A few weeks ago, I heard Tree Swallows twittering overhead and I read a couple reports of them over large lakes. This coincided with the emergence of a small stonefly I see emerging about mid-March and I've even seen this insect on snow. Then there is a lull with few reports of swallows followed by a blizzard of activity.
My guess is that the first wave is traveling up to the Alaska coast where they need to travel farther and start breeding as soon as they can. They are supported by early hatches. Once warmer and other insects are out and about the second wave of swallows arrive and start breeding. This is now the fourth year I've noticed this pattern. We just need to stick a geolocator on a bunch of swallows and sit back.
It was a perfect morning to walk into school - long overdue.