Blog written by Ryan Bourbour, a PhD student in the Department of Animal Science and Graduate Group in Ecology at the University of California, Davis. Read the full paper here.
Picture an autumn day in coastal California, accompanied by hundreds of migrating songbirds, hawks, and falcons. This influx of avian predators and prey along the Pacific coast happens every September and October in the Marin Headlands of California just north of San Francisco. On peak migration days, migrant raptors seem to swarm the hillsides, ambushing their unsuspecting songbird prey to fuel the next leg of their long journey, leading us to wonder: 1) are these raptors tracking flocks of migrating songbird prey as a plentiful food source, and, most importantly, 2) how can we study the diet of migrating raptors?
A migrant Sharp-shinned Hawk that has visible prey remains on its toes and talons.
Migratory raptors survive in three spatially and temporally distinct realms within their annual cycle: the breeding grounds, non-breeding grounds, and the migratory route, with the latter being one of the most dangerous times of a raptor’s life1. Much of the available information on the diet of migratory raptors primarily comes from studies focused on the breeding and non-breeding grounds when conventional diet study methods can be feasibly carried out, (e.g. observations, nest cameras, and analysis of pellets and prey remains), leaving gaps in our knowledge on the foraging ecology of a migrant raptor actively on migration. For raptors that primarily prey on songbirds, such as accipiters and falcons, hunting almost daily during migration is necessary in order to survive the long journey. Understanding what resources are critical for raptors to survive their migratory journey may shed light on the co-evolution of migration strategies between migrant predators and prey2,3, and also has conservation implications in the face of anthropogenic climate change and mismatch of cues4,5 if there are disruptions in trophic interactions along a migratory route. Our primary motivation for our migration research was to develop a new research technique to study raptor diet when direct observations are nearly impossible and then implement this technique to investigate exactly what fuels raptor migration.
We collect prey DNA from the exterior of the raptor’s beak (left) and talons (right) using a nylon swab tip.
Previous research on the foraging ecology of migrating raptors has highlighted correlations between peak migratory movements of migrant raptors and migrant prey6,7, which suggests migrating raptors may be keying in on energetically taxed migrant prey and possibly tracking this food source to increase hunting opportunities2,3. A more modern approach to studying diet was used in New Mexico by collecting visible songbird prey feathers from the talons of migrating Sharp-shinned Hawks (Accipiter striatus), which allowed researchers to identify 50 songbird prey items over 5 years8. However, in order to study the diet of migrant raptors in depth, we wanted to develop a method that could provide a robust sample size within a single migration season. So before we set out to study the foraging ecology of migrant raptors, we had to think outside the box.
The inspiration for our idea emerged while we were banding migrating raptors at migration monitoring stations. We noticed that these migrant raptors often have visible prey blood and tissue (sometimes fresh, sometimes not) on their beaks and talons. Swabbing that “messy beak” or those “messy talons” would mean we could be sampling prey DNA, so naturally, we wanted to test this idea out. We collaborated with the California Raptor Center at the University of California, Davis and designed a controlled study to confirm that when we swabbed a beak or talon we were actually sampling what the raptor had eaten. We sampled three resident raptors that had different diets. We tested for the presence of chicken DNA and only found positive detections on raptors that were fed chicken, which confirmed that our method was ready to try in the wild. The most important and exciting part was that we were still able to detect whether chicken had been eaten even when beaks and talons were visibly “clean”. This led us to believe we would be able to detect prey DNA on any talon or beak, regardless of whether there was visible prey remains or not.
To implement our new method on wild raptors, we collaborated with the Golden Gate Raptor Observatory and their volunteer raptor bander team to collect samples over the 2015 and 2016 migration seasons from over 600 migrating Sharp-shinned Hawk and Merlin (Falco columbarius) individuals. To confirm our swabbing technique, we randomly selected 19 beak and talon swabs and amplified the prey DNA using COI primers developed for songbirds. These primers isolate a specific region in the mitochondrial DNA, known as a DNA “barcode”. We sequenced the DNA barcodes, referenced them to a public barcode database, and identified our barcode sequences to the species level. We detected only probable songbird prey, which was amazing news! The next steps in our migration diet study are to sequence the prey DNA collected from the beaks and talons of hundreds of migrating raptors using DNA metabarcoding (high-throughput sequencing) to describe migration diet in detail, and investigate how important migrant prey are for fueling raptor migration.
Another exciting thing about our beak and talon swabbing method is that it can also be applied outside the context of migration. We are currently using it to investigate and describe the pathways of anti-coagulant rodenticide exposure in wintering raptors on farms by comparing blood samples to dietary (beak/talon swab) samples. Our beak and talon swabbing technique shows promise in helping researchers study the diet of raptors, and even other inconspicuous predators, when documenting prey selection is logistically difficult with traditional methods.
An immature Red-tailed Hawk with prey blood on its beak (left) and Ryan Bourbour swabbing a beak for prey DNA after a blood sample was taken to test for rodenticide exposure.
- Bildstein, K. L. (2006). Migrating raptors of the world: their ecology & conservation. Cornell University Press.
- Kerlinger, P. (1989). Flight strategies of migrating hawks. University of Chicago Press.
- Ydenberg, R. C., Butler, R. W., & Lank, D. B. (2007). Effects of predator landscapes on the evolutionary ecology of routing, timing and molt by long‐distance migrants. Journal of Avian biology, 38(5), 523-529.
- Jones, T., & Cresswell, W. (2010). The phenology mismatch hypothesis: are declines of migrant birds linked to uneven global climate change?. Journal of Animal Ecology, 79(1), 98-108.
- Buskirk, J. V. (2012). Changes in the annual cycle of North American raptors associated with recent shifts in migration timing. The Auk, 129(4), 691-698.
- Aborn, D. A. (1994). Correlation between raptor and songbird numbers at a migratory stopover site. The Wilson Bulletin, 150-154.
- Nicoletti, F. J. (1997). American Kestrel and Merlin migration with green darner movements at Hawk Ridge. Loon, 68, 216-220.
- Delong, J. P., Cox, N. S., Cox, S. W., Hurst, Z. M., & Smith, J. P. (2013). DNA Sequencing Reveals Patterns of Prey Selection in Migrating Sharp-Shinned Hawks. The Condor, 115(1), 40-46.