In reply to Flinticus:
The photo looks more like a crow or a rook than a jackdaw. Jackdaws have quite distinctive grey hoods and blue eyes.
The photo is a bit blurry so it's hard to tell if this is a crow or a rook. Rooks have a bald patch behind the beak which crows don't have. They also generally look scruffier, especially around the tops of the legs, but that's harder to use as an identifying feature unless you see them side by side. If I had to put money on it, I would guess this is a crow.
It is true that crows don't usually flock (although they will under some circumstances), whereas jackdaws form quite impressive flocks. We have lots of them round here and they flock in their hundreds every dusk and dawn. The photo is from a small-ish flock on our neighbour's house at dawn. On mornings when they pick our house, it's incredibly noisy! They do look very similar to crows when you are too far away to see the hood. I would guess the flocks you see in the park are probably jackdaws.
You also sometimes see one or two crows in with a flock of jackdaws.
Post edited at 11:13