A big part of the secret is that many of these 'migrant spots' tend to be in heavily urban/suburban areas. So a migrating bird flying over a lot of houses suddenly sees water and greenery and figures it's a good place for a rest stop. The birds really don't seem to care too much about what might be in the water! There are similar places out in eastern Washington, where the water and greenery is surrounded by unappealing scrubby desert.

My suitcase isn't even started, and I need to go through my manga shelves and make a list of the volumes I need....