Who says this?

"Suweet? Suweet? Suweet?...."

It's a clear whistled phrase, repeated, without other ornamentation.

Whoever it is, they hide well. I heard this today at Pritchard Beach in a willow thicket, at Martha Washington from a oak thicket and from the neighbor's oak tree just now at dusk. I? have yet to see who it is.

Ideas?

Thanks,
Catherine Alexander
Lakewood Neighborhood
South Seattle