The bar-tailed godwit, Limosa lapponica baueri, or kuaka, is a wading shorebird seen on marshy estuaries and wetlands during the New Zealand summer. Populations of this species embark on some of the longest migrations known amongst migratory animals.

Bar-tailed godwits breed in Alaska, spending the non-breeding season in New Zealand and eastern Australia (see map opposite). The journey from New Zealand to Alaska - a distance of up to 13  000 km - is completed in two stages. It begins in March, when the birds fly to South Korea, Japan or China. Here, they spend some time on feeding grounds (called staging sites) that may be shared with other migratory species, before leaving for Alaska in May. The birds return to New Zealand in September and October, taking a direct, non-stop route over the Pacific Ocean - a flight of approximately 11 000 km that takes 6 - 10 days. This southward migration begins about one month earlier than that of other migratoryspecies.

Before each migration begins, flocks of godwits congregate at leaving sites and feed continuously. At the time of departure, up to 45% of their body weight is fat. Adult birds accumulate more fat than juveniles. The timing of the migratory flights coincides with weather systems, in both northern and southern hemispheres, that generate favourable tail winds.

Discuss the biological concepts associated with the migration of the bar-tailed godwit. In your  discussion, consider:

• the biological mechanisms involved in the preparation for, and during the migration
• the benefits and risks of migration to the birds
• how this behaviour may have evolved