I used to survey bird nests along a trail in a state park. I volunteered for the work with my girlfriend. During the months of May, June, and July we would trudge through the un-mowed thigh-high grass and check 20+ nest boxes for Eastern Bluebirds and Tree Swallows.
Swapping out rotations with a few other intrepid volunteers, we watched their eggs turn into blind little baby birds and eventually saw their feathers grow in. Then one day they wouldn't be in the nest boxes anymore. They fledged.
We'd sometimes see the young in the trees and grass around the boxes, begging for food from their parents. Those were the hot quiet days of summer in the fields and woods. We used to duct tape our long pants up to our knees so the ticks would get caught. I've never seen so many in my life...but I digress.
The protocol was to approach the nestbox from an oblique angle (as to not make a path for predators to follow) and then to knock on the box. This would cause any parent on the nest to fly out so we could quickly count and assess the eggs and young inside. We sent our data to national Audubon for collation.
The adults would swoop at us and make little bubbling sounds but we never overstayed our welcome and tried to be done quickly. They'd perch up in the trees and yell at us.
Tree Swallows will always have a special place in my heart because of this experience.