Some birds perform their entire migration with a single bout of flying. This is true of some short-distance migrants which can cover the whole distance within a single day or night and of shorebirds and others whose migration routes lie entirely over water or other unsuitable habitat, and may take up to several days and nights of non-stop flight (Chapter 6). However, the majority of birds break their journeys for hours, days or weeks at a time in places where the food situation can influence their fattening rates, and subsequent breeding or survival (Chapter 27). The term stopover ecology has been increasingly used for studies at stopover sites, where birds are often crowded, present for only a short time, and feeding hard in order to replenish their depleted body reserves and continue their journeys. Attempts are made to estimate the periods that individual birds are present, and by repeated trapping and weighing, to determine their rates of fuel deposition.
Stopover periods are generally hard to measure because one can seldom be sure exactly when particular birds arrive or leave. However, precise measures have been obtained for marked individuals of conspicuous species that can be seen throughout their stay (such as cranes and swans), radio-tagged birds that can be monitored daily, and other birds that visit such small sites that they can be caught within hours of their arrival, and then seen throughout their stay, and where there is no danger of confusing migrants with local residents, as at the garden of St Catherine's monastery in the Sinai desert (Lavée et al. 1991). Otherwise birds are often present in an area before they are seen or captured, and may also stay some time after the last observation or capture. Hence, in most studies stopovers have been estimated as minimum values between first and last sighting or first and last capture. Such estimates can be improved in various ways, for example by use of various statistical methods (Kaiser 1999, Schaub et al. 2001).
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