During some years in the 1960s and 1970s, thousands of evening grosbeaks visited Pennsylvania, often in flocks of 50 or more. But a massive decline has conservationists puzzled.

When I lived in eastern Pennsylvania, I saw evening grosbeaks almost every year. Two or three times each winter, a flock of 10 to 16 grosbeaks appeared at the feeders. They gorged on my seeds, emptied ...