Meanwhile, overhead, was a constant, scratchy warbling call. It was one of those calls that puzzles at first, but then sinks in. Oh! Warbling Vireo!
Though not uncommon in deciduous woods, especially during spring migration, Warbling Vireos often escape being sighted, because they are drably green/grey/yellow, and they move through the densest foliage of deciduous trees (often cottonwoods and aspens), gleaning caterpillars and other invertebrates from the leaves.
They are not a bird to inspire a novice, unless the novice has a thing for subtlety.