It is indeed amazing that any get back to us, one of the wonders of our Earth.
Even 'four and twenty blackbirds baked in a pie' always seemed pointless to me - swallows are tiny, how would one even make a mouthful? What an unnecessary waste of life.
It seems we are so lucky this year with lots of very excited swallows nestbuilding in our barn. Some years only a single pair make it back.
I've just been watching the swallows here taking a 'bath on the wing', or maybe they're just getting a drink from the pond and get wet in the process, but there's a queue of them circling the pond surface, then scooping up the water. Afterwards they perch on the phone wires to have a thorough preening session.
Then I saw something I've not seen before. There's a barrow of molehill soil waiting in the garden to replenish the flower borders; I watched as a swallow hovered for a moment then popped into the barrow and made off with a beakful of soil for nest making. Normally they scour around for puddles but when it's dry, mud can be hard to come by. I shall know in future to leave a load of soil out for them each year at nest building time