House Sparrow egg.
There is a small round hole on one side of the egg (actually visible in 'original' size). Otherwise the shell is intact, though empty.

One less bird in the world...

I want to say it's a sparrow's egg but I don't want to show off my ignorance, so I won't.
(Let's forget we had this conversation, shall we?)

Here is one who made it: http://www.pbase.com/image/29798707