A brief review of the intrapopulation differences in bird movements that are attributed to the phenomena of differential migration is given, and the main hypotheses put forward to explain the biological meaning of these phenomena are provided. Protandry during spring arrival, variations in the timing of autumn migration in birds of different sexes and ages, and intrapopulation differences in the distance of migration are considered. When discussing all aspects of differential migration, primary attention is paid to the conflicting results obtained for different bird species. A possible reason for the apparent inconsistency of the results is that the ringing data from ornithological stations are not sufficiently informative about the territorial distribution of migratory birds from individual populations, since it is not known which populations the ringed birds belong to.
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