New research has linked Wood Warbler population declines to mast seeding events across Europe.
Mast seeding is the episodic, massive production of plant seeds over large areas, and is widely referred to as a 'mast year' when it occurs. The resulting superabundance of seeds represents an increase in resources that can in turn affect animal populations.
Following years of high seed production, the abundance of both seed consumers and their predators increase. This increase in predators means that there is greater pressure on species right across the ecosystem, leading to impacts on non-seed-eating species such as Wood Warbler through increased nest predation in the breeding season following a mast year.
Wood Warbler declines have been linked to increased mast events in some parts of Europe (Paul Coombes).
Over the past 30 years, the frequency of mast years has increased in response to climate change, while Wood Warbler populations have declined in several regions of Europe. A team of researchers hypothesised that increasing mast frequencies may have contributed to the observed population declines by creating suboptimal breeding conditions in years after masting.
To test this hypothesis, the researchers firstly used matrix models to predict population trends based on estimated reproductive output of the birds and mast-year frequency, before then measuring reproductive output of Wood Warblers in four study areas across central Europe: Wielkopolska and Białowieża National Parks in Poland, Hessen in Germany, and Jura in Switzerland.
They found that, in the years following masting, an average of between 0.61 and 1.24 fewer fledgings were produced than in those breeding seasons following non-masting years.
At Wielkopolska National Park and in Hessen, where masting occurred on average every four years, Wood Warbler populations were found to be stable. However, in Jura and at Białowieża, where masting events occurred every two and 2.5 years respectively, populations were declining.
The observed trends matched closely with the modelled predictions, leading to the team to suggest that further increases in mast frequency in the future will lead to further declines in Wood Warbler populations.
Maag, N, Korner-Nievergelt, F, & 13 others. 2023. Wood warbler population dynamics in response to mast seeding regimes in Europe. Ecology. DOI: https://doi.org/10.1002/ecy.4227