Bird or Ice ? Prey or Nice ?

This old mill sits on the bank of the Delaware River in Trenton NJ. It’s now a restaurant - actually has been for over 20 years (my surprise 50th b’day party was held here!), different owners, different themes, but still going strong.
The sea gulls from the river have multiplied over the years. But this evening in the midst of a cold snap they found refuge together along the roof.
Interesting sight.