Լ����Ƶ

After birding several spring migrations on the East Coast, you might start to notice some patterns. Species don’t show up in April and May all mixed up in an avian potpourri; instead, they tend to pass through in a predictable order year after year.

No one knows this better than banders at Ohio’s (BSBO). Situated on Erie’s lakeshore, BSBO is one of the last places migrants can rest before crossing the Great Lakes for breeding grounds in Canada. When they alight to refuel, some hit mist nets set up in the vegetation below. Trained experts remove birds, take measurements, place a band around an ankle, and then send them on their way.

“It’s a whirlwind,” says lead bander Ryan Jacob, who has worked these nets since 2009 when he began as a volunteer. “People’s memories are terrible,” he says, so it’s important to have data. And BSBO has a lot of it: Since 1992 they’ve banded 225,000 spring migrants—an average of 7,000 annually.

From those data the station has detailed a wave theory of avian migration. Each wave has a unique cast of characters that share ecological traits, such as wintering grounds and diet. And within each are two smaller peaks divided by sex: Males tend to migrate ahead of females to claim nesting territories.

The birds’ arrival dates and precise order vary annually. Wind patterns shift. Weather, like storms or late freezes, can interrupt migration, whereas warm breezes and clear skies hasten birds along. Habitat changes on wintering grounds or stopover sites alter routes. But overall, the waves are reliable.

That’s helpful for banders, who must carefully plan to process sometimes 600 birds in a single day. “Our number-one priority is bird safety,” Jacob says. “Wave theory helps us predict when the big pushes of birds are going to come.” It’s helpful for birders, too: By knowing what birds to expect they can better prepare for a day in the field. “It doesn’t take away from the magic of it,” Jacob says. “It’s reassuring to know: They’re going to be here, and I know when they’re going to be here.”

The Three Waves of Migration

Wave 1: Early Birds

Short-distance migrants that winter in the southern United States or northern Mexico dominate the first wave to reach BSBO. When spring’s sunlight hits the Arctic, warmth spreads south, spurring them north. Some eat seeds and thus can find food before insects hatch. Species include: Yellow-rumped Warbler, White-throated Sparrow, Ruby-crowned Kinglet, Hermit Thrush, Swamp Sparrow, and Palm Warbler

Wave 2: Neotropical Travelers

The pace of migration picks up in May. The second wave features many colorful migrants traveling from the tropics. These species depart when they sense changes in day length and other subtle cues. They arrive just in time to glean the first emerging midges and other insects off leaves and branches. Species include: Gray Catbird, Least Flycatcher, Magnolia Warbler, Northern Waterthrush, House Wren, Ovenbird, Tennessee Warbler, and Yellow Warbler

Wave 3: Bug Lovers

The third wave doesn’t come until spring is in full force. Leaves are out, the weather is warm, and there are plenty of bugs. That’s when avian insectivores skilled at catching midair insects show up: flycatchers and certain warblers. Caterpillars are also abundant, supporting vireos and cuckoos. Species include: Willow Flycatcher, Alder Flycatcher, Red-eyed Vireo, Yellow-bellied Flycatcher, American Redstart, Wilson’s Warbler, and Swainson’s Thrush

Does the West Coast Experience Waves?

Songbird migration looks different in the West. So say data from California’s Palomarin Field Station, operated since 1966 by the nonprofit . The banding station, which records some 3,000 birds a year, is situated on the coast just north of San Francisco specifically to study songbird migration: Many species follow the shoreline as they travel north from wintering grounds.

As early as January, plants start greening, and birders begin hearing Allen’s Hummingbird’s distinct wing whistle. But it’s not a starting bell for a massive influx of birds, as in Ohio. Here, spring migrants arrive in two distinct bursts, says avian ecologist Mark Dettling, a Palomarin banding supervisor. In mid-March bug catchers, such as Tree Swallow and Orange-crowned Warbler, arrive for the insect hatch. Then, in mid- April, berry and seed eaters, such as Black-headed Grosbeak and Swainson’s Thrush, make an entrance.

The data don’t form wave “peaks,” however, because most of the birds aren’t visiting for a brief stopover. Some move on to nest farther north. But many stick around and breed near the station. “It’s an elongated experience,” Dettling says. After a few months, those birds depart and are replaced by species escaping the northern winter, such as Fox Sparrow. “We get this turnover in species. It’s not like where I grew up in Michigan, where you get several months where there is a paucity,” Dettling says. “It’s lovely for us.”

Graph: Julie Rossman/Լ����Ƶ. Illustrations: David Allen Sibley

A version of this story originally ran in the Spring 2023 issue as “The Waves of Migration.” To receive our print magazine, become a member by .