We've had bluebirds around for years too and usually they come for the water and my daughter has put out worms for them.. well, this year we put out one of the suet for "woodpecker's" and they have been at it daily..I think they are after the bugs and fruit in it...
We also put out a couple of bluebird house's and they are nesting in one of them... just too nice. We are loving bluebirds this year..