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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. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Raptor Research is the property of Raptor Research Foundation and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Secondary exposure to anticoagulant rodenticides (ARs) through consumption of contaminated prey has been documented worldwide in many non-target species, especially raptors. The Red-tailed Hawk ( Buteo jamaicensis), a raptor that frequents agricultural areas and eats primarily rodents, is particularly susceptible. Because there is documentation of Red-tailed Hawk exposure to ARs in California, this study aims to describe the extent to which migrating juvenile Red-tailed Hawks are exposed to ARs, as well as any sublethal effects of AR ingestion. We collected blood samples and body morphometrics from 97 juvenile Red-tailed Hawks migrating through the Marin Headlands, Marin County, CA, from August to December in 2013 and 2015, and screened samples for the presence of ARs. Eight hawks (8.2%) tested positive for some amount of ARs. We detected first-generation (diphacinone, chlorophacinone) and second-generation (brodifacoum, bromadiolone) ARs. Although some juvenile Red-tailed Hawks are exposed to ARs either along their migration route or as resident birds in the Marin Headlands, we did not find any relationship between body condition and presence of ARs. Although this method of AR sampling of live birds is novel and increases our sampling capabilities, the short half-lives of ARs in blood make it difficult to estimate population-wide exposure rates. Future studies should focus on resident raptors near agricultural areas where AR exposure can be tested over time to better understand how this technique can be used to estimate exposure rates across whole populations. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Raptor Research is the property of Raptor Research Foundation and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

One of the primary assumptions associated with many wildlife and population trend studies is that target species are correctly identified. This assumption may not always be valid, particularly for species similar in appearance to co-occurring species. We examined size overlap and identification error rates among Cooper's (Accipiter cooperii) and sharp-shinned (A. striatus) hawks specific to a raptor migration count station along the Pacific Coast of North America. Illustrating the difficulty of distinguishing between these 2 species, we found overlap in 7 metrics among species-sex groups and in 2 metrics between species, and a principal components analysis revealed a continuum of discrete clusters for each species-sex combination in morphospace. Among juvenile hawks (n = 940), we found the greatest misidentification rate for male Cooper's hawks (23% of the 156 males were identified as sharp-shinned), lesser error rates for female Cooper's (8%, n = 339) and female sharp-shinned (6%, n = 246), and the lowest misidentification rate for male sharp-shinned hawks (0%, n = 199). We observed a similar pattern of misidentification among adult hawks (n = 48). We attempted to use conditional probabilities (identification rates) from calibration data to calculate the true number of adult and juvenile Cooper's hawks and sharp-shinned hawks. Discrepancies between total number of observed accipiters and estimated number using calibration data suggest that daily observer misclassification rates are higher than misclassification rates estimated from calibration data and prevent correction of the raw data. Our results illustrate the importance of testing for and quantifying observer error in species identification in wildlife census and population trend studies particularly when target species may be easily confused with other nontarget species. [ABSTRACT FROM AUTHOR]