Evidence of declines in insect populations has recently received considerable scientific and societal attention. However, the lack of long-term insect monitoring makes it difficult to assess whether declines are geographically widespread. By contrast, bird populations are well monitored and often used as indicators of environmental change. We compared the population trends of European insectivorous birds with those of other birds to assess whether patterns in bird population trends were consistent with declines of insects. We further examined whether declines were evident for insectivores with different habitats, foraging strata, and other ecological preferences. Bird population trends were estimated for Europe (1990-2015) and Denmark (1990-2016). On average, insectivores declined over the study period (13% across Europe and 28% in Denmark), whereas omnivores had stable populations. Seedeaters also declined (28% across Europe; 34% in Denmark), but this assessment was based on fewer species than for other groups. The effects of insectivory were stronger for farmland species (especially grassland species), for ground feeders, and for cold-adapted species. Insectivory was associated with long-distance migration, which was also linked to population declines. However, many insectivores had stable populations, especially habitat generalists. Our findings suggest that the decline of insectivores is primarily associated with agricultural intensification and loss of grassland habitat. The loss of both seed and insect specialists indicates an overall trend toward bird communities dominated by diet generalists.

Declinaciones a Largo Plazo de Poblaciones de Aves Insectívoras en Europa y las Causas Probables Resumen La evidencia de las declinaciones poblacionales de insectos ha recibido recientemente una atención considerable por parte de la comunidad científica y la sociedad. Sin embargo, la falta de un monitoreo prolongado de los insectos complica valorar si estas declinaciones tienen una distribución extensa geográficamente. Como contraste, las poblaciones de aves tienen un monitoreo constante y con frecuencia se usan como indicadores del cambio climático. Comparamos las tendencias poblacionales de las aves insectívoras de Europa con las de otras aves para valorar si los patrones en las tendencias poblacionales de aves son consistentes con las declinaciones de insectos. Además examinamos si las declinaciones eran evidentes para aves insectívoras con diferentes hábitats, estratos de alimentación, y otras preferencias ecológicas. Las tendencias poblacionales de las aves se estimaron para Europa (1990 - 2015) y para Dinamarca (1990 - 2016). En promedio, las aves insectívoras declinaron a lo largo del periodo de estudio (13% en Europa y 28% en Dinamarca) mientras que las aves omnívoras tuvieron poblaciones estables. Las poblaciones de aves que se alimentan de semillas también declinaron (28% en Europa; 34% en Dinamarca), pero esta valoración se basó en menos especies que para los otros grupos. Los efectos de la insectivoría fueron más evidentes para las especies de tierras agrícolas (especialmente las especies de pastizales), para las especies que se alimentan sobre el suelo y para las especies adaptadas al frío. La insectivoría estuvo asociada con la migración de larga distancia, la cual también estuvo ligada a las declinaciones poblacionales. Sin embargo, muchas aves insectívoras tuvieron poblaciones estables, especialmente aquellas generalistas de hábitat. Nuestros hallazgos sugieren que la declinación de las aves insectívoras está asociada principalmente con la intensificación agrícola y la pérdida de pastizales. La pérdida de aves cuya alimentación es especialista en insectos o en semillas indica una tendencia general hacia comunidades de aves dominadas por aquellas con dietas generalistas.

Department of Bioscience Aarhus University Kalø Grenåvej 14 8410 Rønde Denmark

DOF BirdLife Denmark Vesterbrogade 140 1620 Copenhagen Denmark

German Centre for Integrative Biodiversity Research Deutscher Pl 5E 04103 Leipzig Germany

Institute of Ecology Evolution and Diversity Goethe University Frankfurt Max von Laue Street 13 60439 Frankfurt am Main Germany

Norwegian Institute for Nature Research P O Box 5685 Torgard 7485 Trondheim Norway

Pan European Common Bird Monitoring Scheme Czech Society for Ornithology Na Bělidle 34 150 00 Prague Smíchov Czech Republic

Senckenberg Biodiversity and Climate Research Centre Senckenberganlage 25 60325 Frankfurt am Main Germany

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