RÉSUMÉ

Introduction
Resource selection by an animal can be regarded as hierarchical, or ordered (Johnson 1980, Kristan 2006); with geographic range, home range, feeding areas, and food items each constituting different orders of selection.  Within this hierarchy, however, the selection of an appropriate natal site (eg den, nest) is arguably one of the most profound choices affecting an individual’s fitness, and ultimately, a species’ persistence (Walsberg 1981, Li and Martin 1991, Carey et al. 1997, Magoun and Copeland 1998, White et al. 2001).

For any given species, natal site selection is especially important, because failed reproductive efforts may retard population growth (Lack 1954, Ricklefs 1969, Caughley 1977).  For endangered species or geographically isolated populations, reproductive failures can also increase extinction probabilities (Loiselle and Hoppes 1983, Ewens et al. 1987, Harrison 1991, Sieving 1992).
Biologists frequently attempt to assess resource selection at one or more orders. With avian species, this assessment frequently addresses nest site selection (eg Stauffer and Best 1982, Sedgwick and Knopf 1990, Li and Martin 1991, Filliater et al. 1994, Roper 2003).  Although most studies of nest site selection have typically examined selection at the spatial level of the nest site or nest itself (eg Joern and Jackson 1983, Munro and Rounds 1985, Holway 1991, Götmark et al. 1995) increasing numbers of studies have examined avian habitat and nest site selection at multiple spatial scales (eg Gutzwiler and Anderson 1987, Esely and Bollinger 2001, Hardy and Morrison 2001, Kristan 2006), or using multiple statistical methods (eg MacKenzie et al. 1982, Sedgwick and Knopf 1990, Battin and Lawler 2006).

Understanding nest site selection at multiple spatial scales can provide insights into how specific environmental components may interact to ultimately influence reproductive outcome (Nilsson 1984, Li and Martin 1991, Orians and Wittenberger 1991, Doligez et al. 1999, Weidinger 2002).  Such knowledge can be invaluable in the case of intensively managed wild populations, such as those of game species and endangered species.

An example of an intensively managed wild population is the critically endangered endemic Puerto Rican Parrot (Amazona vittata), one of the 10 most endangered birds in the world (Wiley et al. 2004).  Once abundant throughout Puerto Rico, this species has been restricted to less than 20,000 ha of subtropical montane rainforest in northeastern Puerto Rico for over 60 years (Snyder et al. 1987, Wiley et al. 2004).

Research on the nesting ecology of the remnant wild population began in the 1950s (Rodriguez-Vidal 1959).  In 1967, the species was listed as endangered, and active recovery efforts began in 1968 (Snyder et al. 1987).  Intensive nest management to improve reproductive success began in 1973 and continues to date (Wiley et al. 2004).  However, despite all management efforts, the known number of active wild nests has never exceeded 6 in any year since recovery efforts began (White et al. 2005a).

A key component of management efforts for the Puerto Rican Parrot has been the provisioning of artificial nest cavities to augment the paucity of suitable natural cavities in the nesting areas (Snyder et al. 1987, White et al. 2005a).  Artificial nests, combined with intensive monitoring of nesting activities (Lindsey 1992, White and Vilella 2004) have been instrumental in improving individual nest success (Wiley et al. 2004).  Regardless of whether the nest is natural or artificial, nest site fidelity is high, and active nests are normally used for several years (Snyder et al. 1987, White et al. 2005a).

A likely contributor to the observed fidelity is success of previous reproductive attempts (Switzer 1997, Doligez et al. 1999).  A seemingly unique feature of nest site fidelity in Puerto Rican Parrots is that it is directed to the specific nest sites within a breeding area.  Over time, different nesting pairs tend to use the same sites, rather than pioneer new ones.  This situation begs the question - why do these birds consistently use particular nest sites and not others?

Because all known nesting activity by wild Puerto Rican Parrots has, since 2001, occurred in standardized artificial nest cavities (White et al. 2005a), we were provided with a unique opportunity to examine habitat and spatial characteristics at all known nest sites, without the confounding effects of differences intrinsic to the cavities per se (see Belles-Isles and Picman 1986, Lumsden 1986, Finch 1989).

To our knowledge, this also is the first reported nesting study of an avian species in which the entire wild population nests in standardized artificial cavities.  To this end, our objective was to answer the following questions:  1) Are there detectable differences between used and unused nest sites?  2) Can habitat and spatial characteristics be used to differentiate, and thus predict, used versus unused sites?

Correctly identifying biologically meaningful variables that influence nest site selection may provide greater understanding of how management actions may affect reproductive success (Martin 1992, Steele 1993, Schmidt and Whelan 1999).  Thus, we also discuss the management implications of our findings for the continuing recovery efforts for this and other endangered avian species.