In 1976, Poland issued a set of 6 stamps, each featuring one of its National Parks. The 90g stamp shows a view of Wielkopolski National Park and a Tawny Owl, while the 1z stamp shows Wolinski National Park with a White-tailed Eagle. The only other stamp with a bird shows what looks like a gull on the 1.50z value featuring Slowinski National Park.
I recently checked some of my "unidentified" birds and while looking at a web page for Slowinski I noticed that "The symbol of the Park is a seagull (Larus argentatus), an outline of a dune and blue water."
I checked the pages for Wielkopolski where it stated that "The symbol of the Park is the tawny owl (Strix aluco)". Similarly, the page for Wolinski states that "The symbol of the Park is the white-tailed eagle (Haliaeetus albicilla)".
I checked pages for the other 3 stamps and confirmed that each shows the animal which is the symbol of its respective park.
I propose, therefore, that the unidentified gull is best assigned to Herring Gull (Larus argentatus) and will change it on my site during the next monthly update.
If anyone would like to comment on this proposal, please email me.