Alpine Tit and Willow Tit – an example of incipient speciation?

In many bird species, especially passerines, song plays an important role in mate selection, allowing females in particular to distinguish between males of their own species and those of closely related species. For example, Common Chiffchaff and Willow Warbler, two leaf warblers, look very similar, but have a completely different song. From this perspective, it appears likely that populations of a species that differ in song will eventually evolve into two or more distinct species in the medium to long term.

Alpine and Willow Tit, two forms of Poecile montanus that occur in Switzerland, could well be an example of this speciation process. The Willow Tit breeds in the Jura (P. m. salicarius), in the Pre-Alps, and locally on the Central Plateau (P. m. rhenanus), where it mainly inhabits deciduous woods, young woodland and alluvial forests between 800 and 1500 m. The Alpine Tit (P. m. montanus) occupies mixed and coniferous stands in the Alps, mostly between 1300 and 2100 m. Both forms rely on a sufficient amount of standing deadwood to excavate their nesting cavities. Apart from minor differences in diet and size, Alpine Tit and Willow Tit can only be identified by their song. The Willow Tit utters a series of long, descending notes («tyoo tyoo tyoo tyoo»), whereas the Alpine Tit’s territorial song consists of short notes on an even pitch («dee dee dee dee dee»). The distribution of these two forms in Switzerland and the presence of two contact zones in this country and one in Bavaria were described as early as 1962. Other contact zones were later discovered in the Pre-Alps of Fribourg, in Savoie, the Allgäu, the Austrian Alps and the mountains of Bulgaria. In Switzerland, the two forms have never been surveyed separately before. For the first time, the 2013–2016 atlas provides an overview of their distribution.

Overlapping ranges

Interestingly, the breeding ranges of Alpine and Willow Tit described by W. Thönen have remained largely unchanged for more than half a century. They have now been precisely modelled and quantified for the first time. The Alpine Tit is closely confined to the Alps, reaching high densities in some areas. The Willow Tit occurs in much lower densities in the Jura, the Pre-Alps, and some parts of the Central Plateau and the Rhine Valley in St. Gallen and Grisons. Contact zones exist in the Pre-Alps of Fribourg, Bern, Obwalden and Lucerne, in the Alpstein massif, and in the Rhine Valley between Ilanz GR and Altstätten SG. The suspected presence of the Alpine Tit in the Jura could not be confirmed.

Willow Tits in the Pre-Alps have two noteworthy features: they occupy similar habitats to the Alpine Tit, and they occur locally in fairly high densities, despite the distance to the core range in the Jura. How can this be explained? We suspect that the Alpine Tits in this region have adopted the song of the Willow Tit, either by learning or by gene flow (i.e. the exchange of genetic material between two populations). This is a bold hypothesis given the information available to us today, but one that nonetheless merits more detailed investigation.

Further research needed

The data collected so far give us a fairly clear picture of the distribution and population size of Alpine and Willow Tit in Switzerland. Distinguishing between the two forms allows us to track the subpopulations separately. This is an interesting opportunity given the slightly negative trend of the Willow Tit, at least in the lowlands.

As most Alpine and Willow Tits do not appear to recognise each other as conspecifics by their song, there is reason to assume that they may be evolving into two distinct species, as Thönen suggested. However, the occasional observation of mixed singers – Thönen himself notes five cases – indicates that reproductive isolation (i.e. the interruption of gene flow between subpopulations of the same species) is far from complete. Because we now know the exact distribution of Alpine and Willow Tit, additional investigations into the status of the two forms could be pursued. For example, the degree of differentiation between the subspecies could be analysed in birds from the contact zones using a modern population-genetics approach. Preliminary studies in this direction have not been taken far enough to yield this kind of information. In a subsequent step, inheritance and acquisition of song could be examined in order to gain a better understanding of the dynamics that separate the two populations and how they might evolve in the future. Song appears to be largely innate, but additional experiments are necessary to provide support for this hypothesis.

The relationships between subforms of P. montanus provide one of the few opportunities in the Alpine region to study the process of speciation. Hopefully, the 2013–2016 atlas will serve as a basis for future research that will shed light on some of the many questions regarding the populations of Alpine Tit and Willow Tit in Switzerland.