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STUDY AREA We conducted this study in the Caribbean National Forest (CNF), located in the Luquillo Mountains of northeastern Puerto Rico (18º18′N, 65º47′W).  The CNF comprises 19,650 ha of subtropical rainforest, with elevations ranging from 200 to 1,074 m above sea level, and contains four dominant forest types; namely, the tabonuco (Dacryodes excelsa) zone at elevations to 600 m, the palo colorado  (Cyrilla racemiflora) zone above 600 m, sierra palm (Prestoea montana) stands on steep slopes and ravines throughout CNF, and dwarf cloud forest on the highest peaks and ridges (Snyder et al. 1987). Puerto Rican Parrot nests, however, are found mainly at elevations from 500 to 700 m at the transition between the tabonuco and palo colorado forest types, with palo colorado being the primary species used for nesting (Snyder et al. 1987, White and Vilella 2004).  Other principal overstory species in the parrot nesting area include Micropholis garcinaefolia, M. chrysophylloides, Magnolia splendens, Clusia grisebachiana, Cecropia peltata, Ocotea spathulata, Manilkara bidentata, Calycogonium squamulosum, and Sloanea berteriana (Snyder et al. 1987). Precipitation is copious, ranging annually from 200 cm at lower elevations to more than 500 cm at the highest peaks.  Annual temperatures range from 11º to 32º C, averaging 21º C (Lindsey et al. 1991). Growth rates of woody vegetation are relatively slow, due to a mild climate and low soil fertility (Snyder et al. 1987, Weaver and Murphy 1990).

METHODS

Data collection
We grouped all available artificial nest cavities (n = 19) into 2 groups; namely, those that had been used at least once within the past 6 years (2001-2006; n = 7), and those that had not been used in any previous year (n = 12).  We classified a nest as “used” if egg-laying had been initiated.

Data were collected at all nest sites during July-August 2005, immediately following the annual nesting season (February-June).  At each site, we measured 20 variables in two general categories:  1) nest entrance characteristics, and  2) nest site characteristics.  Two additional variables – nest tree species and whether the nest tree was canopy-emergent or not – also were recorded, but not used in later analyses, as they were invariant (ie all artificial nests were in canopy-emergent palo colorado trees).

We focused particular attention on spatial characteristics relative to the nest entrance, because final decisions by Puerto Rican Parrots regarding acceptance of a nest site likely occur at, or near, the nest entrance (Snyder et al. 1987, Lindsey 1992, Wilson et al. 1995, White and Vilella 2004).  Also, with standardized artificial cavities, the nest entrance constitutes the primary spatial nexus between a homogeneous internal environment and the heterogeneous external environment.

Further, we examined variables at five spatial scales:  1) spatial characteristics of the nest entrance itself (eg height above ground, aspect, etc),  2) selected habitat characteristics within 15 m of the nest site,  3) selected habitat characteristics within 30 m of the nest site,  4) selected habitat characteristics within 100 m of the nest site, and 5) spatial relationships between all nest sites within the entire parrot nesting area.

We determined nest entrance height above ground, distance below surrounding canopy, nest tree height, and ground slope at nest site, using a clinometer.  Nest entrance aspect and aspect relative to site slope were determined using a compass.  Aspect relative to site slope was the absolute (+/-) degree of angular deviation of the nest entrance from straight downhill (0º) to straight uphill (180º), and was placed into two categories corresponding to 90º increments (ie downhill, uphill) of angular deviation.

We used a spherical densiometer to determine canopy cover at nest sites.  At each site, we used the mean of 4 densiometer readings oriented along each of 4 cardinal compass directions.  Horizontal visibility at the nest entrance was defined as the horizontal distance from the nest entrance to the nearest point of visual obstruction by surrounding vegetation in each of 8 compass directions (ie 45º increments).

These distances were then used to calculate the total area of the horizontal field of view from the nest entrance.  The horizontal field of view was further divided into two hemispheres (frontal and rear) to evaluate differences in visibility within these regions.  Distance from nest entrance to nearest branch, both horizontally and vertically, also was recorded.

A point-center quarter method was used to determine distance and bearing to the nearest canopy tree in each of four quadrants (Finch 1989).  We defined a canopy tree as any tree whose crown made up part of the surrounding canopy.  We then used these measurements to calculate the area of a polygon representing degree of “openness” around the nest tree relative to spacing of nearest canopy trees.

Visibility and openness at nest sites were examined, because several studies have suggested that, for some species, use of more open nesting sites is an adaptive response to predation (Dyrcz 1983, Belles-Isles and Picman 1986, Finch 1989, Götmark et al. 1995, Koenig et al. in press; but see Joern and Jackson 1983, Alonso et al. 1991, Holway 1991, Martin 1993, Weidinger 2002), and because Red-tailed Hawks (Buteo jamaicensis) are a major predator of Puerto Rican Parrots (Snyder et al. 1987, White et al. 2005b).  Distance to nearest snag (ie dead standing tree) also was recorded, because snags are frequently used as hunting perches by Red-tailed Hawks in the CNF (Snyder et al. 1987, Nimitz 2005).

We used a nested circular plot (using nest tree as center) to determine number of woody stems with a diameter-at-breast-height (DBH) >10 cm within 15 m of the nest site, number of mature (ie flowering, fruiting, senescent) sierra palms within 30 m, and number of canopy emergent trees within 100 m of the nest site.  We enumerated sierra palms because they provide the primary source of food for Puerto Rican Parrots during the nesting season (Snyder et al. 1987, Wunderle 1999).  Finally, elevation and coordinates of each nest site were obtained using a geographic positioning system (GPS) and used to determine distance to nearest active nest site.

Because not all used nests were active in all years, we determined distance to nearest active nest each year at time “t” and “t-1″ (except for year 2001) to account for any autocorrelation that could influence probability of a nest site being used in a given year due to proximity of an active nest the previous year (Switzer 1997, Doligez et al. 1999, Brown et al. 2000).

Although presence and number of attached vines and lianas on nest trees have also been included in some nesting studies (eg Roper 2003, Koenig et al. in press), most such vines become detached and are routinely removed from nest trees during the installation and maintenance of artificial nests for Puerto Rican Parrots.  Also, an horizontally oriented “entrance vine” is affixed to the entrance of all artificial nests to facilitate access by parrots (Vilella and Garcia 1995, White et al. 2005a), further precluding meaningful comparisons between used and unused sites for this characteristic.