Warbler Guy, how do I know if warbler migration is strong? Migrating warblers are more dense on some spring days than other ones?

One that is good: Birdcast

See:

http://birdcast.info

At this link, you'll read about the current week's presence of migrators and predictions.

It is a great resource, given the BirdCast forecast highlights migrant species that you can expect to see in each of several regions: Upper Midwest and Northeast; Gulf Coast and Southeast; Great Plains; and West.