Continent-Wide Drivers of Spatial Synchrony in Breeding Demographic Structure Across Wild Great Tit Populations

I have published a third chapter from my PhD thesis which I defended and passed last Autumn. This paper investigated how the age structure of great tit populations varies across Europe, and particularly whether populations separated by large distances tend to become younger or older at the same time. This kind of synchrony is well studied for changes in population size, but much less is known about whether the demographic composition of populations also fluctuates together across space.
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We brought together long-term breeding data from 32 great tit populations across Europe, spanning 67 years and including more than 130,000 birds. Across these populations, we found that years with larger clutch sizes, colder winters and larger beech crops tended to be followed by younger breeding populations, largely because of differences in the recruitment of young birds into the population.

We also found clear spatial synchrony in population age structure. Populations closer together tended to show more similar changes from year to year, with this synchrony remaining detectable over distances of around 650 km. Interestingly, although several environmental variables were associated with whether populations became younger or older, most did not appear to explain the geographical scale over which these changes were synchronised. Beech masting was the main exception.

The results show that changes in the age structure of animal populations can be coordinated over surprisingly large geographical scales, while also highlighting how local ecological and density-dependent processes can alter the way populations respond to broader environmental variation.

This was also a particularly enjoyable paper to develop because it involved collaborating with many other researchers working on long-term great tit populations across Europe. Bringing together datasets and expertise from so many different study systems was a very different experience from my other papers, which have generally been developed with collaborators within my immediate research group, so it was great fun to lead! The paper can be found here.