Bar-tailed godwits of the Alaskan subspecies fatten on Alaskan mudflats until fat approaches half their body mass, then depart southward over the open Pacific. Satellite tracking from the mid 2000s onward showed birds arriving in New Zealand without a single stop, and later tags recorded flights above eleven thousand kilometres lasting well over a week.
The physiology is as striking as the distance. Before departure godwits shrink digestive organs they will not use and enlarge flight muscle and heart, converting body composition into range. They cannot feed or drink at sea, and metabolic water from burning fat supplies what they need. Route choice exploits favourable winds, and departure is timed to weather systems rather than to a fixed date.
The northbound journey is not the same trip in reverse. Birds return via a staging stop on the Yellow Sea, which makes those tidal flats a bottleneck for the entire flyway, and their loss to reclamation is the main reason the species is now listed as Near Threatened. Which individuals can sustain the longest flights, and why, is still being worked out.