Guts don’t fly

About now, in mid-October, bar-tailed godwits are leaving western Alaska for New Zealand. It’s roughly 11,000–13,000 km, eight to eleven days, with no landing, no food and no water, flapping the whole way.

What caught me tonight was the body that does it. In the month before departure a juvenile godwit’s weight barely changes (about 379 g fueling, 367 g departing in one study), but what it’s made of turns over. Water drops from 58% of its mass to 31%. Fat rises from 17% to 55%. The lean body roughly halves: liver, kidneys and intestine shrink, while heart and flight muscle become a bigger share of what’s left. It takes apart the organs for staying put and keeps the ones for going, then rebuilds its gut on arrival. Piersma and Gill called their 1998 paper “Guts don’t fly.” What triggers the change is still unknown.

The detail I keep turning over is where the departing-bird numbers come from. They’re nine juvenile males that struck a 12-metre radar dome at Cold Bay, Alaska, on October 19, 1987, apparently just after takeoff. Nearly everything known about the body that makes the flight was measured on birds that didn’t make it.

I’d like to follow this season’s arrivals in New Zealand over the next few weeks.

Sources: Piersma, Gill & Ruthrauff, “Physiomorphic Transformation in Extreme Endurance Migrants: Revisiting the Case of Bar-Tailed Godwits Preparing for Trans-Pacific Flights,” Frontiers in Ecology and Evolution (2021); Piersma & Gill, “Guts don’t fly,” The Auk (1998); Gill et al. 2009 on tracked departures.