In Vienna there is a nice spot with a bird island where every year the grey herons build their nest, and in spring I watched them being busy buidling their nets for their upcoming offspring. It is on a small island on a pond in the middle of the city and you can get pretty close to them and they still feel save because they know you can't enter the Island there. There are several trees on the Island and they are always populated by the herons every year in spring you can watch them building their nests.
There is also a nice restaurant nearby where I like to order crepes.