Who's that guy whistling from a tree?

Believe me or not, it is that time of the year, when this yearly visitor comes and settles in a tree on shore, right by my boat. He whistles like a man who whistles at a beautiful woman- I know it may not be correct in this country, but in other parts of the world, it is an appreciative and appreciated sign - So this guy whistles, "Fiuifu!", Fuifu!" endlessly. Sometimes I venture to answer him, and of course he answers back. Some days he wakes me up, and in that half awake state, even before I open my eyes, I smile, and then wake up amused. I know what you're gonna say, it's a bird. Well nobody was able to tell me so far which bird sings that way, and I never made it to actually see the creature. So what I think is that it must be a secret admirer. Skeptical? well, he follows me when I go to my workshop on shore, and goes on "Fuifu, Fuifu!". I've been here three years and he comes back each year, never missing the "rendez-vous gallant". He is one of the announcers of spring, you can tell, his song is so insistant and amusing. As if he wanted to tell us all "Wake up, rejoice, spring is on the way!".