Violet-green swallows hunting the mayfly pulse

The air above San Francisco Bay shimmers with movement this morning. Thousands of mayflies (Ephemeroptera) rise from the water in translucent clouds, their wings catching the light like scattered glass. They emerge all at once, synchronized by temperature and daylight, transforming from aquatic nymphs to flying adults in a single coordinated event that has been building underwater for months.

Violet-green swallows (Tachycineta thalassina) streak through these aerial swarms with surgical precision. Their metallic green backs flash as they bank and dive, mouths open wide to scoop insects from the air. Each swallow can capture dozens of mayflies in a single hunting pass, their throat pouches bulging with the soft-bodied prey. The birds have timed their return from winter grounds in Central America to coincide exactly with this explosion of food. They arrive in San Francisco's parks and riparian corridors just as the mayflies begin their brief adult lives, which last only hours or days.

The mayflies themselves have spent the past year as nymphs in the bay's muddy bottom and in creeks flowing through Golden Gate Park and the Presidio. They filter algae and detritus from the water, growing slowly through multiple molts. When water temperature and photoperiod align in late spring, entire cohorts rise to the surface simultaneously. They shed their nymphal skins, unfurl gossamer wings, and take flight in massive synchronized emergences that can darken the sky. These mayfly pulses feed not just the swallows but also bats, dragonflies, and fish that leap from the water to snatch insects from the air. The female swallows are especially hungry now, converting this protein windfall into eggs. They nest in cavities in the valley oaks (Quercus lobata) and California buckeyes (Aesculus californica) that line the park's edges, where white flower clusters now hang heavy on the buckeye branches. Each female will lay four to six glossy white eggs, and both parents will make hundreds of foraging trips to feed their nestlings. The mayfly emergence provides the caloric foundation for this reproductive effort. A single brood of swallow chicks requires thousands of insects, and the parents must capture them one by one in aerial pursuit.

The mayflies continue their ancient dance above the water, males forming mating swarms that rise and fall like living smoke. Their bodies are impossibly delicate, built for this single reproductive moment. Most will die within hours, their brief adult phase complete. But the swallows remain, their own breeding cycle just beginning, sustained by this pulse of ephemeral abundance. Listen for their liquid chatter as they return to their nests, beaks full of the morning's catch, wings still trembling from the hunt.