Data on the infection of birds with Leucocytozoon were suggested to be used for investigation of the structure of hosts' populations in the regions isolated by ecological barriers where breeding places of bloodsucking flies (Simuliidae) are absent. Conducted parasitological investigations have shown that when migrating through the Courland Spit birds of more northern populations extensively infected with Leucocytozoon either do not merge with local populations of chaffinch, willow wren and icterine warbler or their portion is quite negligible. This phenomenon is indicative of the relative stability of the above Courland Spit populations of birds.