Ice spike!

A surprise this morning! The bird bath in my garden has produced an ice spike.
From Wikipedia:
An ice spike is an ice formation, often in the shape of an inverted icicle, that projects upwards from the surface of a body of frozen water. Ice spikes created by natural processes on the surface of small bodies of frozen water have been reported for many decades, although their occurrence is quite rare. 
Spikes tend to form in containers such as bird baths and pet drinking bowls, where the water freezes quickly, rather than in large bodies of water such as lakes and ponds.