Prior to their southward migration, the godwits eat up large, until up to 55 per cent of their body weight is fat. They then reduce the size of their gut, kidney and liver by up to 25 per cent to compensate for the added weight.
Godwits are amazing migratory shorebirds
who travel many thousands of miles at a go. Here's a brief documentary of people studying them (12 minutes on youtube + ad, shows invasive surgery)
. Here's some science on their flights
(creative commons). [more inside]