We conducted an analysis of the ecological compatibility the Oriental cuckoo with known and potential host species on Sakhalin Island, and compared oological parameters of this brood parasite and its hosts on both the mainland and island regions of the Russian Far East. Variation of oological parameters of Oriental cuckoo and host eggs were examined here for the first time. We documented fundamentally-similar egg coloration among Oriental cuckoo and Pallas’s leaf warblers, which supports the assertion by N.N. Balatskii that there is a distinct race that parasitizes Pallas’s leaf warbler nests. The high level of similarity among Oriental cuckoo eggs collected from the mainland and island regions of the Russian Far East reflects broad parasitism of this shared host species, and is evidence of the common origin of these populations.