UC Davis

Bird migration is one of the most complex natural phenomena to occur on a global scale. Every fall, billions of birds migrate thousands of kilometers from their breeding territories to wintering grounds. Within migration corridors around the world, avian communities coexist spatially and temporally, competing for resources at stopover sites, and even interacting as predators and prey. Migrations of raptors and songbirds often overlap in space and time, creating interactions that shape their behaviors and migration strategies. However, the wide-ranging movements of migrating avian communities make ecological interactions logistically challenging to study, especially when many migrating raptors hunt regularly along a migration route. Literature on the diet of raptors that feed en route is mainly based on opportunistic observations and correlations between peak movement activity between predator and prey migrants. Research about diets during migration requires innovative approaches to systematically document prey selection by migrating predators. This dissertation developed and demonstrated the utility of a research framework to investigate the foraging ecology and coevolution of migrating raptors and their prey. Firstly, this dissertation developed and tested a new method to obtain robust dietary datasets for migrating raptors. In Chapter 1, we tested a new technique to collect trace prey environmental DNA (eDNA) from the exterior of a raptor. When raptors feed, they grasp and tear their prey with their sharp talons and beak, leaving residual traces of prey material that can be retrieved by swabbing the exterior of the beak and talons. The residues of prey can be identified to species through application of DNA barcoding techniques. Our results indicated that DNA identified on the swab is directly linked to a previously consumed meal. In Chapter 2, we employed our swab method to study the diet of migrating Merlins (Falco columbarius). We swabbed migrating juvenile Merlins during two fall migration seasons at a long-term migration monitoring station operated by the Golden Gate Raptor Observatory (GGRO) in Marin County, California, a non-profit organization powered by volunteer community scientists. Using eDNA metabarcoding techniques, we detected the presence of 40 distinct prey species derived from 210 individual prey detections on 63 of the 72 (87.5%) Merlins sampled. Our results supported the hypotheses that describe migratory prey as abundant food sources of bird-eating raptors and suggest raptors select abundant prey species during fall migration. In Chapter 3, we applied our swab method to migrating Sharp-shinned Hawks (Accipiter striatus). Using eDNA metabarcoding techniques at GGRO, we obtained prey species detections from 94.1% of the hawks sampled (n=525) comprised of 1396 prey items and 65 prey species. To gather prey availability data, we extracted weekly abundances within our study region from the publicly available eBird Status and Trends data. By combining hawk diet data and songbird abundance data collected through two community and citizen science organizations, we were able to use discrete choice models to test how prey traits influence the interactions between raptors and songbird communities during fall migrations. This dissertation demonstrates that the logistical challenges of documenting raptor diet and prey availability within a migration corridor can be overcome by combining eDNA metabarcoding and big data generated by citizen science platforms, like eBird. Taken together, this dissertation contributes a framework for revealing the ecological and evolutionary relationships between raptors and songbirds that have remained elusive in migratory systems around the world.