Fitness of two competing species of flycatchers and their hybrids in term of nestling growth

Speciation can be seen as the establishment of reproductive barriers among species. Hybrid zones provide us with an ideal situation to study the process of speciation because the evolution of reproductive isolation is still in progress. Hybrids usually suffer reduced fitness (i.e. leading to post-zygotic isolation) although they may have higher or similar fitness as the parental species in some cases. In this paper, we want to investigate the fitness of hybrids in terms of nestling growth compared with the parental species. Pied flycatcher (Ficedula hypoleuca) and collared flycatchers (F. albicollis) are two closely related passerine species. The natural hybrid zones of these two species include central and eastern Europe and two Swedish islands in the Baltic, Gotland and Öland. Populations of collared and pied flycatchers on the two Swedish islands have been well studied during the last two decades. In this study, we raised some offspring hybrids of pied and collared flycatchers and swapped them into the natural nests of the parental species. Begging behaviours of offsprings were recorded and analyzed. Results showed that nestling collared flycatchers begged more intensively than F1 hybrids (p = 0.0293) and nestlings of pied flycatcher tended to beg less intensively than F1 hybrids (p = 0.3177). The intermediate begging rate of hybrid offsprings between two parental individuals may indicate the intermediate offspring need and growth potential of hybrids. Moreover, long-term data showed that heterospecific pairs of female collared and male pied flycatchers had intermediate fitness sensitivity to the seasonal changes of environment. In addition, hybrids’ lower begging intensity compared with collared nestlings’ may help the collared mothers to reduce the cost of hybridization when the mothers have pure collared nestlings sired by conspecific extra-pair males in the same brood. Our conclusion is that hybrids of collared and pied flycatchers fall between their parental species in terms of nestling growth and begging behaviour, which in turn influence their relative fitness in different environments.