You've been sitting quietly in an old field among some hawthorn bushes and small sumacs. A small yellow bird lands on a branch nearby, throws back its head and sings. The first note is a whistle, but the second note sounds more like and insect call than a bird song. After about a minute the bird repeats the two notes. A second bird that looks exactly like the first appears, carrying some dead insects, and flies down to a clump of grass nearby. Just before it disappears among the grasses you notice the black line through its eye. What kind of bird are they?