Dynamics of a clinal hybrid zone and a comparison with island hybrid zonesof flycatchers (Ficedula hypoleuca and F-albicollis)

Breeding data for pied flycatchers Ficedula hypoleuca and collared flycatch ers F. albicollis, from 47 localities in the Czech Republic and Slovakia, w ere analysed. We show that co-existence and hybridization were mainly restr icted to a rather narrow, latitudinal cline. The distribution of the two fl ycatchers coincides with topography and habitat, collared flycatchers domin ating in warmer habitats than pied flycatchers. Maintained co-existence wit hin the same locality was the exception rather than the rule and most matin gs occurred in allopatry. In sympatry hybridization occurred at a frequency that was much lower than expected from random mating. Hybrids had low hatc hing success but some hybrids were apparently fertile. Shortage of conspeci fic mates may explain why individual birds engage in mixed species pairs. T he proportion of pied flycatchers that was involved in mixed pairs was high and increased with decreasing relative frequencies of pied flycatchers in the population. The proportion of collared flycatchers involved in mixed pa irs was low, reflecting the higher relative frequency of this species in mo st mixed populations. Comparisons suggest that fewer hybrids are breeding i n the Central European, clinal hybrid zone than in the isolated hybrid zone s of the Baltic Isles. Moreover, hybrid fertility was apparently higher in the island zones than in the clinal zone. We suggest that more extensive in trogression in the isolated island populations has resulted in an increase in hybrid fitness by an accumulation of fertile individuals of mixed ancest ry (i.e. F2, F3...Fn-hybrids) acting as bridges for gene exchange. Differen ces in the dynamics of the two classes of hybrid zones, especially in patte rn of gene flow, may explain these differences in frequency and fertility o f hybrids.