Thanks to Matty-O for sending me the link to this website.
Edward Soldaat has curated a fantastic reference collection of seabird skulls, which can be accessed through his Seabird Osteology Pages website. From Loons and Grebes to Albatrosses and Shearwaters, to Gulls and Terns and Penguins and Alcids, this site is full of great skull specimens of over 100 species. But don’t take my word for it, check it out for yourself!