An estimated 7,500 to 10,000 purple martins are roosting near Presque Isle Bay.

Joe Siegrist never tires of watching purple martins swirl above the cattails south of Erie's South Pier.

Some nights, the swift, skilled flyers — North America's largest swallow — soar by the hundreds or thousands high in the sky. Other times, they show off their agility and maneuvers close to the ground.

They show up earlier on some evenings and later on others to perform their aerial acrobatics before they settle into their roost.

"It's a great show every night," said Siegrist, 42, president and CEO of the Erie-based Purple Martin Conservation Association. "Each night is different depending on weather conditions. It just varies. You never really know how they are going to behave."

After spending most of their day searching for food, the purple martins return to their roost and put on one of nature's best ornithological shows.

"It's spectacular to see them come in during the evening when they're all swirling around and then they plummet to the ground right into the reeds," said Erie resident Jerry McWilliams, founder and director of Presque Isle Hawk Watch and chairman of the bird records committee for the Presque Isle Audubon Society.

"If you haven't seen it, you're missing a spectacle," McWilliams, 65, said. "About 340 bird species have been recorded in Erie County. Purple martins are one of the few species where you can see huge numbers of one kind of bird. You can go out and see 10,000 on some nights."

Those who wish to see the purple martins and learn more about them can attend a free public gathering, hosted by the Purple Martin Conservation Association, on Wednesday at 7:30 p.m. at the South Pier. Siegrist and staff from the Erie nonprofit will be on hand to answer questions.

Each year, purple martins migrate to Erie starting in early April to nest near Presque Isle Bay before they begin their winter migration to South America.

Siegrist estimates the roost near the South Pier at between 7,500 and 10,000 birds. In the next couple weeks, those numbers could grow.

"We're still shy of 10,000," Siegrist said. "Next week it should swell. Last year we had close to 20,000 here. It will probably peak next week or a little bit after that and really tail off by the end of August. Historically, we've had up to 30,000 in the past."

Studies have determined that purple martins migrate to the Erie roost in the spring and summer from 200 to 300 miles in all directions, Siegrist said.

During the past three months, the Erie colony has grown steadily.

"There are probably a few hundred purple martin roosts across North America," Siegrist said. "Probably the closest one is right across Lake Erie at Long Point, Ontario. We put GPS trackers on them as part of our research and we see them going back and forth, mixing. It's a bigger roost than ours. The lake is not an impediment. They fly miles and miles every day."

Purple martins feed on larger flying insects such as dragonflies, butterflies and moths, McWilliams said.

"They like close proximity to the water along Lake Erie and they like to eat dragonflies, which you find at the water," he said.

Siegrist said the purple martin colony here has enjoyed a good breeding year.

"Sometimes really rainy springs are a problem if there are a lot of rainy days in a row without a break," Siegrist said. "Luckily, we've had breaks here and there. A lot of rain makes a lot of insects to eat. A lot of insects means they lay more eggs, and a lot of insects means there's more food for the babies. We really had a bumper crop this year."

During the day, the martins spread out and search for food all over the region.

"When they leave in the morning, they show up on Doppler radar," Siegrist said. "You can see how far they really go. They will leave their roost before sunrise. It's amazing how far they go in a day to feed and just to come right back."

Their primary predatory threats are hawks, owls, and peregrine falcons.

In 2017, purple martins roosted near Leo's Landing on Presque Isle State Park, then relocated to the South Pier area in 2018. Before 2017, the colony roosted for nearly 20 years in the cattails at the head of Presque Isle Bay.

"I think that the original spot in the west part of the bay where those cattails were was kind of a victim of its own popularity," Siegrist said. "There were a lot of kayakers that maybe didn't respect the distance and would paddle straight into it. Also, the high water we've had over the last few years has thinned out those cattails. That species of cattail on that end of the bay is actually an invasive one that doesn't tolerate deep water, so it's really thinning out there. It's been part of the reason I think that they have come to the South Pier. They just drive on instinct, and these guys like to go where there's other birds."

Within a month, most purple martins will have departed on their migration to South America.

"It's a slow burn," Siegrist said. "They don't migrate together in these big flocks. They'll leave in small groups. They just kind of all go their own way. By mid-September, most are gone."

Ron Leonardi can be reached at 870-1680 or by e-mail. Follow him on Twitter at www.Twitter.com/ETNLeonardi.