There are many collective names for birds - a host of sparrows; a murmuration of starling and my favourite - a charm of goldfinches: http://baltimorebirdclub.org/gnlist.html
Apparently, it is the 14th centaury writer Juliana Bernes who is credited with most of these: http://en.wikipedia.org/wiki/Juliana_Berners
She coined the term 'a siege of herons', but I've only seen solitary ones - like this one (x40 zoom) on longmead lake:
And there is no collective name for my favourite small bird - the long tailed tit. These endearing little birds in their chirping their flocks can raise a smile on the most melancholic day - even if they are rather frustrating to photograph - perhaps it should be 'a chirruping of long tailed tits?'
Now the trees have shed their leaves, the flocks of woodpigeons are also more evident, maybe this should be 'a sulk of woodpigeons'?