By Keith L. Bildstein, Ph.D., Sarkis Acopian Director of Conservation Science
Hawk Mountain Sanctuary
17 March 2017
Most studies of bird migration focus on the movements of birds that ornithologists refer to as “complete migrants,” species whose global populations evacuate their breeding grounds each year while traveling to geographically distant wintering areas. Although Arctic Terns, Black-and-white Warblers, and Broad-winged Hawks, along with a small percentage of other species of migratory birds, do follow this pattern, most migratory species of birds are “partial migrants,” species whose populations include both sometimes-migratory and non-migratory individuals, and whose migratory populations sometime overwinter in areas already occupied by non-migratory members of the same species. It turns out that partial migration is a lot more complicated than complete migration, and although the former is little studied compared with the latter, its complexity makes it a lot more intriguing, as well as a lot more challenging, both to investigate and to understand.
New World Turkey Vultures are a good example of partial migrants, with some populations consisting entirely of migratory individuals, whereas other populations consist of both migratory and non-migratory individuals, and still other populations consisting entirely of non-migratory individuals. The degree to which different populations of Turkey Vultures interact, and the effects they have on each other is not well known. One place where such interactions have been explored is in the Llanos, the enormous freshwater wetland of central Venezuela, where Turkey Vulture migrants from western North America, which are larger and more massive than the region’s non-migratory, year-round residents, displace the latter from the best available habitats and gain weight after their arrival while the supplanted residents simultaneously lose weight. Indeed, the so-called Llanos “residents” appeared to “migrate reciprocally” to avoid competing with of the larger North American birds.
But what happens in areas in which some breeders migrate south in autumn, whereas other stay put, while still others from more northern breeding sites migrate into the region and over-winter there? Such is the case in the Sonoran Desert of southwestern Arizona, where members of the breeding population include both migrants and non-migrants of the aura race of Turkey Vultures, whereas as the winter-only individuals are migrants from the larger and more massive meridionalis race from farther north. (Note: There are six subspecies of Turkey Vultures, three that breed in North America, along with two that breed in Central America, and three that breed in South America.)
We began satellite tracking members of the Sonoran Desert breeding race in 2014 and have followed the movements of nine birds since then. All but one have proved to be migratory, with some individuals overwintering consistently in Mexico, whereas other have overwintered consistently either in Guatemala, El Salvador, Panama, or Colombia. In late January of this year my Hawk Mountain colleague, Dr. Jean-Francois Therrien, and I traveled to our tapping site outside of Phoenix and successfully trapped two winter-only individuals of the meridionalis race of Turkey Vultures. Although we don’t know exactly where these two breed, as of early-March one was on Vancouver Island in south western most Canada, and the other was in Death Valley California in late March, with both north of their Arizona trapping site.
Tracking the movement of all of these Sonoran birds for several years will allow us to determine if the northern breeders crowd out the smaller year-round residents from Arizona, and if so, whether or not the smaller southern migrants leave in autumn in advance of the arrival of the larger northern migrants and time their returns to avoid the departing northern migrants in spring.
Although this may seem a bit esoteric to some, understanding the extent to which the two subspecies interact in the Sonoran Desert has important implication for conservation. Suppose, for example, that in Arizona the larger northern breeders consistently dominate the smaller southern breeders. If so, the size of the former’s populations could limit the size of the latter’s, particularly in winter when food may be limiting. If so, the fate of the latter would be affected by the fate of the former. Thus, if global change were to affect the former–either positively or negatively–the latter might “respond” as well, albeit indirectly, in-kind. Although, all of this remains highly speculative, the extent, if any, to which populations of the two races merits examination, and I very much look forward to doing so over the course of the next few years.
I will keep you posted of the arrivals and departures of both our “northern” and “southern” Arizona migrants this summer.