It is, it has to be said, rather troubling, isn’t it?
I genuinely don’t have any idea… When I first heard about the whole thing, I wondered about possible unusual weather, or possibly air pollution… Volcanoes, for instance, belch vast clouds of noxious gas from time to time. That would tie in with the birds which, apparently, died from a lack of oxygen(!). Apparently mass deaths of certain types of animals do tend to occur from time to time. Certain creatures (sea creatures, for instance) are also very sensitive to changes in their environments. An extended period of unusual weather (such as an unexpectedly cold winter) could cause a die off in populations. Weather conditions could also have to do with the birds. A strong enough updraft could conceivably propel birds up into the jet stream, where there would be insufficient air to breathe. The jet stream, in turn, is a strong enough current that it could quite possibly carry a dead bird halfway around the planet.
I’ve seen a couple of hypotheses thrown about (including movement of Earth’s magnetic poles — though I don’t really see what that could have to do with it), though none seem particularly plausible. I thought about the oil too, but no… accumulation in the food chain would cause a steady decline, not a sudden die off. It’s an enigma. An enigma that so far hasn’t had any rigourous scientific investigation. So far the only connection is that it’s either sea creatures or birds. Trying to correlate the cause of death globally would really help clear things up. I’ll keep an eye on the science blogs though, and if I hear anything, I’ll be sure to post about it!
(And thanks. :)