Bird Alarm Calls

[old'un]

We get a few Sparrow Hawk attacks in the garden and the birds seem to have learnt the alarm call of other birds and dive for cover when one is raised, we also get a fair number of Wood pigeons in the garden and in the large Ash tree at the bottom of the garden, observing these Pigeons it made me wonder how they raise the alarm, have they also learnt other birds alarm call or do they have some other way of warning their mates.

Just the other day there were about 8 pigeons on the lawn and about the same sitting in the Ash tree, they were pecking away at the bird seed and just having a walk around the garden when all of a sudden they all stopped feeding, heads went up on outstretched necks and it was almost like a still picture, within 15 seconds of this a Sparrow Hawk dived between the houses and slammed into a Rhododendron, needles to say all the pigeons in the garden and Ash tree took flight, I have seen this before but as pigeons don't have a vocal alarm how do they know there is danger before it is even seen? Apart from alarm flight of Pigeons does anyone know how they communicate danger?