The Fossil Lake deposits hold the earliest known specimens from the frigatebird family, which today includes five tropical and subtropical species found worldwide. Modern frigatebirds are all seabirds, but this freshwater Fossil Lake species (Limnofregata azygosternon) likely felt right at home, given the lake's wide expanse and warm climate.
Frigatebirds make their living by swooping low over the water to scoop up surface fishes in their long beaks. At Fossil Lake, they likely fed on the huge schools of herring-like Knightia, as well as hatchling turtles and amphibians common in the area. Living frigatebirds are also famous for harassing gulls and terns to give up their catches.
Poor swimmers, walkers, and waders, frigatebirds have a huge wingspan-to-body ratio that allows them to stay aloft for days, landing only to roost or breed on trees or cliffs.
Specimen: Limnofregata azygosternon, FMNH PA755
Locality: Fossil Butte Member of Green River Formation
Credit: © The Field Museum, GEO86607_04d, Photographer John Weinstein