Connectivity between countries established by landbirds and raptors migrating along the African-Eurasian flyway

Each year, more than two billion birds migrate along the African-Eurasian flyway. The increasing availability of tracking technology, involving attaching very small devices to migrant songbirds, has provided greater insight into these movements and highlighted the diverse range of threats the birds face en route. Unfortunately, the results of these tracking studies are often stored in ways that are fragmented, inaccessible or subject to biases, making it challenging to apply them for conservation action.

This international study, involving BTO, addressed the problem by combining data from 132 studies, covering 43 species, into a comprehensive review of published African-Eurasian flyway migratory records. The researchers quantified the degree of linkage between breeding and wintering grounds, with the strength of each link dependent on the proportion of those individuals from a country’s breeding population that wintered in a particular country in Africa. Using this technique, the researchers identified how countries are linked through the process of migration, creating an opportunity to foster greater international cooperation and develop conservation policy in relevant regions.  

Data from 1,229 individual birds and 71 countries were included in the study. Importantly, however, the researchers found that published data were only available for a tiny fraction of the relevant species. This was particularly true for land birds, for which only 7.5% of the available species were, on average, tracked per country. Smaller-bodied birds and records from eastern European countries were also underrepresented. Despite this, the study provides some fascinating insights into intercontinental migration. It reports an extensive network of 544 migratory links, connecting the countries within Europe and Africa. Based on the available data, Sweden had the most migratory links for its breeding populations (63), while Spain featured the most individual species (14). In contrast, Mali was the most significant African country for both number of migratory links (47) and species (21). Although based on limited data, this exercise demonstrates that valuable findings can be uncovered when scientists collaborate to ensure data are freely available.  

The study revealed a number of interesting patterns. Land birds were found to rely more heavily on central and southern African countries compared to raptors, but the differences became even more apparent when examined at the species level. In some species (e.g. the Great Reed Warbler), a small number of individuals settled in many different African countries, while for others (e.g. Montagu’s Harrier), a substantial proportion of the European population travelled to a single location. Different populations of the same species also employed different strategies. For example, Finnish Ospreys travelled to diverse locations across Africa while, in contrast, 62% of their British counterparts were concentrated in just Senegal and The Gambia.

As the majority of migrant species could not be included because of the lack of data, the study greatly underestimates the number of migratory links that exist along the flyway. By identifying the most critical gaps in the dataset, this research provides direction and incentivises the collection of more targeted tracking data from locations which simultaneously increase coverage and prioritise key species (e.g. Cuckoo in Poland, Stone-curlew in Ethiopia). This study provides policymakers with a valuable tool, informing international conservation plans, and will undoubtedly facilitate conservation efforts along the flyway and improve the long-term prospects of many declining migratory species.

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