Coulee Region Audubon Society Bird Quiz

The trail widens a little as it enters a large open field, overgrown with grasses and a few clumps of a dogwood of some kind. Perched in one of the small shrubs is a small brown bird. As you stand watching it, it lifts its head and sings. The song is quite musical, each slurred note a little shorter than the last, with the song speeding up until it ends in a trill. You can see another bird of the same color and size nearby with what looks to be green caterpillars in its bill. This second bird flies down into the grass nearby. What do you think these birds are?