A number of good complete and partial skeletons, some with feather impressions, are known of the type species, Limnofregata azygosternon,[2] and L. hasegawai is known from two skulls and most of one torso.

The bill of L. hasegawai was notably larger still than that of its congener, whereas its feet were smaller, echoing a pattern found in the extant Fregata species.

[4] The Eocene frigatebird genus Limnofregata comprises birds whose fossil remains were recovered from prehistoric freshwater environments, unlike the marine preferences of their modern-day relatives.

[3][5] They inhabited the freshwater or brackish lakes that in time formed in today's Green River valley due to the uplift of the Rocky Mountains, feeding on smaller vertebrates and probably harassing other shorebirds for food and feasting on dead fish (e.g. Knightia) during summer dieoffs due to oxygen depletion in the eutrophic lakes.

[5] Modern frigatebirds show pronounced sexual dimorphism in size, apparently to avoid competition due to different soaring behavior of male and female wings.