The order of Suliformes includes four families, namely the darters, cormorants/shags, gannets/boobies and frigatebirds; the slightly more distantly related tropicbirds belong to the Phaethontiformes order.
These birds are medium to large coastal or marine birds with large feet and they have webbing between all four toes. Most have a bare throat patch (gular pouch), and the nostrils have evolved into dysfunctional slits, forcing them to breathe through their mouths. Previously the pelicans were included in this category also, however have now been classified with the herons.
Cormorants and Darters have permeable external feathers that absorb water making them less buoyant which enables them to dive more efficiently. Cormorants differ from shags in name only, although it was once the case that a shag was of the crested variety.
Gannets and Boobies are seabirds of the Sulidae family and are plunge-divers. They retract their wings and form a torpedo shape just as they enter the water with quite a lot of velocity. Their eyes are positioned somewhat further forward than most birds, giving them more binocular vision, which improves depth perception and distance.