ACTA ORMTHOLOGICA Vol.32 (199D No.2 : Wieslaw WRLeNKswrczl , Cezary MITRUSt, Dorota Czpszcznvttr', Piotr M. JawotGrct Is the Pied FlycatcherFicedulahypoleuca overcompeted by the Collared Flycatcher Ficedula albicollls in the natural forest of BialowieLa? Walankiewicz W., Mitrus C, Czeszczewik D., fablorftski P. M, 1997 . Is the Pied Flyc atchet Ficeilula hypoleuca overcompeted by the Collared Flycatcher Ficedula albicollis intre natural forest of Bialowieia? Aclaorn 3L,2J3277. Abstract. We studied the fluctuation n 1975-7997 numbers of two species: the Pied Flycatcher Fiuduk hypoleuu and the Collared Rycatcher F. albimllis breeding in natural cavities in the primeval deciduous stands of the Bialowie2a National Park (NE Poland). Both sh-rdied birds are commonly regarded as competitors if sympatric. Densities of two flycatchers are positively correlated within 7 plots (pooled data) investigated by a territory mapping method over 22 years (r = 0.77, P < 0.05) and 36 ha plot studied very intensively through 9 years (r = 0.35, p < 0.0$. We found no evidence that the Pied Flycatcher density is negatively affected by the dominant Collared Flycatcher when that stronger species has a higher density. Clearly, both species fluctuated in a parallel way which contradicts an earlier generalization. The Pied Flycatcher breeds in Bialowieia deciduous stands in much lower densities than the Collared Flycatcher does. Key words: interspecific competition, Collared Flycatcher Flceduls albimllis, Pied Flycatcher Ficedula hypolua, nahrral cavities, BialowieZa National Park. ' Agricultural and Pedagogic University, Department of Zoology, Prusa 12, 08-110 Siedlce POLAND, e-mail: wwalan@wsrp.siedlce.pl ' Sienkiewicza 29 / 21., 05-8'25 Grodzisk Maz., POLAND Received - Sept. 197, Accepted - Nov. 197 INTRODUCTION The Collared Flycatcher Ficedula albicollis and Pied Flycatcher F. hypoleuca are small tropical migrant passerines,basically simple-brooded monogamous thoughwith somecases of simultaneous polygyny and polyterritorialism. Both species originally breed in natural tree cavities, sometimes reaching very high densities. "Competition for nest sitesis markedly asymmetric in that there are clear winners and cleat losers" (Newton 1994). The model species of European omithology, the Pied Flycatcher, is commonly regarded as a bird which, if sympatric with the Collared Flycatcher, is being overcompeted by that latter species (Alerstam et aL 1,978, Ldhrl 1955). Sucha situationwas described ascompetition for nest sites, of which there is often a shortage. Once all available nest sites were occupied by the Collared Flycatcher, the Pied Flycatcher breeding density was limited. Furthermore, in sympatry in western or cenkal Europe the Pied Flycatcher breedsmore often in coniferousstandsthan the Collared Flycatcher, which populates old deciduous stands (Glowacinski 1975, Lundberg & Alatalo 7992). It is noteworthy that this is always regarded as the result of the Pied Flycatcherexpulsion from the beststandsto the poorer onesby the Collared Flycatcher (Lohrl1955, Lundberg& Alatalo 1992). Habitat distribution of the two Ficedula flycatchers in natural stands of Bialowieza National Park (hereafter BNP) is dissimilar one. Both species breed mostly in the deciduous stands. The Collared