Morphological characteristics and Systemmatics

According to the Hackett Taxonomy, loons, penguins, storks, and as well as Suliformes and Pelecaniformes, all seem to have evolved from a common ancestor. The Order Suliformes is a proposed order by the International Ornithologist's Union. Suliformes includes 4 families, 6 genera and 15 species according to Eastern North America. Members of this order are Cormorants (Family: Phalacrocoracidae), Anhinga (Family: Anhingidae), Booby and Gannets (Family: Sulidae), Frigate birds (Family: Fregatidae). They are medium to large sea-birds usually found over tropical oceans. They prefer fresh water wetlands, ponds, rivers for habitat. They are mostly fish and small aquatic animal eaters. They have webbed feet. 

Anhinga's feet

                  For Cormorants and Anhingas, A major characteristic that unites them is the lack of an oil gland for waterproofing their wings. Thus they are often seen with their wings spread out to dry their feathers. The Anhinga is frequently seen soaring high in the sky overhead. It is a graceful flier and can travel long distances without flapping its wings, much in the manner of a Turkey Vulture.  Anhinga swims lower in the water than many other birds due to its reduced buoyancy a result of wetted plumage and dense bones. They have impressive fishing technique: diving and chasing fish underwater with powerful propulsion from webbed feet.

           Sulidae have long, narrow and pointed wings, and a quite long, graduated and rather lozenge-shaped tail whose outer feathers are shorter than the central ones. Their flight muscles are rather small to allow for the small cross section required for plunge-diving, and thus their wing loading is high.

                                                  Male Frigatebird on top, female on bottom

A long-winged, fork-tailed bird of tropical oceans, the Magnificent Frigatebird is an agile flier that snatches food off the surface of the ocean and steals food from other birds. Frigatebirds produce very little oil and therefore do not land in the ocean. The gular sac is used as part of a courtship display and is, perhaps, the most striking frigatebird feature. The males have inflatable colored throat pouches. the females have a white patch on her underside.

Sources
google images
wikipedia