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.