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DISCUSSION Our overarching goal was to attempt to predict site selection based on characteristics of the nesting sites available to a very small breeding population.  To do so, we used non-traditional statistical techniques to obtain meaningful inferences due to the sparseness of the data, finite nature of the selection process and temporal correlation evident in site selection. Our exact test has several appealing properties.  First, it does not depend on any distributional assumptions (normality, homogeneity, etc).  Second, the temporal correlation in site selection is implicitly handled.  Third, the experimental design is implicitly controlled for.  Finch (1989) argued that if nest sites are scarce, nesting preferences might be difficult to detect, due to use of suboptimal sites by some individuals.  In our case, the number of available unused nest sites exceeded both the number of used sites and the number of wild nesting pairs.  Moreover, artificial nest cavities were abundant, occurring throughout the known nesting area (approx. 30 ha) at a density of approximately 1 per 1.6 ha. Thus, we believe that nests used represented those nests actually selected by nesting pairs.  Further, we used a predictive model that considered the number of times each nest was used in “weighting” the differences in relevant habitat characteristics.  This approach is effective in discerning site selection because it uses the nest site as the experimental unit, rather than the nesting pair (Finch 1989).

Used and unused nest sites of Puerto Rican Parrots differed in several aspects (Table 1).  Nests used by Puerto Rican Parrots occurred in more open sites, relative to horizontal and vertical visibility from the nest entrance, than unused nests.  Entrances of used nests also tended to face westerly, and in a downhill direction.  Further, used nest sites were typified by a greater abundance of mature sierra palms than unused sites.  In contrast, greater visual obstruction from the nest entrance, uphill and eastward oriented nest entrances, and fewer mature sierra palms characterized unused nest sites.

Rodriguez-Vidal (1959) and Snyder et al. (1987) reported that most natural nest cavities used by Puerto Rican Parrots also faced in a downhill and generally westward direction, and attributed this to greater frequency of such cavities in palo colorado trees, the predominant species used for nesting by Puerto Rican Parrots.  The nonrandom occurrence of such cavities was further attributed to inclination angles of palo colorado trees on slopes in the CNF, resulting in higher incidence of limb breakage and thus cavity formation on the lower (ie downhill) sides (Snyder et al. 1987).

However, orientations of artificial cavities in this study were not dependent on such factors and were, in fact, random when considered as a group.  Also, we found no difference when we compared mean vector for natural nest cavity entrances reported by Snyder et al. (1987) within the same nesting area (ie South Fork) with that of used artificial cavities.  Moreover, the proportion of downhill facing natural nest entrances reported by Snyder et al. (1987) was identical (6/7) to this study.

Thus, based on observed patterns, Puerto Rican Parrots apparently are selecting against eastward, uphill facing nest cavities.  Nonrandom orientations of nest cavity entrances also have been reported for Hispaniolan Parrots (Amazona ventralis; Snyder et al. 1987), Williamson and Red-naped Sapsuckers (Sphyrapicus thyroideus, S. nuchalis; Crockett and Hadow 1975), Common Flickers (Colaptes auratus; Inouye 1975), and several other cavity nesters (Connor 1975, Stauffer and Best 1982, Korol and Hutto 1984, Hardy and Morrison 2001).  In those studies, thermoregulatory advantages and protection from inclement weather were most often cited as factors influencing cavity orientations.

Because of the geographic location and topographic features of the CNF, prevailing winds and rain blow primarily from a northeasterly direction, and sweep down ridges into adjacent valleys (Snyder et al. 1987, Ostertag et al. 2005).  For Puerto Rican Parrots, cavities that provide greater protection from the frequent rain and winds (ie westerly, downhill facing) may also have greater adaptive value for successful nesting.  Westerly-facing cavities can also provide increased solar exposure during late afternoon, before ambient temperatures begin to decline at nightfall, thereby reducing the range of daily temperature fluctuations within nest cavities (McComb and Noble 1981, Hardy and Morrison 2001, Wiebe 2001).

For some avian species, nests located within food-rich habitat patches are more productive than those within areas of sparse resources (eg Hussell and Quinney 1982, Finch 1989, Li and Martin 1991); yet for others, nest sites providing greater protection from predators are more adaptive (eg Belles-Isles and Picman 1986, Martin and Roper 1988, Alonso et al. 1991, Wightman and Fuller 2005).  However, these factors are not mutually exclusive.

Selection by Puerto Rican Parrots for nest sites in patches with greater densities of mature sierra palms likely reflects three mutually compatible adaptive strategies.  First, by nesting in areas with higher concentrations of a key food source (eg sierra palm), nesting parrots may maximize foraging efficiency and minimize time spent away from the nest, thereby increasing incubation and brooding efficiency.  Indeed, Puerto Rican Parrots that are more attentive to incubation and brooding activities are more successful nesters than parrots that spend more time away from the nest (Wilson et al. 1995, 1997).

Secondly, by reducing their foraging radius, nesting adults may simultaneously reduce their own exposure to Red-tailed Hawk predation, and more effectively foil potential nest predations and usurpations by Pearly-eyed Thrashers (Margarops fuscatus), their primary nest predator and competitor in the CNF (Wiley 1985, Snyder et al. 1987, White et al. 2005a).

Finally, once chicks have fledged, adults and fledglings normally remain near (<100m) the nest site for several days, until fledglings are capable of more extensive forays (Snyder et al. 1987, T. White, pers. observ.).  During this period, close proximity of foods such as sierra palms may constitute a survival advantage for the family group by allowing parrots to forage with minimal movements, potentially reducing predation risk from raptors (Snyder et al. 1987).

Predator avoidance most likely also explains parrots’ selection for nest sites with greater visibility from the nest entrance.  Götmark’s et al. (1995) “trade-off hypothesis” of nest site selection stated that where predation is a major selection pressure and where thermoregulation is of minor importance, nesting birds should benefit from selecting more open nest sites over more concealed sites.

Götmark et al. (1995) also listed four potential benefits accrued to nesting birds from good visibility at the nest.  Among these, avoidance of predation on adults and watching for nest predators may be the most important benefits for Puerto Rican Parrots.  Nest sites with a clear view of the immediate surroundings can provide nesting birds the opportunity to effectively scan for potential predators and react appropriately (see Dyrcz 1983, Belles-Isles and Picman 1986, Finch 1989, Li and Martin 1991).  This is particularly true for cavity nesters such as parrots, as they can be highly vulnerable to predators when entering and exiting nests (see Gnam and Rockwell 1991, Brightsmith 2005, Stahala 2005).

Nesting behavior of Puerto Rican Parrots is characterized by extreme vigilance at and near nest sites, with circumspect nesting birds normally spending several minutes quietly scanning the surroundings before entering or exiting nest cavities.  At the slightest disturbance, nesting Puerto Rican Parrots will either quickly retreat back into the nest interior, or immediately vacate the nest vicinity if outside the nest entrance (Snyder et al. 1987, Wilson et al. 1995, White, pers. observ.).

Similar nesting behavior has been reported in Black-billed Parrots (A. agilis; Koenig 2001), and observed in Hispaniolan Parrots (A. ventralis) and Bahama Parrots (A. leucocephala bahamensis; T. White, pers. observ.).  Intuitively, nest sites that facilitate nest vigilance for such species should provide an adaptive advantage in more effective predator detection and avoidance.

Adaptive preferences in nest site selection reflect the influence of species-specific selection pressures (Nilsson 1984, Brightsmith 2005).  For the Puerto Rican Parrot, predation on adults and juveniles by Red-tailed Hawks, and nest predation and usurpation by Pearly-eyed Thrashers, have been major factors limiting productivity and population growth in the CNF (Snyder et al. 1987, Wiley et al. 2004).  Accordingly, adaptive responses of Puerto Rican Parrots to these factors should result in behaviors, such as nest site selection, designed to maximize individual fitness and consequently, species persistence (Brightsmith 2005).

Not surprisingly, both the current artificial and natural nest sites used in the past by Puerto Rican Parrots share some common characteristics, suggestive of nest site selection influenced by a temporally constant selection pressure.

Based on this study and prior work, we believe predation pressure to be the primary factor influencing nest site selection by Puerto Rican Parrots, which respond by selecting nest sites that provide advantages in predator detection and avoidance at all stages of the nesting cycle.  Our findings are consistent with Götmark’s et al. (1995) trade-off hypothesis of nest site selection in explaining current nesting behavior of Puerto Rican Parrots in the CNF.

CONCLUSION

The recovery plan for the Puerto Rican Parrot includes re-establishment of additional wild populations, the first of which is in the Rio Abajo Commonwealth Forest in north central Puerto Rico (US Fish and Wildlife Service 1999, Wiley et al. 2004).  Because the Rio Abajo area is, like most of Puerto Rico, predominately secondary forest, natural cavities are limited and the provisioning of artificial nest cavities will be essential to insure initial availability of suitable nest sites for reintroduced parrots.

Given our findings, management strategies for future reintroduced populations in Puerto Rico should consider placement of artificial cavities at sites with similar site-specific functional characteristics as those used by parrots in the CNF.   For example, suitable nest sites should facilitate predator detection and avoidance and foraging efficiency for nesting adults, provide maximum protection from inclement weather, and promote fledging success and survival of young.

For endangered birds, improved nesting success can yield dividends in lower extinction probabilities.  Identifying biologically relevant variables that influence nest site selection is an essential step toward developing effective management strategies for such species.  In cases where nest site availability limits productivity, efforts to identify and replicate the “nest gestalt” of successful nesting sites may be an effective tool in promoting population growth, and potentially avoiding extinction.