Yesterday morning I was almost hauled out of bed to 'Come and see the bird bath, it's amazing'. Yikes, and so it was too:
At first we pondered whether it could have fallen from a passing aircraft, but we're well away from the flight path. Had a bird dropped it? Unlikely as it was 4" tall and well embedded at about a 60 degrees angle into the frozen bird bath.
Then, little Miss Marple began searching the web for ideas and 'bingo' it was an ice spike. In other words - an upwards growing icicle or ice crystal!!
From what we can gather, they aren't exactly your every day occurance. It was quite beautiful and just like a piece of clear crystal. They grow due to 'super cooling' and need quite specific conditions to form it seems, our bird bath must have fitted the bill (so to speak).
We just put it down to being a fluke that we'd been lucky enough to see for ourselves, and watched sadly as later in the afternoon it melted right away - Len wouldn't let me put it in the freezer to keep :)
Here's some info. on Wikipedia I found.