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The 1.4-ounce female Northern cardinal squawked, squirmed and bit Brian Rademacher hard enough to leave a red mark on his hand. Minutes later, she flew free with a tiny metal band around her leg.

Rademacher described the bite as a “severe pinch.” But the bird’s return was welcome: She was a recapture, first caught and banded at Occoquan Bay National Wildlife Refuge three weeks earlier.

At the height of spring migration in April and May, volunteers operate mist nets near Marumsco Creek from 6 a.m. to noon three mornings a week. The annual project began in 2001 and has continued every spring except 2020.

During a typical six-hour session, the team catches 20 to 30 birds, with about 500 banded over a season. The white-throated sparrow is the species caught most often.

By April 27, BirdCast estimated that 45 million birds had crossed the region since early March. Birds migrate in search of food, favorable climates and breeding grounds, while their movements are shaped by weather, wind, habitat, human disturbances and conditions on their wintering grounds.

“A calm, cold night without rain or a big headwind is ideal for migration,” project leader Suzanne Miller said. Songbirds generally travel high overhead at night, while hawks use rising columns of warm air known as thermals.

Five Minutes to Gather the Data

The team follows Monitoring Avian Productivity and Survivorship protocols and does not attract birds with seeds or recorded calls. Volunteers inspect 17 fine, black mist nets every 30 minutes.

When a bird is caught, banders carefully remove it from the same side on which it entered, reducing the risk of a broken wing or leg. They place the bird in a cloth bag and carry it to the station, where they measure its wings and tail, record its weight, and determine its age and sex.

The goal is to complete the process in about five minutes to limit stress.

Banders also blow gently on the breast and belly to examine areas hidden beneath the feathers. Paul Napier said they look for a brood patch, where bare skin can reveal the stage of egg incubation; a cloacal protuberance associated with breeding condition; and fat that indicates the bird’s energy reserves.

The cloaca is the shared rear opening for birds’ digestive, reproductive and urinary systems. A pronounced protuberance can show that a male is ready to breed, while one on a female may indicate that she is carrying eggs.

Fat levels offer clues about the bird’s recent journey and surroundings. Little fat may mean a bird used significant energy while traveling. Greater reserves can point to productive habitat with plentiful insects, grubs and inchworms.

Recaptures Build a Longer Record

Volunteers carefully document birds that return to the nets. Since 2001, the project has recaptured 74 of the 652 cardinals it has banded, a rate of 11.3 percent.

Northern cardinals are year-round Northern Virginia residents and generally remain close to where they hatched, Miller said. Recapture records can help researchers estimate age and track survival.

All collected information goes to the U.S. Geological Survey’s Bird Banding Laboratory. Scientists use banding data to study bird behavior, populations, distribution and survival rates.

Napier, a Falls Church resident, leads the Cape May Raptor Banding Project in New Jersey. He has banded raptors since 1988 and songbirds since 2005, and said every bird teaches him something new.

Beverly Arnold travels from Fredericksburg to participate. She called the work “a labor of love” and said it allows her to contribute to citizen science. Springfield resident Janet Martin described the experience simply: “It’s so cool.”

Because the work involves capturing and handling wild birds, banding is regulated by federal and state law. The team conducts its banding, marking and sampling under a federal permit issued by the U.S. Geological Survey. The refuge also holds permits from Virginia and the U.S. Fish and Wildlife Service.

Occoquan Bay National Wildlife Refuge covers about one square mile at the mouth of the Occoquan River on Belmont Bay. Its tidal marshes, grasslands and forests are crossed by four miles of trails. More than 650 plant species and 200 bird species have been documented there.

The banding station is open to visitors from 7 a.m. to noon through Memorial Day weekend. Groups of more than 10 people need a permit.