The house martins have built their homes.
It is mid-May. I am standing in shade beneath the eaves of the Visitor Centre, watching them gather insects high on the wing. The microphones are on a wall, directly beneath two nests.
Hurriedly, both parents return to feed their young: a puff of air as they swoop in tightly; jumbled, bubbling phrases; then gone.
One of the birds returns a few minutes later.