Wightman, Catherine S. – Patterns associated with habitat selection by Peregrine Falcons in central west Greenland. 2001.
Identifying patterns and features associated with selected habitats can indicate which combinations of resources are most suitable to animals. Animals, however, may select habitats as suitable even if resources in those habitats do not provide measurable fitness benefits. It is therefore necessary to evaluate the fitness benefits associated with selected habitats to identify features that are important in the quality of a habitat. In an increasing population, the patterns with which animals occupy habitats also provide information on their behavioral strategies associated with habitat selection. Members of the Greenland Peregrine Falcon Survey collected occupancy and productivity data on an expanding population of peregrine falcons (Falco peregrinus) in central West Greenland between 1972 and 2000. Using these 28 years of data, I analyzed patterns of peregrine falcon occupancy and distribution among breeding sites, and evaluated whether productivity was associated with those patterns.
To identify characteristics associated with selected breeding sites, I measured 29 habitat features at 67 occupied and 38 unused breeding sites in summer 1998 and 1999. For each occupied site, I found that the nearest neighbor was significantly farther than the nearest cliff and the nearest historically occupied cliff. Although some unused cliff sites may be unavailable in a given year because of peregrine falcon spacing requirements, physical characteristics may make these cliffs unsuitable regardless of their availability. I used logistic regression to identify those features that are useful for predicting occupancy by peregrine falcons. Peregrine falcons chose cliff sites with eyrie ledges that provided some vertical overhang protection and were inaccessible to ground predators. They also chose ledges with a sand or dirt substrate rather than an available common raven (Corvus corax) stick nest. The apparent avoidance of stick nests may result from selection for moderate rather than complete overhang protection, the latter being characteristic of common raven stick nests in the study area. Peregrine falcons also occupied breeding sites with tall cliffs and open views.
To evaluate patterns of distribution and selection among occupied nest sites, I used occupancy and productivity data from 1972-1999. Variance in productivity was significantly lower at traditionally (n=29) rather than recently (n=37) occupied cliff sites, which suggests that traditionally occupied cliff sites are better quality. This also indicated that peregrine falcons occupy breeding sites according to a pattern of despotic distribution (resource defense) rather than an ideal free distribution (resource exploitation). Consistently occupied cliff sites (n=40) provided higher average productivity than inconsistently occupied cliff sites (n=18), and thus, these sites are also better quality. Using the habitat features measured at occupied cliff sites, I created two logistic regression models to predict traditional and consistent occupancy at cliff sites. Features of good quality habitat included the height or elevation gain of the cliff and protection from weather on the eyrie ledge. Spacing of suitable and occupied cliff sites also was an important feature, and the best cliffs generally were more isolated. I suggest this association with isolation results from the benefits of decreased intra-specific competition from neighbors and floaters. The similarity between features associated with suitable and good quality breeding sites suggests that unused sites are unsuitable because they provide poor reproductive benefits to the occupants.
To assist managers in recognizing suitable and good quality breeding habitat for peregrine falcons in Greenland, I developed three easily implemented models. Using logistic regression, I created a management model for predicting occupancy, and thus suitability, of cliff sites for peregrine falcons. Features in this model included cliff height and elevation of hill across the valley. I reserved a set of habitat features measured in 1999 and 2000 from occupied (n=9) and unused (n=11) cliff sites to evaluate the accuracy of the management model. The model correctly predicted occupancy at 75% of the validation cliff sites, which indicates this model is useful for estimating the number of suitable cliff sites in central West Greenland. I also used logistic regression to create two management models using habitat features of traditionally and recently occupied cliff sites to predict traditional occupancy by peregrine falcons, and thus better quality cliff sites. Habitat features included in one model were cliff height, ledge depth, and horizontal exposure of the eyrie ledge. The second model included cliff height, distance to nearest historically occupied cliff, and distance to nearest neighbor. Classification of cliff sites using these models, in conjunction with local sampling, can function as an index to the peregrine falcon nesting population size and stability throughout central West Greenland.