Nets 'filled with birds' this fall surprise Pelee Island field bander after quiet spring season

Within a half hour of opening the 10 Pelee Island Bird Observatory nets, field supervisor Sumiko Onishi said many of them they were "filled with birds" — some with 15 to 20. 

This caught Onishi off guard. 

While fall banding season is always busy, Onishi said this year's spike was unexpected because the spring season had been the "quietest banding season" she had experienced in her 15-year career. Fluctuations in migratory birds are common, but this year's appeared a bit more drastic to those on the ground — some of who have spent years monitoring Pelee Island's bird species. 

"In the spring time, I kept thinking 'maybe tomorrow it's busy, maybe tomorrow it's busy,'" Onishi said with a laugh. But the day never came. 

Back in June, bird banding assistant Alexandra Wilcox told CBC News that while the station usually bands between 600 and 700 birds in the spring season — this year saw half of that. 

At the time, they could only hypothesize about what might have happened, such as bad weather during peak migration or, on the more extreme end, a significant population drop-off. Now with such a busy fall, Onishi said it raised even more questions. 

Between mid-August to Oct. 31, Onishi and her assistant banded close to 2,000 birds. Though this is relatively on par with past years, it was still an increase. 

"This is maybe the first time I saw such a big difference between spring and fall," she said.

While it's hard to know for certain, there's a few hypotheses that could explain the jump. 

For one, Onishi said there was a Spruce budworm outbreak in the boreal forests, in particular those in Northern Ontario and Manitoba. The worm outbreak is cyclical, according to Onishi, and happens every few years.

This sudden increase can drive up bird species that feed on the insect, including the Bay-breasted Warbler, the Tennessee Warbler and the Cape May Warbler. 

According to Canadian Bird Migration Network's director of migration ecology Stuart Mackenzie, weather patterns and breeding season can also cause fluctuations. 

"It may have been a good breeding season in the areas of the north where we monitor and so that means in the fall we are detecting not only adults, but also all of the young that were born over the summer as well," Mackenzie said.

"So typically in the fall we're monitoring double if not more than the pure number of birds that are migrating through any area."