At 530 this evening, three Tree Swallows zipped by. My first of the year. I noticed this pattern in the past too. For a few days, I see them high overhead only. Then I see a few on the river - some even land on the boxes. Then for a few days - no swallows. Then we get dozens with them quickly claiming boxes and lots of swallow shenanigans. They raise their brood and are gone again by July. Then during fall migration there are a number that pass by. Cool and mysterious.
Glad we were able to get out and check the boxes!