Spring birds

Spring migration is interesting because of both who is arriving and who is leaving. I still have a small group of Golden-crowned Sparrows in my yard here in Oregon, and they are supposed to be on their breeding territories somewhere in British Columbia and Alaska in just a few weeks. How do they know their patch of boreal forest is still snow-covered? But also today a House Wren was checking out a nestbox here. I wonder if he wintered in California, or possibly in Costa Rica, and what adventures he had getting here.