Pallid Swift; Vencejo Pálido

Rather than try to write what is a long story, check this link: http://www.rspb.org.uk/wildlife/birdguide/name/s/swift/identification.aspx When they are all flying together the differences soon become apparent and you can learn which is which when you see them alone. Separating the different Swifts is another story, it is very difficult in some cases as shape is the same and size can only be seen by comparison. Pallid Swift is much paler on top of the wings if you get to see them like you can on this picture.