A public outcry led to an eventual ban of DDT, the main culprit, in the United States. And many of those countries are where our songbirds migrate during the winter. Unfortunately for many songbirds, increased pressure to change habitats has resulted in the destruction of the birds' homes causing their populations to crash. Many organizations are involved with observing changes in bird populations. Locally, the Point Reyes Bird Observatory is extremely active with banding and monitoring programs.
Banded birds are only helpful if the bands are recovered.