Birds

Recently at about 8 in the morning we have been visited by a large group of crows.  I believe the collective name for them is a "murder."  I wonder how that came about?  They are very big so perhaps they are ravens.  Maybe someone out there could let me know how to tell, or is it just a question of size?

I managed to take this picture out of my window, although they are quite sensitive to people and will fly off rapidly.  They are very large birds and have glistening black feathers.  My father used to call them "swaggering black rascals," and it seems they are.  The one here on the right has a piece of trash and we have to keep the lids on our trash bins or they will tip them over and spread the contents around.
In an aside, during our trip to London we went to the Tower.  Part of the tradition is the protection of the ravens there.  For it is said that if the ravens ever leave the Tower, England will fall.  During WWII, the Germans tried to poison them but the attempt failed.
My brother-in-law sent me this link, which is a short movie of an encounter two girls had in Ireland with a massive flock of starlings. http://www.boreme.com/posting.php?id=31278   The collective name for them is a "murmuration."  The one for Flamingos is, I believe a "flamboyance."