The merlin (Falco columbarius) is a small yet sturdy falcon. It is slightly larger than the more familiar American kestrel (Falco sparverius). It can be differentiated from the kestrel by its lack of prominent facial markings and more aggressive and powerful flight. In North America, it breeds mainly in the boreal forests of Canada and Alaska. It occurs in California as a passage migrant and winter resident, mainly from September through April. Although most individuals winter in North America, some migrate as far south as Venezuela, Colombia, Ecuador, and northern Peru. This species is fast and agile on the wing. It preys mainly on small birds, which it catches in midair. The merlin is such an adept flier that it even snatches dragonflies in flight, consuming them on the wing during migration.
Over the past several years, colleagues from the University of California Davis (UC Davis), U.S. Geological Survey (USGS), Hawkwatch International, Golden Gate Raptor Observatory, as well as independent raptor biologists and I have been investigating the genetic relationships of the three North American subspecies of merlin. These subspecies include the black merlin (F. c. suckleyi) (Figure 1), taiga merlin (F. c. columbarius) (Figure 2), and prairie merlin (F. c. richardsonii) (Figure 3). These subspecies have distinct plumage characteristics and breeding ranges. The black merlin is the darkest of the subspecies and breeds in the Pacific Northwest and southeast Alaska. The taiga merlin is intermediate in plumage and breeds from Newfoundland to Alaska. The prairie merlin is the palest of the subspecies and breeds in central Canada and the north-central United States. This geographic plumage pattern is an example of what ecologists call Gloger’s rule. Gloger’s rule states that among warm-blooded animals, darker forms tends to occur in more humid environments, and paler forms tend to occur in more arid environments.
For genetic analysis, we collected small contour feathers, which we gently plucked from captured birds at 13 locations across the United States. These locations represent the major migratory flyways—Pacific, Intermountain, Midwest, and Atlantic—which merlins use on their way to their wintering grounds.
As part of this effort, I collected merlin feather samples during three fall migration seasons along the Atlantic flyway at Fisherman Island National Wildlife Refuge in Virginia. Fisherman Island is a 1,850-acre island just south of the Delmarva Peninsula. The geographic position of the island makes it an ideal location to study raptor migration.
Most raptors avoid open water during migration because flying over water is more energetically demanding than flying over land. Solar radiation off the earth’s surface creates updrafts of warm air, which allow migrating raptors to glide and maintain height without flapping their wings excessively. As migrating raptors make their way down the Delmarva Peninsula, they encounter a tapering landmass, which funnels them to Fisherman Island (Figure 4). When merlins and other raptors reach the island, they are faced with a difficult migratory decision—head back north and find an alternative route over land or make the energetically demanding 15-mile flight across the mouth of Chesapeake Bay. At this critical decision point, large numbers of migrating raptors “pile” up on the island before continuing their southward journey. This pileup can be spectacular, with up to several hundred merlins passing through on a single day during the peak of migration. As individuals are largely solitary during the rest of the year, seeing these kinds of numbers in one place is mind-blowing.
The merlin feather samples were processed at a USGS lab in Anchorage, Alaska, and population genetic analyses are currently underway at UC Davis. By analyzing the feathers we hope to better understand how merlin subspecies originated and what aspects of this falcon’s ecology, behavior, and evolution help maintain its genetic diversity.
—Joe Medley, Staff Scientist