I don't know, but I remember when I was a ickle kid, one of our fave things to do was to peel bubbly off the road and eat it! Many's the time I've popped a beautifully pancaked slither of tyretracked chewy with assorted bits of grit in my mouth... Happy days!!!
Perhaps the birds pick it up and swallow it and choke to death (like our mothers warned us we would) and this explains why you don't see them falling out of the sky due to old age as queried in a another question.
Couldn't agree more pinotage, I hate to see people chewing the damn stuff endlessly, and, aaagghhh the noise some people make when they chew with their gobs open... Biggest culprit, Kate from last year's Big Brother!!!