Different lineages of the Eurasian Nuthatch have different evolutionary histories.
When you see a bird crawling head down on a tree, it is probably the Eurasian Nuthatch (Sitta europaea). This small passerine is found throughout Europe and Asia across a wide range of climatic niches. Surprisingly, the number of distinct lineages in this species is unknown. A recent study in the journal Molecular Ecology explored this mystery.
Yu-Chi Chen, Masoud Nazarizadeh and their colleagues collected samples across Eurasia and sequenced several molecular markers to reconstruct the evolutionary history of the Eurasian Nuthatch. The genetic analyses revealed six distinct lineages corresponding to different geographical regions, namely Europe, North Asia, East Asia, Caucasus, Northern Iran and Southwestern Iran.
The geographical distributions of these six lineages are immediately adjacent to each other but do not significantly overlap (i.e. parapatric distributions). To understand why these lineages do not mix, the researchers turned to Ecological Niche Modelling. These models predict the past geographical distribution of a species based on environmental data. These analyses indicated that different lineages of the Eurasian Nuthatch have been influenced by different ecological and evolutionary histories.
The northern lineages (Europe and North Asia) experienced frequent range contractions and expansions driven by the fluctuating climate during the ice ages. The southern lineages (East Asia, Caucasus, Northern Iran and Southwestern Iran), on the other hand, show a more stable history.
The parapatric distribution of these lineages raises an intriguing question: why did the lineages not merge? The researchers think that different processes are preventing the mixing of lineages in the north and the south. Northern lineages have been geographically isolated during the ice ages. During this separation, they adapted to different climatic conditions. When they came into secondary contact, they could not invade the range of the other lineage. This is an example of local adaptation.
The situation in the south was slightly different. These lineages were not geographically isolated and could thus freely mix. Why didn’t this happen? The answer could be competitive exclusion, which refers to the situation where two species competing for the same limiting resource cannot coexist. In this case, the limiting resource might be tree holes which Nuthatches use as nesting sites. More research is needed to test this hypothesis.
In summary, the parapatric distribution of the Eurasian Nuthatch lineages have been shaped by local adaptation in the north and competitive exclusion in the south. Whether this is a common pattern among Eurasian bird species remains to be determined.
Chen Y., Nazarizadeh, M., Lei, F., Yang, X., Yao, C., Dong, F., Dong, L., Zou, F., Drovetski, S.V., Liu, Y., Huang, C. & Hung, C. (2019) The niches of nuthatches affect their lineage evolution differently across latitude. Molecular Ecology, 28(4): 803-817.