Thursday, 14 July 2011

Crowned eagles

African Crowned Eagle, Arusha NP Feb 2011

Montane race of East African Black & White Colobus, Arusha NP, Feb 2011
Driving around Arusha NP a couple of weeks ago we were busy searching for Black and White Colobus (among other things, of course) - my favourite monkey of the forests around here. And we did eventually find a troop, though they weren't playing as well as they often do. But as we were heading down the mountain I spotted a crowned eagle sitting in the top of a tree, and pointed it out to my colleagues (botanists), noting its probably the main predator of the colobus and other monkeys in this forest. It was a male, and sitting at the top of a tree, so not as big as they can appear and William was pretty amazed that they could catch monkeys at all, let alone carry them any distance (which, admittedly, they'll only do for smaller individuals up to about 6kg). Like most raptors, females are rather bigger than males (up to c. 5kg according tot he usual handbooks), but there's no doubt that these birds are the major monkey predator across most of their range (check here for some details of the impact on monkey populations). I knew all this, but was busy reading an article by one of the absolute experts on these birds today, and even I was pretty impressed by his desciptions of them killing Bushbuck! With an average mass of 36kg for a female, that means the bird can kill something 6 times it's own mass, and surely puts it in the top predator category!
Female bushbuck, Arusha NP Feb 2011

For animals of this size, apparently the birds will swoop in fast, hit, grab, squeeze and release. With a 10cm hind talon, they can easily puncture a lung and do serious internal damage then just sit and wait until the animal keels over. Once this happens, if it's not yet dead, they'll have a good stomp and squeeze some more until it bleeds to death. Not bad for a bird!

But now think about this - if a crowned eagle can kill prey 36kgs in weight, and shows a distinct liking for primates, doesn't that put rather a lot of children in the picture? And, indeed, the crowned eagle is the only extant (i.e. not extinct) bird that is known to kill humans (check here for one such event in Uganda reported in the scientific literature). And now think back a few million years to when our ancestors were rather more similar to monkeys than we are now - Crowned Eagles (or their ancestors, at least) were quite possibly our major predators, just as they are today on medium sized primates. And what's more, there's even fossil evidence of just once such predation event among our fairly recent relatives, Australopithecus africanus from South Africa. Certainly to our early (and somewhat smaller) ancestors these birds must have exerted a major selective pressure - it's even suggested that because predation by crowned eagles exerts a considerable selective pressure on primates to get larger and be able to fight off eagles (again, there's a record of a Sanje Mangabe killing a would-be eagle predator), they might be what forced our early ancestors to get bigger and have to leave the trees in the first place. Speculation, of course, but interesting to think that a bird may well have been a major part in our early evolution...

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