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(The following ramble was inspired by and is for Ken of Birding is NOT a crime!!! - thanks for all your help and back-up in Chicago this week.)
I've been spending far more time than can possibly be good for me on aeroplanes this week, going from London - Newark - London - Chicago - Houston - Chicago - London in eight days. It may help explain the following post (lack of oxygen, dehydration, jet-lag etc...)
Anyway, I've always had this pet theory that Turkey Vultures, contrary to what the experts might tell me, don't actually feed on carrion at all. I think they probably feed on flying insects instead. I say this because despite seeing many hundreds of Turkey Vultures in many parts of North America I have never actually seen one eat anything. Thinking about it, I've actually hardly ever seen one on the ground. Sitting in trees or on fences occasionally, but mainly I see them spiralling around high above city centres or above empty grassland where's there's nothing to eat at all - except butterflies and bees and perhaps a late-to-bed moth. Or termites perhaps or flies or wasps. But when have you ever seen roadkill floating around the sky at 10,000'?
Turkey Vulture Cathartes aura, downtown Miami, Nov 2004 - what's it looking for?
I know it all sounds a bit odd, but think about it. When the European settlers began swarming over what would become the cities and suburbs of the US they found huge numbers of American Bison Bison bison - an estimated 30 million of them. It must have been one huge, movable feast for the nation's Turkey Vultures.
Inevitably some Bison would die - old age, injury, times of drought or during winter for example - and how many vulture families would a near 2000lb carcass feed? Lots more than one probably...
American Bison, Panoche Valley, California
Relying on Nature to provide a meal is all very well, but just imagine the celebration in the vulture world when those same settlers began blasting merrily away at every Bison they saw, peeling the hides off, and leaving the flesh behind as they simply couldn't eat (or even carry away) the vast mountains of meat their happy hunting was producing. Turkey Vultures probably never had to do much more than stagger with full bellies from one corpse to the next. If they flew at all it would be to flop heavily from one pile of dead Bison to the next pile of dead Bison. It would have been a 24hour buffet of meat, muscle, and offal with little more to interrupt them than having to pause to spit out the odd lead pellet.
One result of all this food would have been a vulture population explosion. Adults would have been riotously healthy (Bison is a very low-cholesterol meat). Chicks would survive
in unprecedented numbers. Boom-time for the bald-faced birds with the disgusting eating habits...
Of course - as the Europeans discovered a number of times over the next two hundred years - no matter how seemingly abundant an animal, you can only destroy a certain percentage of its numbers before there's a total collapse. Slaughter, say, 29 and a half million out of 30 million Bison and the inevitable happens. Communal behaviour evolved over millenia breaks down as herds become far smaller and more fractured, migration patterns change (particularly as land is claimed by rampant land-speculators), and breeding rates drop off the chart. Bad news for Bison, Native Americans - and Turkey Vultures.
The decline in the Bison was remarkable really. It took less than a hundred years to almost wipe them out. For a bunch of very fat vultures this must have been pretty catastrophic. Used to waddling around stuffing their gullets on prime steak there must have been a sense of panic amongst the country's scavengers as the kitchen closed and the meal's dried up. In fact, they must have had to look for an alternative source pretty quickly. And I think I know what they turned to: migrating Monarch butterflies.
For a hundred thousand years (give or take) millions upon millions of Monarchs have migrated from wintering grounds in Mexico and southern California to breeding grounds right across North America. Despite the attempts of loggers and developers to "do a Bison" on them, they still do. Remarkable really, considering how fragile they are. Still, whilst the Europeans knocked off the Bison (and the Passenger Pigeon and the Carolina Parakeet and the Ivory-billed Woodpecker etc etc) the Monarchs must have seemed like quite a plausible alternative food supply to the suddenly-deprived vultures.
It meant that vultures had to learn to fly again - even to imitate the teetering flight of their prey, to hold themselves on wings held in a slight dihedral much like a migrating butterfly. Of course, butterflies don't provide quite the same amount of protein as a dead Bison, but they're energy-packed snacks, and with little alternative what choice would the vultures have had? Other mammals? With the Bison gone, the settlers must have begun to eat anything that moved - beavers, muskrats, deer, those bunnies with the huge ears...
There would have been virtually nothing left for a long while until the livestockers brought in their scrawny cows to take the place of all the Bison their fore-runners had destroyed. Of course, this time the Vultures weren't welcome at the same picnic table. This time around the farmers and the cowboys weren't going to allow a bunch of "old buzzards" (they were cowboys rather than birders so it would have been an easy misidentification to make) to share the spoils, and would have turned their guns on the uninvited guests. Very quickly natural selection would have come to favour those vultures that were able to pick off airborne insects over those who insisted on a more hearty dinner of half-starved Longhorn at the point of a rifle. And thus arose generations of vultures that rarely land, that still drift over the cities looking for insects caught in the updrafts from skyscrapers, that endlessly circle over patches of milkweed in otherwise barren grasslands hoping - just hoping - that a few choice tidbits will emerge from their chrysalises before starvation sets in...
And that was my theory. A tad fanciful perhaps but how else to explain the fact that I'd never once seen a vulture on terra firma?
And then - out of the blue as it were - I came across the amazing sight I captured in the photos below: a Turkey Vulture feeding on what looked like the remains of a Raccoon. It may have been that the raccoon's body was riddled with maggots and some had already hatched into flies which were what the vulture was after, but I'm pretty certain that it was tearing into the flesh itself. Just like the old days...
Turkey Vulture Cathartes aura, near Chicago, Illinois 15 March 2006
Maybe after a good night's sleep I'll come up with a different explanation, but right now I'm beginning to wonder if the Bison isn't perhaps on the brink of a come-back and the vultures are starting to get the taste of rotting meat again? It's just a theory of course, but you never know...
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