Site Tenacity
photographs of a Long-billed Curlew (top) and Barn Swallow by Rohan Kamath; photographs of a Bank Swallow by Tom Grey, and painting of White-throated Swifts (bottom) by Carel Brest van Kempen	The tendency to return each season to the same nest site or breeding colony is known as "site tenacity," "site fidelity," or "philopatry." The realization that site tenacity exists grew out of ornithologist Oliver Austin's long-term studies of Common Terns in Massachusetts. Austin found that individual terns tended to return to the same colony site and often to the same nest site within the colony. From banding studies, he discovered that this tendency increases with the age of the bird. Subsequent field studies have shown that this behavior occurs to varying degrees in a wide variety of North American birds including Common Goldeneye, Least Tern, Black Skimmer, Spotted Sandpiper, Long-billed Curlew, Broad-tailed Hummingbird, Bank Swallow, and Barn Swallow. Avian biologists think the major advantage of returning to an established breeding site is that the bird's familiarity with the area results in reduced susceptibility to predation and other adverse conditions. Studies have shown that sex and age of the nesting bird, prior reproductive success at the particular site, and physical stability of the nest site are all important factors affecting site tenacity. For all of the aforementioned species, birds that successfully rear young are more likely to return to the nest site the following year than birds that are unsuccessful. A finer discrimination is made by Black Skimmers, which are more likely to abandon a colony site following predation than following flooding. Presumably future failures as a result of predation are more predictable than those resulting from flooding. One very important factor influencing the evolution of site tenacity is the degree of predictability or physical stability of the nest site. This is demonstrated by cliff-nesting Black-legged Kittiwakes, other gulls, and many alcids, which return repeatedly to the same sites (cliffs tend to be there year after year). At the opposite extreme of physical stability are nest sites on open sand or mud beaches, river sandbars, and the banks of watercourses subject to periodic inundation. Species nesting in these places, such as Least, Royal, and Sandwich Terns, exhibit little site tenacity (since the sites themselves are ephemeral). Barn Swallows and Bank Swallows provide another good example of well-developed versus poorly developed site tenacity in two related species that select very different sorts of nest sites. Barns stand for many years; banks are prone to erosion. The repeated use of a nest or colony site is a behavioral property of the individual bird. Specific nest sites, however, may be used over time by a succession of individuals for a variety of reasons that are inherent
properties of the site, leading to recognition of that site as a quality nest location. Such properties include physical stability, protection from predators and inclement weather, and association with a rich food supply. Ornithologist Raleigh Robertson and his colleagues in Ontario found that in any given year the breeding success of Eastern Kingbirds in a particular territory was related to breeding success there in the previous year. Pairs nesting in previously successful territories were twice as likely to fledge young as pairs nesting in previously unsuccessful territories. Of course, scarcity of other suitable nest sites in an area may also promote site tenacity. Occupation of "traditional" nesting locations and roosts over many generations will often occur in species exhibiting site tenacity. Such behavior is prominent among swifts. Black, White-throated, and Chimney Swifts in North America, as well as Chestnut-collared and Short-tailed Swifts in Trinidad, West Indies, are known to use sites over periods that far exceed the life expectancies of individual birds. Long-term use of a single site in North America is exemplified by White-throated Swifts that David Dobkin and colleagues found nesting in the same rock outcrop in Nevada's Toiyabe Range where they had been recorded nesting 54 years earlier by naturalist Jean Linsdale of the University of California's Museum of Vertebrate Zoology.
SEE: Habitat Selection; Coloniality. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.