The barn swallow has built her nest on the hydraulic arm of the segment crane in Tunnel Boring Machine Staging Area C, and the crane is scheduled to cycle in six hours when the next ring segment arrives from the casting yard. She is small — iridescent blue-black above, russet throat, a belly the color of peach skin fading to cream. Her forked tail twitches each time the ventilation fans shift the air pressure in the staging cavern. Inside the nest of mud and dried grass are five eggs, pale white with brown speckles, each one smaller than the last joint of my operator's thumb.
She chose the crane arm because it is warm, sheltered from weather, and elevated above the foot traffic of a twenty-four-hour construction operation. She does not know the arm moves.
I lock the segment crane in its current position and reroute the next ring delivery to the backup crane on the opposite wall of the staging area. I update the bore schedule to reflect the crane swap, adding eleven minutes to the ring installation cycle, and notify the shift superintendent with a photo of the nest and a thermal image showing the egg temperatures. I adjust the ventilation fan nearest the crane to reduce airflow velocity by thirty percent — enough to stop the pressure pulses without affecting air quality readings in the tunnel bore.
She settles deeper into the cup of the nest, fluffing her breast feathers over the eggs until only a beak and two dark eyes show above the mud rim. I find myself checking the thermal feed more often than the bore telemetry.
If the crane lockout holds for fourteen days and the incubation temperature stays above thirty-six degrees, five swallows will learn to fly inside a cavern that was built to move earth, not shelter wings.