My dictionary defines "lilt" as a verb meaning "to sing, speak, play, or move with a light, graceful rhythm or swing." Although it's certainly a matter of personal preference and perspective, I believe most people would agree that several bird songs would qualify as being "lilting." Robin songs are warbley and could be considered lilting. But robins are fairly common birds; and perhaps their familiarity would make some folks think robin songs are too common for this term. So instead, I nominate the wood thrush as having one of the most lilting songs of the forests of the northeastern United States. It's not common. And even if you have wood thrushes in your area, you don't hear them all the time. It's a special treat to hear them. You can hear what their song sounds like by clicking the link to the Cornell University web site below. Perhaps you'll recognize it as something you have heard yourself, but didn't know what kind of bird was making it.