Habitat quality is best measured by habitat-specific demography (Johnson 2007
). However, inference of habitat quality depends upon the scale and level of demographic analysis (Pidgeon et al. 2006
). When we partitioned reproductive success among separate but nested scales, variation was attributable to nest box, neighborhood, and population scales, and also to variation in individual quality (Table ). This resulted in a “lumpy” landscape (Fig. ), with higher and lower quality sites and neighborhoods located within populations that also differed in quality. The best sites and neighborhoods in the less productive lowland population were as productive as their counterparts in the more productive upland. Moreover, variation in individual or female quality mediated differences among sites, neighborhoods and populations, further complicating the fitness landscape and habitat selection decisions.
Habitat determinants of reproductive success were clearly evident (Fig. ). Higher vegetative cover was associated with lower productivity at nest sites in both populations. Predation is probably an important cause of this result. Higher rates of predation were associated with greater vegetative cover in the lowland but not in the upland population (Fig. ). Nest predators are typically more common in forested areas comprising the lowland at our study site (Eisenberg 1979
). The benefit of a decreased risk of predation by nesting in a box with less vegetation was partly reduced by infanticide, which occurred more often at less vegetated sites (Fig. ). Infanticide occurred more often in the upland than in the lowland population, which may have accounted for differences in the effect of predation between these two populations. Nest boxes were also more productive when surrounded by standing water in the lowland but not in the upland. Deep water deters nest predators (Collias and Collias 1984
; Leonard and Picman 1987
; Jobin and Picman 1997
). Water may not have exerted strong effects on upland productivity because ground elevation is high enough that standing water during the rainy season never reaches most upland nest sites (Fig. ), even in the rainiest years. Nevertheless, local food (seed) abundance was unrelated to productivity, suggesting that either it is not an important factor or it acts at larger spatial scales. Starvation commonly causes brood reduction in parrotlets, which results in an unequal distribution of food among nestlings of tremendously different size and competitive ability rather than a shortage of food, but rarely causes complete nest failure (Stoleson and Beissinger 1997
; Budden and Beissinger 2009
Some inaccuracies in the relationship between habitat cues and productivity are introduced by comparing habitat conditions measured in 2004 to productivity estimates made from 1994 to 2003. However, changes to the llanos habitats around parrotlet nest sites over this time period were not large. Vegetative cover adjacent to nest sites experienced only minor changes, although the canopy has become more closed in the forests surrounding the lowland. Annual variation in rainfall is large and would affect measures of water depth made in 2004. However, relative differences in water depth among nest sites should remain the same.
Movements of adults within populations and dispersal of juveniles between populations indicate that parrotlets may identify habitat cues related to high-quality nest sites and preferentially disperse to them. Parrotlet pairs generally have high nest site fidelity (Waltman and Beissinger 1992
) and adults rarely move between populations after they have initiated breeding (Sandercock et al. 2000
; Beissinger 2008
). When adults did change nest boxes, they more often chose boxes in their population with greater productivity. Moreover, parrotlets disperse primarily as juveniles and juvenile dispersers preferentially chose to settle in the population with higher productivity (the upland). These results are compatible with several prevailing models of population distribution, as discussed below.
Parrotlet adult and juvenile dispersal patterns meet two important assumptions underlying IFD theory (Fretwell and Lucas 1970
). Variation in habitat quality is at least partially predictable, since nest sites in the open and with more water experienced less predation than boxes with vegetation cover, near forested habitats, and with less water. Also, parrotlets dispersed preferentially to nest boxes with higher productivity (i.e., quality). Under these conditions that are characteristic of “ideal animals”, population distributions should become evolutionary stable and converge to an IFD (Cressman and Křivan 2006
). However, predator–prey dynamics have the potential to disrupt IFD (Abrams 2007
). Multiple predators prey on parrotlet nests (Stoleson and Beissinger 2001
), which may complicate the temporal and spatial dynamics of the parrotlet populations.
Two features of the parrotlet population suggest that pre-emptive or ideal despotic occupancy may also be occurring (Pulliam and Danielson 1991
; Oro 2008
). Boxes with nests that were initiated earlier in the season were also the most productive (Fig. ) and high-quality nest sites experienced elevated intraspecific competition reflected by infanticide attacks (Fig. ). The patterns of pre-emptive site occupancy and habitat heterogeneity within and between populations prevalent in parrotlets also met two conditions necessary for site-dependent regulation (Rodenhouse et al. 1997
). Moreover, as population size increases, parrotlets should increasingly nest in lower quality sites, which will lower overall reproductive success and population growth. Site-dependent regulation is a density-free process, but the spatial constraints prevalent due to limited nesting sites (nest boxes) may facilitate increased infanticide. Rates of infanticide can be positively related to population density (Møller 2004
), which appears to occur in parrotlets (unpublished data), but this is not always the case (Boonstra 1980
; Rankin and Kokko 2007
). Interference competition in the form of infanticide in parrotlets decreases the advantage of nest boxes that are less likely to experience predation (Figs. , ).
Our results show that parrotlets meet three important assumptions of the IFD. First, heterogeneity in productivity, or site fitness, existed at multiple scales. Second, productivity varied reliably with habitat quality, providing parrotlets with dependable cues for choosing good sites (e.g., canopy cover and water). Third, parrotlets actively dispersed to sites of higher fitness. Predation and infanticide were important processes that influenced “ideal free” choices, creating spatial structure in productivity within and among parrotlet populations.