—By Mike Brasher, Ph.D.; J. Brian Davis, Ph.D.; and Douglas Osborne, Ph.D.
For many of us, the urge to return to places of origin or familiarity is common. It's what John Denver was singing about in his classic hit "Take Me Home, Country Roads." The impulse to return home is common in wildlife as well, having been documented in a variety of species, including long-distance travelers like waterfowl.
Termed "philopatry," this phenomenon is defined as the tendency for an animal to return to a particular area in subsequent years. For waterfowl, it is best understood with respect to adult birds returning to prior breeding sites ("breeding philopatry") or young birds returning to their place of hatch ("natal philopatry"). A related term, "site fidelity," refers to this same behavior, but as it occurs when birds return to previously used migration or wintering sites.
In most species of North American migratory birds, males exhibit higher rates of philopatry to breeding sites, whereas in waterfowl it is the female that is more philopatric. This pattern emerges largely because pair formation in waterfowl, especially ducks, occurs on wintering and migration areas, with the male subsequently following the female to her nesting destination. With other migratory birds, such as songbirds, males are the first to return to breeding grounds, where they establish territories and attract later-arriving females.
Geese and swans are somewhat different from ducks because they pair for life, and both pair members display high rates of philopatry, with males accompanying females back to breeding sites over multiple years. Rates of breeding philopatry are typically greater for geese and swans and are higher for adult birds. Most data reflect return rates to a general study area or nesting colony, but stories of females returning to the same nest bowl or brood-rearing wetland in subsequent years are quite common.
Why did breeding philopatry develop and why does it vary among species or individuals? One of the most common explanations is simply that returning to an area of prior experience enables a bird to benefit from familiarity with the local environment, such as nesting habitat, brood-rearing wetlands, predation risks, and disturbance. Known as the "local-knowledge hypothesis," the logical premise is that knowledge provides survival or reproductive advantages. This is supported by observations of females that nested successfully in one year returning to the same area at higher rates the following year compared to unsuccessful nesting females. Additionally, philopatry to breeding sites should be higher among species that use habitats (wetlands) that are more predictable from year to year.
Ponder for a moment the interesting parallel that exists between philopatry as described here and a person's natural habit of returning to the same desk in a classroom, the same seat in multiday meetings, or the same parking spot day after day. Acceptable experiences on one day increase the tendency to return to that location the following day, whereas unpleasant or disruptive experiences almost guarantee that we will seek a different seat or location. Similarly, variations in breeding philopatry and winter site fidelity are likely driven by a desire to avoid adverse situations and to seek out beneficial experiences and resources.
We know less about site fidelity on migration and wintering sites than we do about breeding philopatry. In most cases, site fidelity is less common than breeding philopatry, which is not surprising given the mobility of waterfowl and the seasonality of many wetlands during the nonbreeding period. However, some general patterns still hold. Geese and swans are more predictable annually than ducks. Adults are more philopatric than juveniles. Also, species that use migration and wintering habitats that are highly predictable between years show greater fidelity, as do those that depend on food resources that have limited distribution. Excellent examples of this include sea ducks using marine waters along the Pacific Coast, black ducks using Atlantic Coast salt marshes, or redheads using shoalgrass beds in saline waters of the Gulf Coast.
Documented rates of site fidelity to wintering or migration sites vary widely. Examples include 40 percent in Central Valley pintails, 85 percent in Atlantic Coast black ducks, 75 percent in canvasbacks on San Francisco Bay, and more than 75 percent in Barrow's goldeneyes in Pacific Coast waters. These and other estimates of nonbreeding fidelity, which are typically measured from band recoveries or resightings, have been obtained at relatively large scales, such as counties or ecoregions, but they reveal little about fidelity to specific management areas or individual wetlands. Lacking better information about this behavior across finer scales of space and time, it is challenging to identify the exact causes of variation in winter and migration fidelity. Nevertheless, it is easier and perhaps instructive to speculate based on our general understanding.
By extension of the local-knowledge hypothesis and earlier analogies to human behavior, we should expect high rates of fidelity to migration or wintering sites when experiences at those sites yield some combination of abundant food resources, low disturbance, availability of potential mates, and low mortality risk. The converse naturally applies such that high disturbance and low food value should reduce residency time or annual return rates. Waterfowl are highly mobile animals and well adapted to explore and exploit beneficial resources across vast landscapes.
Past study methods have not permitted close examination of factors influencing migration and winter site fidelity at local scales, but new technologies are helping us learn more. Ongoing research that uses solar-powered GPS telemetry is breaking new ground in our understanding of patterns of fidelity for individual birds, multiple species, and different habitats and landscapes. Transmitters now deliver highly precise locations and have multiyear life spans.
Early insight from research on white-fronted geese conducted by Louisiana Department of Wildlife and Fisheries Biologist Paul Link and partners has revealed philopatry to individual nest bowls in the Arctic, predictable stops at fall migration sites, and returns to individual farms in southwestern Louisiana. In the near future, researchers across North America will be answering important questions about winter site fidelity, what influences it, and its effects on survival, productivity, and many other behaviors or strategies. Ducks Unlimited is eager to extend this new knowledge to conservation actions and recommendations that benefit waterfowl, hunters, and others that enjoy the resource.
Dr. Mike Brasher is a waterfowl scientist at DU's national headquarters in Memphis, Tennessee. Dr. J. Brian Davis is an assistant professor of waterfowl and wetlands ecology at Mississippi State University. Dr. Douglas Osborne is an associate professor of wildlife and wetland management at the University of Arkansas at Monticello.
Insights from Winter Banding
Researchers with the Arkansas Agricultural Experiment Station initiated a winter banding program in February 2015 to assess patterns in migration, distribution, and survival of mallards in the Lower Mississippi Alluvial Valley. The research team, led by Dr. Douglas Osborne, has banded 12,638 ducks, of which 10,569 have been mallards. The effort is also yielding information on winter site fidelity. Thus far, nearly 52 percent of band recoveries within one year after banding have occurred within 50 kilometers of the banding site, suggesting mallards exhibit a high level of winter site fidelity to the region. Anecdotal evidence further suggests that winter site fidelity may be quite strong, even to a single block of habitat, as more than 20 banded mallards from this study have been harvested on the same property where they were banded.