You just cannot tell by the appearance of some animals, certainly not by that of some birds. As far as I know, there are only a few ways to tell the sex of a Bank Swallow at a distance: song (male), behaviour (burrow digging and mating), and who has a swollen cloacal protuberance (males during breeding season).
The male Bank Swallow often attempts to mate with fertile females other than its mate. That is probably what is happening here, where the sexes of the birds are worked out by looking at a sequence of pictures.
A male Bank Swallow approaches a female waiting invitingly.
He is about to alight,
when a rival spots the assignation and drives off the intruder.
After a short time, the first male returns,
only to be driven off again. This time, his swollen cloacal protuberance confirms his intentions.
Out of here. 
At the speed these birds fly I am impressed any of these photos are in focus.