Terns

Caspian Terns with the orange beaks and black capped heads
Common Terns with the black beaks and whiter heads

The Caspian Tern (Hydroprogne caspia, formerly Sterna caspia; Hydroprogne tschegrava) is a species of tern, with a subcosmopolitan but scattered distribution. Despite its extensive range, it is monotypic of its genus, and has no subspecies accepted either. In New Zealand it is also known by the Maori name Taranui.

The Common Tern (Sterna hirundo) is a seabird of the tern family Sternidae. This bird has a circumpolar distribution breeding in temperate and sub-Arctic regions of Europe, Asia and east and central North America. It is strongly migratory, wintering in coastal tropical and subtropical regions. It is sometimes known as the sea swallow. The term "Commic Tern" refers to birds which are not seen well enough to positively determined if they are Common or Arctic Terns.

Source: http://www.wikipedia.org/