One in - one out...
I noticed that these birds appeared to have a 'one in, one out' method when feeding the young. I guess it was because the nest is so small, 2 adults and a baby (or babies?) couldn't all fit in there so a number of times the foraging adult would get back to the nest only to turn away and try again in a short while.

The western house martin is a migratory passerine bird of the swallow family which breeds in Europe, north Africa and across the Palearctic; and winters in sub-Saharan Africa and tropical Asia. It feeds on insects which are caught in flight, and it migrates to climates where flying insects are plentiful.