This is old news for veteran birders but the unsettled weather over the last week has brought us a lot of migrants - and the forecast for the coming few days looks to be more of the same. The basic idea is that birds initiate migration after dusk (most of our songbirds are nocturnal migrants) and then are grounded (i.e., they stop migration for the night and make do where they are) if they run into rain or even a strong weather system with very little rain but keep migrating if the weather is good. In my experience in Idaho, this phenomenon is much more pronounced in spring migration than in fall - and May is the peak migration for many Neotropical migrants.
For example, on a quick neighborhood walk with the dog this AM, I eBirded 26 Western Tanagers (probably a conservative #), 3 Western Wood-pewees, & more migrants. In contrast, last weekend Heidi & I birded after a clear night and found relatively few migrants.
Take home message - get out birding when you can during breaks in the weather & you;ll be rewarded! I would guess there are currently many hundreds, if not well over a thousand, Western Tanagers in and around Boise - and just imagine the #s across southern Idaho.
Enjoy spring migration,