The national and nature parks along the Adriatic coast are all excellent areas for bird watching:

Island Cres: for it’s population of Griffon vultures
NP Kornati and NP Telascica both comprise an archipelago of numerous islands with steep cliffs and shallow bays, which make it ideal for breeding of Shags, Swifts and birds of prey.
NP Biokovo: A high rocky mountain area with birds of prey, Swifts, Rock Trush and many more.
NP Mljet: A green island and an oasis for several bird species like Shag, Audouin’s gull, Rock Partridge and Nightjar.
NP Vrana Lake : a shallow lake ideal for all kinds of waterbirds, it is a perfect birding site
NP Krka: important area for waterbirds, birds of prey and also an important region for spring and autumn migration
Neretva: a unique habitat as the fresh water of the Neretva end in lagoons and salt marshes before it enters the Adriatic sea it has the largest breeding population of bitterns in Croatia.