Overland flight by seabirds at Isla Isabela, GalÁpagos

Oceanic seabirds are generally thought to avoid overland flight, perhaps limiting larger-scale distribution, but examples exist to the contrary. We asked whether Blue-Footed Boobies Sula nebouxii, Brown Pelicans Pelecanus occidentalis, or frigatebirds Fregata spp. crossed the Perry Isthmus (width 12.25 km, minimum height 23 m), a low point in the wasp-waist shape of Isla Isabela, Galápagos. Except for the Perry Isthmus, Isla Isabela presents an elongated, high-elevation challenge to movement between the central and western waters of Galápagos. Daytime observations over 3.5 d in June 2012 revealed ≥48 crossings by boobies and more than two crossings by frigatebirds. Overland crossing of a terrestrial barrier of this size may be facilitated by the seabirds’ ability to see water across the Perry Isthmus from an altitude of 42 m looking west and 43 m looking east, altitudes that both the boobies and frigatebirds attain during foraging.