plasticity in the hindlimb of the lizard Anolis sagrei. Evolution, 54, 301-305.
Here, Losos points out that:
"Among Bahamian populations of both A. sagrei and A. carolinensis, which have probably been separated for several thousand years, a relationship exists between mean perch diameter and mean relative hindlimb length (Losos et al. 1994); a similar relationship exists among populations of A. sagrei experimentally established 20 years previously in the Bahamas (Losos et al. 1997). The similarity in these patterns manifested both intra- and interspecifically over time scales ranging from 20 years to more than 20 million years suggests not only that natural selection is the agent responsible, but also that no distinction might exist between micro- and macroevolutionary processes; the latter simply may be an extrapolation of the former manifest over long periods of time.
However, this conclusion rests on a critical assumption, namley that the differences observed among intraspecific populations reflect genetic differentiation. An alternative hypothesis is that different phenotypes are the result of phenotypic plasticity."
In the case of Darwin's finches, can we be sure that something similar isn't operating here (Losos found that it was indeed PP accounting for variation)?
PS; love the blog!
Very captivating series of posts. Do you also plan to cover evolution of brood parasitism?
You say that none of the species you've mentioned are in the habit of making their own wounds. One bird you haven't mentioned (and it's maybe a bit off topic, because it's supposed to have a taste for fat, not blood), is the kea, the mountain parrot of New Zealand, which is reputed to perch on the back of a sheep, bite a hole, and eat its kidney-fat.
The pro-kea camp claims that it actually never happened quite this way, that keas only latched on to sheep that were already dead, or at least down. Nevertheless, the local sheep-farmers were convinced enough to impose a bounty for beaks, and hundreds of thousands of dollars were paid out over the best part of a century. (Keas are now quite rare, and protected.)
Re the kea - yes, some individual kea will attack live healthy sheep, digging into their backs. A few years back ago the NZ Natural History Unit obtained evidence of one in the act, via a night-vision camera. The Dept. of Conservation responds to farmers' reports of kea attacks on sheep, and seeks to identify and remove the offending bird(s) before the behaviour catches on with other individuals and causes a wider-scale problem.
Black-billed American magpies Pica hudsonia are intensely disliked by ranchers in the western U.S., as they pick at the scabs from branding range cattle. They may enlarge the wounds to make them actively bleed, similar to what oxpeckers do in Africa. Before screwworm flies Cochliomyia hominivorax were eradicated in the U.S., these wounds then provided an ideal entry for their damaging maggots.