A new study explored how changes in moth abundance affect insect-eating birds in Finland. The findings reveal that the impact of moths as a food source varies between bird groups and is generally stronger in northern areas.

The study found a positive association between the abundance of moths (Lepidoptera) overwintering as eggs or adults and the abundance of resident or long-distance migrating forest birds, which are reliant on caterpillars as food source during their breeding season in the spring. The results indicate that early-season moth abundance significantly influences bird abundance, and particularly in the north-boreal zone. This highlights how species interactions depend on their environment and suggests that declines in insect populations could lead to declines in their bird predators.

However, a similar dependency of bird abundance on moth abundance was not found in southern and central parts of Finland, or for moths that are available for the birds later in the season.

“The reason why moth abundance affected bird abundance only in northern Finland is that in the north bird breeding seasons are short, food webs are relatively simple, and temporal moth abundance fluctuations from year to year are naturally strong. Therefore, abundance changes of a particular group of insect prey more likely reflect in bird abundance in northern Finland than in central or southern Finland”, explains Mahtab Yazdanian, the lead author and doctoral researcher at the University of Oulu, Finland. “In other words, birds in the north are more reliant on moth preys compared to birds in the central and southern regions of Finland.”

The study drew on 26 years of monitoring data. “This study highlights the value of long-term monitoring programmes. We are fortunate in Finland to have detailed abundance data for both moths and birds, which our study combines in full for the first time.”, explains Tuomas Kankaanpää, a postdoctoral researcher at the University of Oulu.

Such comprehensive temporal and spatial datasets are rare worldwide. “Complexities in the ecosystem, the food web, and the insect populations' fluctuations cascading effects on higher trophic levels like birds are really difficult to systematically study in nature where everything affects everything”, Yazdanian explains. One of the difficulties is that temporal moth abundance fluctuations from year to year are naturally strong. Large variations in abundance of insect prey are needed to reveal its importance for bird survival and reproduction.” Finland’s exceptional long-term monitoring schemes have provided this study with valuable data on almost 80 bird and 400 moth species dating back to 1993.

Deeper insights into ecological dynamics are gained, and the complexities of natural systems are better understood through the investigation of these datasets. The new study demonstrates that using long-term data in combination with information on birds’ migratory strategies and seasonal availability of different moth species facilitates uncovering regional differences in interactions between moths and their bird predators.

“Further investigations of insect abundance effects on bird populations are needed, especially in areas where insect numbers are declining”, Kankaanpää continues. The next research, currently underway at the University of Oulu, aims to identify which moth species are more important as a food source for specific bird species. Technological advances in DNA meta-barcoding helps to identify multiple species from a mixed sample, for example in diet analyses, currently ongoing by another research group at Ecology and Genetics Unit.

The outcomes of the new study address the concern of insect declines and their cascading impact on ecosystems amidst global changes. The results suggest that conservation strategies should consider the entire food web, including the preservation of insect populations, to support bird conservation efforts.

The research team was led by the University of Oulu and included experts from Finnish Environment Institute (SYKE), Finnish Museum of Natural History (LUOMUS), University of Lapland, and Vrije Universiteit Brussel.

The research article Evidence for bottom-up effects of moth abundance on forest birds in the north-boreal zone alone was published in the highly esteemed Ecology Letters journal in December 2024.

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