3.1. The bird species and populations studied

The tits and chickadees are small cavity-nesting passerine birds forming the family Paridae, genus Parus. They are mostly found in deciduous, evergreen and mixed woodlands, including parks and gardens (Cramp & Perrins 1993). The genus Parus contains 30-40 species, of which many occur in wide geographical ranges. Many species share similar habitats and their distribution ranges coincide (Snow 1954). The genus is divided into 5-12 groups depending on authors (eg. Snow 1954, Eck 1988, Cramp & Perrins 1993). The Fennoscandian tits can be allocated into five of these groups following Snow (1954): Major (great tit, P. major), Cyanistes (blue tit, P. caeruleus), Lophophanes (crested tit, P. cristatus), Periparus (coal tit, P. ater) and Poecile (marsh tit, P. palustris, willow tit, P. montanus and Siberian tit, P. cinctus). The phylogeny of these species was examined in study I, added with three American chickadees, of which two belong to the Poecile group: P. atricapillus (black-capped chickadee) and P. carolinensis (Carolina chickadee). The third, P. inornatus (plain titmouse) belong to the Baelophus group and was used as an outgrop.

The population genetic structure of three of these species was studied more closely (the willow tit, P. montanus in study II, the great tit, P. major in study III and the blue tit, P. caeruleus in study IV). These species are territorial during the breeding season but form flocks outside the season. The adults are highly sedentary after natal dispersal but juveniles may perform irruptive autumn movements (Tiainen 1980, Koivula & Orell 1988, Gossler 1993, Cramp & Perrins 1993, Orell et al. 1994). All three species are mainly monogamous but occasional extra-pair fertilisations may occur (Gullberg et al. 1992, Kempenaers et al. 1992, Orell et al. 1997).

The distribution area of the willow tit covers a zone from north-eastern France through the central and northern Europe and Russian Taiga to the Pacific coast and over the Bering strait into Alaska (Cramp & Perrins 1993). The great tit’s range is more southern, reaching the Mediterranean Sea, the north coast of Africa, India and Indonesia in the south and northern Norway through Sweden and Finland to southern Russia and northern China in the north, restricted by the Pacific Ocean in the west and Atlantic

Ocean in the east (Gosler 1993). The blue tit is the most southern and western of these three species. In the south, it occurs in the African north coast and on the Canary Islands. This species reaches the Atlantic coast in the west and the Caspian Sea in the east and the northern limit is in central Fennoscandia and southern Russia (Cramp & Perrins 1993).

The two willow tit populations sampled in this work were located at the western side of the species’ distribution area, representing the subspecies P. montanus borealis. Similarly, the eight populations sampled from the great tit were all from the western edge of the species range and all belonged to the nominate subspecies P. major major. Five blue tit populations belonged to the nominate subspecies P. caeruleus caeruleus and the sixth, most south-western population was from an area where the nominate subspecies occurs together with another subspecies P. caeruleus ogliastrae (Figure 4).