I gave up on the swallows after almost 2 weeks of not seeing them….and then suddenly today they are back at the box! Hope they stick around this time. Now that the weather is warm in PA, perhaps they’re finally ready. If I can’t get bluebirds this is second best for me.
Everybody, including TRES, slows down nesting activity when the food supply is scarce. Since TRES feed completely in the air, they have to wait until insects are flying. Your cool weather is probably what delayed them.