Comparing population trend estimates of migratory birds from breeding censuses and capture data at a spring migration bottleneck

• Published in: Ecology and evolution Vol. 11; no. 2; pp. 967 - 977
• Main Authors: Maggini, Ivan, Cardinale, Massimiliano, Favaretto, Andrea, Voříšek, Petr, Spina, Fernando, Maoret, Francesco, Ferri, Andrea, Riello, Sara, Fusani, Leonida
• Format: Journal Article
• Language: English
• Published: England John Wiley & Sons, Inc 01-01-2021; John Wiley and Sons Inc; Wiley
• Subjects: Bird migration; bird ringing; Birds; Breeding grounds; Breeding seasons; Breeding sites; Catchment areas; Census; Ecology; Ekologi; Estimates; Islands; Migratory birds; migratory bottleneck; Monitoring; Original Research; PECBMS; Ponza; Population decline; population indices; Populations; Species; Spring; Supranationalism; Trends; Tyrrhenian Sea; Ponza; Europe; population indices; PECBMS; Ponza; bird ringing; migratory bottleneck
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.7110

Europe has a well‐established network of breeding bird monitoring that is used to produce supranational indices of population trends for many species. However, a comparison of breeding bird censuses with other methods may be beneficial to confirm the validity of such indices. The aim of this study was to assess the value of standardized capture data of migratory birds at migration bottlenecks as an indicator of the effective breeding populations. One limitation to this method is that several populations are co‐occurring at these bottlenecks and their catchment areas need to be clearly identified to allow extrapolation of population indices. Here, we used standardized trends in capture numbers of 30 species on the island of Ponza, a migration bottleneck in the central Mediterranean, and compared them to population trends estimated in the putative catchment breeding areas between 2005 and 2016. The catchment areas were identified through the analysis of ring recoveries during the breeding season of birds passing through Ponza. Our results show an agreement between the population trends observed on Ponza and those in the breeding areas in 15 out of 30 species. The correlations were strongest in species with a more robust definition of the catchment areas, that is, species with more than 10 recoveries, and for which the recoveries were most likely of breeding birds. The main reason for disagreement between the two indices in the remaining species might be related to different intensity of sampling in different areas. This issue can be solved by further developing monitoring projects in underrepresented countries, as well as by intensifying monitoring through ringing, both in the breeding grounds and at migration bottlenecks. These results show that spring migration monitoring at bottlenecks has the potential to provide a valuable complement and an independent control of breeding bird surveys, allowing raising early warnings of population declines and contributing to their conservation.
We compared trends of 30 migrant Passerine bird species on passage through a migration hotspot with population trends in their breeding areas. We found good agreement between trends in half of the species. This shows how integrating different count methods could benefit monitoring and conservation.