used are not necessarily the same in autumn and spring and may also differ between
experienced (adult) and juvenile birds on their first migration. For conservation
actions it may also be of interest to know the degree of ‘migration connectivity’
among different populations, that is, the movement of individuals between differ-
ent summer and different winter populations (Webster et al. 2002). A number of
approaches are available.
7.2.1Mark-recapture
Most current knowledge about migration routes and wintering areas has been
accumulated over the last century by national ringing (banding) programs, and
recently presented as migration atlases by a few countries (e.g. Fransson and
Pettersson 2001; Werham et al. 2002). Recent interesting results on ringing recov-
eries are usually published as annual reports by the different ringing schemes, such
as the BTO recoveries published in Ringing & Migration. However, even though
millions of birds have been ringed, our knowledge of certain species is still limited
or lacking. By way of example, to get one recovery in Africa south of the Sahara of
Willow Warblers Phylloscopus trochiluscaught in Finland, no less than 16,000 had
to be ringed (Hedenström and Pettersson 1987). Ringing recoveries not only
reveal routes and wintering areas, but they are also useful when analyzing migra-
tion speed, strategies, and orientation mechanisms.
Because the slow rate of generating migration maps based on ringing recoveries,
migrationists have invented supplementary techniques. In addition to a numbered
metal ring, combinations of color bands are used for identification of birds using a
telescope. This method is useful in birds like shorebirds using specific habitats
along the migration routes. Larger birds, such as swans and geese, can be fitted with
numbered neck-collars, likewise checked by telescope. Compared with ringing,
this method allows repeated registration of individual birds along the migration
route and hence improved temporal resolution in data on migration rate and
stopover duration. However, neck-collars are limited to large birds and can accu-
mulate ice during cold weather. Also, it is only possible to get records from locations
visited by observers.
7.2.2Morphology
Individuals from different populations and geographic origin typically vary in
some respects, for example, color, size, wing length, and shape. Hence, morpho-
logical data can be used to distinguish populations at a migration site such as a
bird observatory. However, since this approach is statistical it is a rather blunt
research tool, but combined with genetic or stable isotope markers the number of
individuals that can accurately be assigned to the correct population increases.
162 |Migration