Swallows gather insects over Shelby Bottoms water

The air above Walter S. Davis Boulevard Marsh shimmers with movement. Tree swallows and barn swallows cut precise arcs through the warming air, their wings catching the late spring light as they work the space between water and sky. The Cumberland River bottomland spreads wide here at Shelby Bottoms, and the marsh holds the morning coolness even as the day builds heat.

Both swallow species arrive at this same airspace for the same reason: insects are lifting off the water in steady pulses. Midges, mayflies, and caddisflies complete their aquatic larval stages and emerge as winged adults, rising from the marsh surface in loose clouds. The tree swallows take the higher hunting lanes, their iridescent blue-green backs flashing as they bank and turn. Barn swallows work closer to the water, their deeply forked tails acting as rudders for tight maneuvers around the cattails and emergent vegetation. Each species has carved out its feeding territory in three dimensions.

The timing is precise. These aquatic insects have spent months or even a full year developing in the marsh sediments and shallows. As water temperatures climb and daylight stretches longer, they emerge in synchronized waves. The swallows have timed their own nesting cycle to match this abundance. Tree swallows nest in the wooden boxes and natural cavities around the marsh edges, while barn swallows construct their mud cups under the nearby bridges and building overhangs. Both species are feeding young now, and a single brood requires thousands of insects over the course of two weeks.

The swallows' hunting techniques reveal the insects' behavior. Tree swallows climb and dive in loose spirals, following the thermal currents that carry insects upward from the warming water surface. Barn swallows skim and dart just above the marsh grass, intercepting insects before they gain altitude. Each swallow can catch and swallow an insect every few seconds during peak emergence periods. They return to their nests with their throats bulging with compressed prey, then immediately return to the hunt. The red-winged blackbirds call from the cattail stands below, and the green heron waits motionless at the water's edge, but this aerial feast belongs to the swallows.

The marsh surface reflects the morning sky, broken by the expanding rings where insects push through the surface tension. Somewhere above, a swallow banks into another turn, its shadow racing across the water before disappearing into the light.