ИСТИНА

Zones of secondary contact and hybridization in birds attract attention for many decades (Mayr, 1968; Panov 1989; Rheind and Edwards, 2011). The study of hybrid zones allows to see the «evolution in action» (Panov, 2001) and provides unique opportunities for understanding the processes of populations differentiation and speciation. A variety of hybrid zones in birds is difficult to reduce to a single scheme (Panov, 1989). Different classifications are proposed based on the extent of divergence between taxa, length and width of the zone of hybridization and the fraction of phenotypic or genetic hybrids (Kryukov, 1990; Short, 1969; Price, 2008). In the present paper, we briefly discuss the main problems associated with hybridization in birds. We concentrate mainly on the field investigations conducted over the past two decades using molecular genetics methods. We examine the following problems based on our own studies and the results obtained by other researchers. Are there barriers to gene flow and selection against hybrids? Are mixed singers males hybrids or they may have acquired their song by coping a heterospecific tutor? We also discuss the phenomenon of stability of species-specific phenotype in the face of introgression of alien mtDNA and migration routes of the hybrids and parental forms. As examples we discuss hybridisation zone of leaf warblers (Phylloscopus collybita, Ph.trochiloides), nightingales (Luscinia megarhynchos - L. luscinia), hermit and townsend’s warblers (Dendroica occidentalis - Dendroica townsendi) and some others.