Local biologists said the birds' behavior is the first in memory for the Pacific Northwest.
"I've never seen anything like it," said Bud Anderson, a longtime raptor biologist.
The heat wave, which is responsible for at least 125 people's deaths in Washington state, sent Seattle temperatures above 100 for three days, buckling roads and overwhelming emergency rooms. Climate scientists say the event would have been virtually impossible if not for climate change.
Animal rehabilitation centers were flooded, too, with an influx of baby birds, including some with broken wings, others with dehydration or heat stress.
In the aftermath of the heat wave, the PAWS Wildlife Center in Lynnwood received 31 raptors, including eight bald eagles, three barn owls, 11 Cooper's hawks, five merlins, three red-tailed hawks, and a sharp-shinned hawk.
It took in 44 swallows, which are insect-eating songbirds, and also 53 Caspian terns, which are seabirds with distinctive orange bills, also, making the summer — usually a busy season — "nonstop," said Dr. Nicki Rosenhagen, a PAWS veterinarian.
The terns — which live in large colonies — arrived from a group living on top of a vacant industrial building south of downtown Seattle. As the temperature peaked, dozens of baby birds jumped from the roof to escape the heat, Rosenhagen said. Dozens died.
Many that survived "had burns — foot lesions on the bottom of their feet," Rosenhagen said. "A number had broken wings and broken legs."
The raptors at PAWS — all juveniles — fared better. Most were suffering from dehydration or were in poor physical condition, but otherwise uninjured.