7. Migration
Susanne Åkesson and Anders Hedenström
7.1 Introduction
The flights of some migratory birds are among the most impressive phenomena
in nature. The migration of the Arctic Tern Sterna paradisaeabetween arctic
breeding sites and Antarctic wintering areas (19,000 km one way) is a classic
example, while the 12,000 km non-stop flight between Alaska and New Zealand
by the Bar-tailed Godwit Limosa lapponicais perhaps even more astonishing. At
the other end of the spectrum we find the trickle migration strategy adopted by
many passerines, in which short flights are alternated by refueling episodes.
Adaptations for migration are equally important in all migratory species, as an
integral part of their life histories and annual cycles. They include the morphol-
ogy of body and wings, flexibility of metabolic organs, accumulation of fuel (fat
and protein), sensory capacities for direction finding (orientation and naviga-
tion), as well as an ability to make correct decisions about when to depart and
when to stop. In flight, the bird has to know at what speed and altitude to fly for
best economy, how to maintain its intended flight direction and how to deal with
varying winds.
The questions asked by students of bird migration are very diverse and require
an exclusive “toolbox” of techniques and approaches. Often the research is inter-
disciplinary, using techniques borrowed from fields such as mathematics,
physics, physiology, sensory biology, and morphology. In this chapter we give
some examples of questions asked and research techniques used in a modern bird
migration laboratory.
7.2 Migration systems
Knowledge about population-specific breeding and wintering areas, and the
migration routes between them, is fundamental to migration studies. The routes