7.3.5Stopover
An important part of the migration process and strategy is the stopover periods
during which energy is accumulated. Methods of estimating the duration of
stopover phases and to test if animals of different groups (age and sex classes)
differ in their stopover behavior require appropriate statistical modeling (Schaub
et al. 2001). Simultaneous estimates of fuel accumulation can be made using
individually color-marked birds and a remote operated electronic balance baited
with food or from recaptures of ringed birds. Fuel deposition rate is given by the
fuel deposition divided by stopover duration.
7.4 Physiology of migration
The migration process involves alternate flight and stopover periods during which
energy is consumed and accumulated, respectively. During flight the metabolic
rate is among the highest encountered in animals, and can be sustained for tens
of hours by birds crossing ecological barriers. Also, the fueling interludes require
efficient foraging and metabolic machineries to cope with high rates of food
intake, assimilation, and conversion to adipose tissue in birds flying long nonstop
flights these alternate performance requirements—flight versus fueling—are
associated with rapid physiological changes in the alimentary tract and associated
organs (Piersma and Lindström 1997). During refueling the stomach is enlarged,
the intestine is elongated and surface area increased, and the liver is enlarged. In
flight, however, all unnecessary payload is costly to carry, and therefore organs used
during the fueling period can be reduced to a minimum for best flight economy.
7.4.1Body composition
The most notable change in migratory birds is the accumulation of fat; sometimes
a bird about to undertake a long flight where it cannot feed can double its body
mass due to fat accumulation. Subcutaneous fat (adipose tissue) can be scored
visually (see Chapter 4), with knowledge of body mass and help from regression
analysis, the lean mass and fat mass can be estimated (although the fat class
scoring does not reflect the amount of fat linearly). On dead birds (see Chapter 8),
the fat mass can be determined by chemical extraction using a Sohxlet apparatus
and petroleum ether in a mixture of ethyl alcohol (typically 3 : 1 mixture of 95%
ethyl alcohol and petroleum ether is used for fat extraction). However, it is often
of interest to study the rate of fat (and protein) accumulation in individuals,
in which case working with dead birds is not feasible. A technique that
looked promising was the measuring of total body electrical conductivity
Physiology of migration| 169