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.

Austrian Ornithological Centre Konrad Lorenz Institute of Ethology University of Veterinary Medicine Vienna Wien Austria

Cisca Italian Centre of Research and Conservation of the Environment Cassino Italy

Czech Society for Ornithology Prague Czech Republic

Department of Aquatic Resources Institute of Marine Research Swedish University of Agricultural Sciences Lysekil Sweden

Department of Behavioural and Cognitive Biology University of Vienna Wien Austria

Department of Zoology and Laboratory of Ornithology Faculty of Science Palacky University Olomouc Olomouc Czech Republic

Institute for Environmental Protection and Research Ozzano dell'Emilia Italy

Riserva Naturale Statale Isole di Ventotene e S Stefano Ventotene Italy

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