Winter ecology of raptors in cover-cropped agroecosystems in western Indiana
thesisposted on 02.12.2019, 17:22 by Megan E Zagorski
Intensive row-crop agriculture in the Midwestern United States has radically reduced and fragmented wildlife habitat. In agroecosystems, the distribution of foraging raptors is influenced by the availability of perches and prey abundance. Cover crops are a recent trend in agriculture that could increase the abundance and distribution of raptor prey including small mammals. However, they also benefit agricultural pest species such as voles (Microtus), which have damaged cover-cropped soybean (Glycine max) fields in Indiana. I tested the feasibility of attracting raptors, which are natural predators of voles, to cover-cropped fields by supplying artificial perches from which to hunt. Perches were erected at 3 different distances from the field edge: 50, 125, and 200m. Raptors used 82% of the perches, and perch use was greatest at 200m. However, even at peak use, my best model predicted a low probability of overall perch use for all 3 species. Although raptor perch use by itself is unlikely to control vole populations in cover-cropped fields, artificial perches could form a valuable tool as one part of an integrated pest management system. I also assessed the habitat use of 4 species of raptors, American kestrel (Falco sparverius), red-tailed hawk (Buteo jamaicensis), rough-legged hawk (B. lagopus), and northern harrier (Circus hudsonius), testing for selection of areas near cover-cropped fields. I constructed resource selection functions within a use-availability design to evaluate raptor habitat use with a series of weighted logistic regression models at 2 scales (transect and landscape) and using 2 definitions of available points (random and constrained by hunting method). American kestrels were the only species to be strongly associated with cover-cropped agricultural fields. Across the species, random models identified potential perches, while constrained random models identified more subtle habitat preferences not included in the random models. Modeling resource selection with constrained random availability will work best for well-studied species with discrete, easily mapped habitat features. Finally, although northern harriers are important vole predators, they were unaffected by the artificial perches and were encountered infrequently on my transects. Therefore, I collected and dissected regurgitated pellets to analyze the diets of wintering northern harriers at communal roosts in western Indiana. Additionally, because there is no uniformly adopted method for determining the minimum number of individuals (MNI) contained in a pellet, I also assessed 11 whether the method used to determine MNI influenced our conclusions about pellet contents between months and roosts. Pellet contents differed between years, months, and roost sites. Microtus was the most commonly occurring prey group at all roosts and in both years (range: 45- 73%), but was encountered less frequently than reported by other studies in the Midwest. Other important prey groups included Peromyscus (5-16%), Reithrodontomys (4-14%), Blarina (8- 18%), and birds (7-19%). Relative to other studies, shrews and Reithrodontomys were more important components of harrier diets in western Indiana. The 4 methods of counting MNI produced no discernible effects on our conclusions concerning pellet contents between months or roosts. Northern harrier diets in western Indiana are highly flexible and likely reflect local prey populations. Considering teeth in addition to bones may prove beneficial for pellet-based diet studies of other diurnal raptors.