Falcon fuel: metabarcoding reveals songbird prey species in the diet of juvenile Merlins (Falco columbarius) migrating along the Pacific Coast of western North America

• Published in: Ibis (London, England) England), 2021-10, Vol.163 (4), p.1282-1293
• Main Authors: Bourbour, Ryan P., Aylward, Cody M., Tyson, Chris W., Martinico, Breanna L., Goodbla, Alisha M., Ely, Teresa E., Fish, Allen M., Hull, Angus C., Hull, Joshua M.
• Format: Article
• Language: English
• Subjects: Autumn; Beak; Beaks; Bird migration; Birds; Birds of prey; community science; Deoxyribonucleic acid; Diet; DNA; DNA metabarcoding; Eating; Falco columbarius; Foraging; Foraging behavior; Fuels; Hypotheses; migrating food web; Migration; Migratory species; Passeri; predator–prey interactions; Prey; Songbirds; Species
• ISSN: 0019-1019; 1474-919X
• DOI: 10.1111/ibi.12963

During autumn migration, bird‐eating raptors are thought to rely on flocks of migrant songbirds (Passeriformes) as a critical resource to fuel the energetic demands of long‐distance migration. However, this hypothesis has been challenging to investigate, and the foraging ecology during migration of most migrant raptors remains unexplored. To address these knowledge gaps, our objective was to document the diet of a bird‐eating falcon on active migration. We swabbed visible and trace prey residues from the exterior surface of beaks and talons of migrant juvenile Merlins Falco columbarius in the autumn of 2015 and 2016 at a raptor migration monitoring station positioned on the Pacific Coast of western North America. We used a DNA metabarcoding approach and detected the presence of 40 distinct prey species derived from 210 individual prey species detections on 63 of the 72 (87.5%) migrant juvenile Merlins sampled. We detected an average of 3.3 ± 1.6 prey species on individual Merlins. We found that juvenile males selected smaller prey on average compared with juvenile females. Of the prey species detected, over 80% were migratory songbird species within the Pacific Flyway. In 2015, we detected a greater proportion of irruptive migrants in juvenile Merlin diet compared with 2016. In 2016, we found that the proportion of annual migrants consumed by Merlins corresponded to the timing of peak annual songbird migration in the Pacific Flyway. This study represents one of the first detailed descriptions of songbird prey species consumed by a migrating raptor and supports the hypothesis that migrating juvenile Merlins rely on migrant songbirds to support the energetic demands of migration.