Visual Obstruction Readings Research Articles

Adaptive grazing and animal density implications for stocking rate and drought in northern mixed-grass prairie

Grazing management is critical for ruminant livestock enterprises affecting both animal performance and land sustainability. Intensification of grazing management is important for increasing food production, however, linear stocking rate models are not ideal in rangeland environments due to complexities such as drought. We conducted a study in a semiarid northern mixed-grass prairie in Wyoming, USA from 2017 to 2021. Grazing treatments were moderate rotational grazing (MRG; 4 heifers), ultra-high density rotational grazing (UHD; 33 cow-calf pairs), and no grazing (NG) organized in a randomized complete block design using 0.4 ha paddocks. Baseline vegetation was measured prior to grazing each year to determine grazing time available using a modified Robel pole coupled with forage biomass clippings. Planned and realized stocking rates were then calculated for all MRG and UHD treatments and potential stocking rates for NG and relativized to effective precipitation (March 1 to timing of sampling in mid-June). For the UHD treatment, cattle time in paddocks ranged from 0.29 to 1 d while for the MRG treatment, cattle time in paddocks ranged from 4.5 d to 12.5 d Effective precipitation ranged from 130 mm in 2020 to 251 mm in 2018 with a five year mean of 205.4 mm. Planned/potential stocking rate ranged from 1.40 to 5.17 Animal Unit Months per hectare (AUMs ha−1) and realized stocking rate ranged from 1.38 to 4.87 AUMs ha−1 across paddocks and years. Vegetation visual obstruction and stocking rate did not differ by treatment any of the years of the study (all p-values > 0.05) with the exception of 2020 when the NG treatment was higher than the MRG and UHD treatments (p < 0.05). Planned and realized stocking rates for both grazing treatments had highly correlated (both r2-values ≥ 0.80) and significant relationships (both p-values < 0.05) with effective precipitation. Five year mean planned or realized stocking rates did not differ by treatment. Accomplishing planned stocking rates proved to be difficult for the UHD grazing scheme that requires rapid moves as indicated by the broader range of ΔAUMs ha−1 (Planned – Realized) in that treatment compared to the MRG treatment. The effect of precipitation on stocking rates, and specifically relative to potential stocking in the NG treatment, was most influential during the significant deficit year but was non-existent during average years or surplus years. Using annual visual obstruction readings (VOR) to adapt stocking rates may be useful for more intense grazing management.

Resource Selection and Herbaceous Biomass at Foraging Sites of Translocated Bighorn Sheep

Bighorn sheep (Ovis canadensis) historically inhabited the Black Hills region of South Dakota, but the species was extirpated from the area in the early 1900s. We translocated 26 bighorn sheep from Alberta, Canada to the northern Black Hills. We predicted that translocated bighorn sheep would exhibit similar resource use and selection as populations native to comparable habitats, specifically steep slopes with low overstory cover and high visibility. We used resource selection functions to estimate habitat selection by bighorn sheep and Robel pole and hand-clipped biomass to investigate the correlation between forage estimate methods. Bighorn sheep selected habitat types at varying levels, but were most strongly associated with barren and grassland landscapes while negatively associated with forest landscapes. Bighorn sheep had a positive association with slope and selected for fire-disturbed landscapes and elevation seasonally. Visual obstruction readings and herbaceous biomass at foraging sites (n = 78) were positively associated (r2 = 0.62). Herbaceous biomass ranged from 302.07 to 2,487.43 kg·ha−1. Foraging sites were typically located in areas with little overstory tree canopy cover (mean = 8.41%, standard error [SE] = 1.85), shorter distances to escape terrain (mean = 24.00 m, SE = 3.21), and low amounts of woody debris (mean = 0.25 kg·ha−1, SE = 0.07). The study area (Deadwood region of South Dakota) provided sufficient landscape attributes and herbaceous biomass to support the newly established bighorn sheep herd. Post-translocation assessments of resource use are crucial for evaluating conservation actions and potential success of future large mammal translocations.

Refining Robel Pole Applications for Grazed Idaho Fescue Grasslands in the Big Horn Mountains, Wyoming

Administrative oversight of grazing on public land requires efficient and reliable rangeland monitoring. In a portion of the Bighorn National Forest of Wyoming, the US Forest Service uses the modified Robel pole as a monitoring tool to measure residual herbaceous biomass after grazing. Questions about the vegetation standards have persisted for decades. From 2018 to 2020 we quantified relationships between herbaceous biomass and visual obstruction across spatial, temporal, management, and topoedaphic gradients in 11 grazed pastures selected mutually by stakeholders. Initial standards were developed on the basis of 1 yr of research, and state visual obstruction readings (VORs) were correlated with biomass with an R2 = 0.81, yet our research resulted in R2 values of approximately half (ranging from 0.39 to 0.45) with significantly different slopes 2 of 3 yr. Moreover, the intercepts for each year individually, when averaged, or when combined were always greater than the relationship used to develop initial standards, resulting in 140−564 more kg per ha of biomass at the 5-band standard level, or situations where lower band levels (2.5−3.9) would yield similar biomass levels. In addition to VOR and biomass, slope, aspect, plant and ground cover, weather, and livestock management variables were assessed with information theory models for predicting residual biomass. The top model included VOR, slope, exotic grass, and temperature. This suggests that greater knowledge of the site will increase understanding of biomass production on the site and help to mitigate disagreements among stakeholders by tailoring expectations to variable sites. Finally, the higher intercepts in all of our models may explain the persistent skepticism about the application of the original standard and provide the opportunity to refine standards to better match postgrazing monitoring with resource objectives, particularly if functional group dominance on these sites have changed over time.

American Black Duck Nesting Ecology in North Carolina

ABSTRACTNorth Carolina, USA, is the southernmost extent of the American black duck's (Anas rubripes) breeding range; however, little is known about their nesting ecology in this region. We located and monitored 140 nesting black ducks over 2 years (2017–2018) to quantify preferred nesting habitat and assess nesting productivity within coastal North Carolina. We located nests in brackish marshes (75%) and man‐made dredge spoil islands (25%) at a density of 1 nest/22 ha. Black ducks selected high marsh and nested an average of 21.81 m from open water at a mean elevation of 1.36 m. In preferred nesting habitat, visual obstruction readings were 0.50 m with a maximum mean vegetation height of 0.81 m and land cover consisted largely of grasses (84.6%). Apparent nest success rates varied from 31% (2017) to 63% (2018) across years. The majority (72.2%) of variability in nest success was best predicted by nest location (mainland marsh, natural island, or spoil island), vegetation density, maximum vegetation height, and year. Management for breeding black ducks in coastal North Carolina should focus on promoting selected nesting habitat and reducing nest predators. Prescribed burns, used to set back succession on spoil islands and in brackish marshes should be conducted in the winter or in the early growing season not to exceed the twenty‐fifth quantile date of black duck nest initiation (2 Apr). © 2021 The Wildlife Society.

Sharp‐tailed Grouse in the Nebraska Sandhills Select Residual Cover Patches for Nest Sites

ABSTRACTWe evaluated selection and availability of residual cover (dead standing herbage) by sharp‐tailed grouse (Tympanuchus phasianellus) at time of nest‐site selection in an intact and annually grazed grassland. We used radiotelemetry in 1988–1990 to locate 147 nests in the sandhills of Nebraska, USA, and classified 121 as initial nests and 26 as renests. We used visual obstruction readings (VOR) to measure the height and density of residual cover at nests and 373 landscape‐scale transects around leks (trap sites). We excluded 77 nests from vegetation analysis because green herbage or early livestock grazing compromised residual cover measurements. Most females selected nest sites with residual cover, mostly warm‐season grasses, taller and denser than surrounding vegetation. Visual obstruction readings at 70 nests ( = 7.1 cm, SE = 0.4, range = 1.0–19.0) averaged almost twice the VOR of residual cover within 12 m of nests ( = 4.0 cm, SE = 0.3, range = 0.9–11.8) and almost three times the landscape VOR ( = 2.5 cm, SE = 0.1, range = 0.5–7.9). As further evidence of the importance of residual cover, &gt;52% (n &gt; 37) of the females (initial nests) in 1988 and 1989 completed egg‐laying and were incubating before green herbage began contributing to nest cover. More than 88% (n &gt; 42) of the females relied on residual cover through egg‐laying in 1990 when annual drought delayed foliar development. Interested ranchers and land managers can enhance residual cover through livestock grazing management to attract females and presumably increase nest density, a key component of annual sharp‐tailed grouse productivity. © 2020 The Wildlife Society.

Timing of vegetation sampling does not influence associations between visual obstruction and turkey nest survival in a montane forest.

Evaluating relationships between ecological processes that occur concurrently is complicated by the potential for such processes to covary. Ground‐nesting birds rely on habitat characteristics that provide visual and olfactory concealment from predators; this protection often is provided by vegetation at the nest site. Recently, researchers have raised concern that measuring vegetation characteristics at nest fate (success or failure) introduces a bias, as vegetation at successful nests is measured later in the growing season (and has more time to grow) compared with failed nests. In some systems, this bias can lead to an erroneous conclusion that plant height is positively associated with nest survival. However, if the features that provide concealment are invariant during the incubation period, no bias should be expected, and the timing of measurement is less influential. We used data collected from 98 nests to evaluate whether there is evidence that such a bias exists in a study of wild turkey (Meleagris gallopavo) nesting in a montane forest ecosystem. We modeled nest survival as a function of visual obstruction and other covariates of interest. At unsuccessful nests, we collected visual obstruction readings at both the date of nest failure and the projected hatch date and compared survival estimates generated using both sets of vegetation data. In contrast to studies in grassland and shrubland systems, we found little evidence that the timing of vegetation sampling influenced conclusions regarding the association between visual obstruction and nest survival; model selection and estimates of nest survival were similar regardless of when vegetation data were collected. The dominant hiding cover at most of our nests was provided by evergreen shrubs; retention of leaves and slow growth of these plants likely prevent appreciable changes in visual obstruction during the incubation period. When considered in aggregate with a growing body of literature, our results suggest that the influence of timing of vegetation sampling depends on the study system. When designing future studies, investigators should carefully consider the type of structures that provide nest concealment and whether plant phenology is confounded with nest survival.

Habitat Characteristics and Selection by Ornate Box Turtles in the Sandhills of South Dakota

The ornate box turtle (Terrapene ornata Agassiz) is a species of greatest conservation need in South Dakota. Habitat loss through agricultural development and fragmentation is the main threat to the species throughout its range, which extends from Wisconsin and northern Indiana through the central Great Plains, and from southern South Dakota to Arizona, northern Mexico, and the Gulf Coast of Texas. The objectives of this study were to determine the ornate box turtle's preferred vegetation characteristics (microhabitat) compared to the available habitat (macrohabitat) on the Pine Ridge Reservation, South Dakota Sandhills region, during 2010–2011. In both years, using a modified Robel pole method, we determined that turtles selected microhabitat with greater visual obstruction readings (VORs) than those within the random available macrohabitat (P < 0.01), with means of 22 cm and 15 cm, respectively. Higher VOR values indicate greater vegetation height and/or density. Canopy cover results showed that ornate box turtles exhibited high selection (P < 0.01) for sand sagebrush (Artemisia filifolia Torr.) coverage (38%) but selected lower cover than available within the macrohabitat for total grasses (37%), total forbs (19%), and bare ground (14%). Shrubs, such as sand sagebrush, are an important component of box turtle microhabitat, as they facilitate thermoregulation by providing cool areas during the summer and favorable hibernation sites during the winter. Shrub coverage is highly reco mmended for consideration when developing habitat management plans that aim to increase or sustain ornate box turtle populations in the Sandhills ecological type.

Characteristics of Four Agricultural Crops Established as Northern Bobwhite Brood Habitat

Small plots of agricultural crops are often planted in the Southeast for northern bobwhite (Colinus virginianus) management. Often these are viewed as primarily winter habitat, and assumed to provide summer habitat. We evaluated the macroinvertebrate and vegetative structure of millet, sorghum, wheat, and soybean plots on a cotton farm to assess their value as bobwhite brood habitat. During June and July 1999 and June, July, and August 2000, we studied 5 blocks, each planted with all 4 agricultural crops. We measured invertebrate abundance along a 15-meter transect in each plot using vacuum sampling and height/density of vegetation. Visual obstruction readings (VOR) were highest in millet and sorghum, followed by wheat and then soybean (P < 0.001). Macroinvertebrate numbers differed among cover types (P < 0.001), but macroinvertebrate weights did not (P = 0.14). Among important Orders, Coleoptera, Hemiptera, Diptera, and Homoptera were found in greater numbers in millet. Numbers of Hymenoptera did not differ among crops. In most cases, millet yielded the highest biomass and numbers of macroinvertebrates, followed by sorghum. Soybeans and wheat had fewer macroinvertebrates among the crops studied. On our study area it appears that millet provides the best brood habitat, although sorghum appears to provide a second useful crop. Thus, among these crops we recommend use of millet plots as brood habitat for northern bobwhite chicks.

Habitat Use by Breeding Northern Bobwhites in Managed Old-Field Habitats in Mississippi

To better understand the proximate and ultimate cues associated with habitat selection in breeding northern bobwhites (Colinus virginianus), we compared habitat use vs. availability at 2 spatial scales equivalent to Johnson's (1980) 2nd and 3,d order selection. We conducted the study in managed old-field habitats in Mississippi, from 1994 to 1996. We also estimated habitat use by broods with respect to availability, and quantified micro-habitat characteristics (4th order selection) at brood-rearing sites and nesting sites. Breeding bobwhites did not establish home ranges at random or allocate resources among patches in proportion to their availability. Breeding bobwhites, given a mosaic of seasonally manipulated old-field habitats, consistently used burned fields, disked fields, and areas with advanced woody succession to define breeding season home ranges. Bobwhites allocated their time and resources more to woody areas and fields that had received a combination of burning and disking. Broods consistently used burned/disked fields in proportion to availability; consistently avoided row crops and pastures; and generally preferred woody corridors. Vegetation characteristics at nest sites did not differ from random sites located within the same patch of habitat. Characteristics among nest sites were similar, yet successful nests were located in the proximity of more bare ground and less litter cover than unsuccessful nest sites. Brood site habitat characteristics were similar to nest sites; however, woody canopy (44.3%) and visual obstruction readings (59.0cm) at brood sites were significantly greater than nest sites (26.6% and 32.5cm).

Effects of Sediment Removal on Prairie Pothole Wetland Plant Communities in North Dakota

ABSTRACT This study assessed the effects of sediment removal on Prairie Pothole Region wetland plant communities in North Dakota in order to determine if this management tool improves vegetation structure that benefits wildlife. Three categories of seasonal wetlands were evaluated: reference condition, converted cropland, and excavated. Vegetation surveys were conducted in the wet meadow and shallow marsh zones of 39 seasonal wetlands located in Benson, Eddy, Towner, and Wells Counties, North Dakota. Visual obstruction readings were taken at various heights in order to assess habitat quality. There were no geographic differences in the reference wetland plant communities. However, there were significant treatment differences between the plant communities of the wet meadow and shallow marsh zones. In general, excavated wetlands showed vegetation trends similar to reference wetlands, whereas the converted cropland wetlands tended to be more cattail-choked. Visual obstruction scores and mean percent cover of...

Visual obstruction as a method to quantify herbaceous biomass in southern African semi-arid savannas

Biomass of aboveground vegetation is a useful descriptor for studies of grazing, fire and wildlife habitat use in grassland systems. The traditional method to estimate biomass, hand-clipping, is time intensive and other indices of biomass have been used successfully. In southern Africa, the disc pasture meter has been the tool of choice and the use of visual obstruction has been much less prevalent than in North America. Our goal was to determine if visual obstruction could be used as a correlate to aboveground grass biomass in grassland systems in Namibia. We gathered clipping and visual obstruction samples at three study sites in Highland Savanna and Woodland Savanna in northern Namibia. Dry biomass of grass was correlated with visual obstruction readings when samples from all study sites were pooled (r 2 ≤0.64, P<0.0001), but the strength of the relationship varied among the three study sites. We also evaluated the number of samples needed to characterise biomass at a study site. Variation in the cumulative mean was very low at sample sizes of 15–20 samples. Visual obstruction can be a useful method to evaluate biomass of grassland systems in a quick manner.

Monitoring mid‐grass prairie in southwestern South Dakota and northwestern Nebraska, USA

AbstractIt is expensive and difficult to make precise measurements of standing herbage and to monitor changes in rangeland pastures. This study used a modified Robel pole with 1.27‐cm alternating white and gray bands to estimate standing herbage on adjacent Buffalo Gap and Oglala National Grasslands in western South Dakota and northwestern Nebraska. We determined the relationship between visual obstruction readings (Robel pole) with clipped standing herbage and developed resource guidelines for monitoring standing herbage. Clipped standing herbage ranged from 72 to 4953 kg dry matter (DM) ha−1 with a mean of 1784 kg DM ha−1. Visual obstruction bands (band = 1.27 cm) averaged 6.9 bands and ranged from 0.1 to 22.9. Visual obstruction reliably predicted standing herbage (R 2 = 0.80, P = 0.001, SE = 485 kg DM ha−1, n = 182 transects). Cluster analyses grouped visual obstruction readings (bands) and standing herbage into three resource categories; short, intermediate and tall. Band 6 corresponds to residual standing herbage of 1626 kg DM ha−1 and is recommended as a monitoring guideline for minimum residual herbage. Monitoring of residual standing herbage is accurate, precise and easy to apply. The monitoring protocol and guidelines developed based on bands and/or standing herbage provide information for managers to determine compliance with desired plans to maintain diversity for plants, wildlife and livestock use that sustain grassland communities.

Monitoring standing herbage on granitic soils in the Big Horn Mountains, Wyoming, USA

AbstractMonitoring annual herbage use in rangelands is desirable for scientific management but problematic due to the difficulty and expense of making direct measurements of sufficient precision. We used a Robel pole with 1.27 cm alternating white and gray bands to indirectly measure standing herbage. This technique enables a smooth transition in the basis of annual use monitoring from percent use to residual herbage and overcomes the difficulties alluded to above. Our study was conducted in the southern Bighorn National Forest, Wyoming on granitic soils and within the Idaho fescue (Festuca idahoensis Elmer) vegetation type. The objectives were to develop and validate the relationship between Robel pole visual obstruction readings and clipped standing herbage, and develop guidelines for monitoring and estimation of optimum sample size. Clipped standing herbage ranged from 134 to 3792 kg dry matter (DM) ha−1 with a mean of 1558 kg DM ha−1. Visually obstructed bands (band = 1.27 cm) averaged 6.1 and ranged from 0.2 to 19.5. Approximately 75% of the variation in standing herbage was explained by linear regression on visual obstruction measurements. Both intercept and slope of the regression were highly significant (P = 0.001). The standard error of the estimate for a single observation was 458 kg DM ha−1. Cluster analyses grouped the visual obstruction readings into four management categories. This monitoring tool provides pertinent information for managers to develop guidelines based on band and/or standing herbage for natural areas, wildlife and livestock use.

Monitoring Idaho Fescue Grasslands in the Big Horn Mountains, Wyoming, with a Modified Robel Pole

The reliability of monitoring visual obstruction and estimating standing herbage with a modified Robel pole was examined for high-elevation meadows in sedimentary soils on the Bighorn National Forest, Wyoming. Our objectives were to (1) test a modified pole graduated with 1.27-cm (0.5-inch) bands for estimating standing herbage based on linear regression of visual obstruction readings (VORs) on standing herbage, (2) validate the derived regression, (3) provide sample size estimates, and (4) develop guidelines for monitoring mountain grasslands. Each transect had 20 visual obstruction stations spaced 10 meters apart with 4 visual obstruction readings at each station. At 4 stations, vegetation within a 0.25-m2 area was clipped at ground level. VORs and clipped standing herbage were averaged by transect for analysis. Visual obstruction reliably predicted average standing herbage (dry weights) for mountain meadows (r2 = 0.81, s𝑥̄= 382 kg ·ha−1). Standing herbage ranged from 387 kg ·ha−1 to 3930 kg ·ha−1, with a mean of 1742 kg ·ha−1. A validation data set of 13 transects sampled across the range of variation in standing crop showed that 85% of transects fell within the 90% prediction limits. We recommend a minimum of 4 transects for monitoring key areas or small areas up to 259 ha (640 acres) when managers need to consider differences in VOR bands and address multiple objectives. Cluster analyses (ISODATA) applied to the pole readings resulted in 4 visual obstruction categories: short, short-intermediate, tall-intermediate, and tall. This tool provides a simple, reliable, and cost-effective (time-saving) alternative to clipping vegetation and obtaining dry weights for monitoring.

MONITORING WITH A MODIFIED ROBEL POLE ON MEADOWS IN THE CENTRAL BLACK HILLS OF SOUTH DAKOTA

Abstract This study using a modified Robel pole was conducted in the central Black Hills, South Dakota. The objectives were to test the relationship between visual obstruction readings and standing herbage, develop guidelines for monitoring, and estimate sample size. The relationship between visual obstruction and standing herbage was linear with 2 segments in a piecewise model. Regression coefficients were highly significant (P ≤ 0.001). Standard error of the estimate for a single mean was 373 kg · ha−1. Herbage ranged from 89 to 3821 kg · ha−1 with a mean of 1416 kg · ha−1. The average number of visually obstructed bands was 6.4 and ranged from 0.21 to 20.4. Cluster analyses grouped the visual obstruction readings into 3 management categories: short, medium, and tall. A minimum of 3 transects (20 stations per transect) is recommended for monitoring areas ≤259 ha (1 section) to be within 20% of the mean at 80% confidence. The protocol developed to monitor standing herbage is accurate, precise, and easy t...

Nest site characteristics of eastern meadowlarks and grasshopper sparrows in tallgrass prairie at the Fort Riley military installation, Kansas

Abstract Knowledge of nest site characteristics and nest success of grassland birds is important in the conservation and preservation of species such as the eastern meadowlark (Sturnella magna) and grasshopper sparrow (Ammodramus savannarum). We documented nest site characteristics of these two species and compared each nest site to a paired random site. We also compared the eastern meadowlark nest sites to the grasshopper sparrow nest sites. Eastern meadowlark nests were significantly further from an edge than random (P=0.049). Grasshopper sparrow nests were significantly further from a shrub/tree (P=0.042) and a perch (P=0.004) than random. Eastern meadowlarks selected sites further from a bare ground patch (≥1 m2; P=0.007), with more litter depth (P=<0.001), smaller bare ground percent (P=0.083) and higher visual obstruction readings (VOR) (P=0.037) than grasshopper sparrows. Military training, conducted before the current breeding season, did not appear to affect the location and success of EAME and G...

Effects of Livestock Grazing on Duck Nesting Habitat in Utah

Periodic vegetation disturbance is an important yet controversial tool for waterfowl managers. Some have reported livestock grazing removes residual vegetation and thus is detrimental to nesting ducks, and others argue that such disturbance is necessary to maintain grassland health. We evaluated the impact of winter livestock grazing on duck nesting at Bear River Migratory Bird Refuge, Utah. During winter 1999, 6 experimental plots were grazed by cattle and 6 were ungrazed; in winter 2000, 8 plots were grazed and 8 were ungrazed. All grazing treatments were conducted during 15 November–15 March and averaged a stocking rate of 9.5 animal unit months/ha. During the following spring nesting season, we measured visual obstruction readings (VOR) on grazed and ungrazed sites. Although VOR on ungrazed sites were greater than those on grazed sites, this difference became less important as the nesting season progressed. Winter grazing impacted the nesting habitat of early-nesting ducks such as mallards (Anas platyrhynchos), but not that of late-nesting species such as cinnamon teal (Anas cyanoptera) and gadwall (Anas strepera). When using livestock grazing to manage grasslands, waterfowl managers should consider their management goals, the species composition of breeding duck populations, and environmental conditions.

Effects of Livestock Grazing on Duck Nesting Habitat in Utah

Periodic vegetation disturbance is an important yet controversial tool for waterfowl managers. Some have reported livestock grazing removes residual vegetation and thus is detrimental to nesting ducks, and others argue that such disturbance is necessary to maintain grassland health. We evaluated the impact of winter livestock grazing on duck nesting at Bear River Migratory Bird Refuge, Utah. During winter 1999, 6 experimental plots were grazed by cattle and 6 were ungrazed; in winter 2000, 8 plots were grazed and 8 were ungrazed. All grazing treatments were conducted during 15 November-15 March and averaged a stocking rate of 9.5 animal unit months/ha. During the following spring nesting season, we measured visual obstruction readings (VOR) on grazed and ungrazed sites. Although VOR on ungrazed sites were greater than those on grazed sites, this difference became less important as the nesting season progressed. Winter grazing impacted the nesting habitat of early-nesting ducks such as mallards (Anas platyrhynchos), but not that of late-nesting species such as cinnamon teal (Anas cyanoptera) and gadwall (Anas strepera). When using livestock grazing to manage grasslands, waterfowl managers should consider their management goals, the species composition of breeding duck populations, and environmental conditions.

Lesser prairie-chicken brood habitat in sand sagebrush: invertebrate biomass and vegetation

Invertebrates are an important food source for grouse chicks, especially within the first 2 weeks of life. Invertebrate abundance is highly patchy and dependent upon herbaceous cover and vegetation structure. We examined the relationship between invertebrate biomass (from sweepnet samples) and habitat structure at lesser prairie-chicken (Tympanuchus pallidicinctus) brood-use and non-use areas during 2001 and 2002 in a sand sagebrush (Artemisia filifolia) prairie vegetation community of southwestern Kansas. We delineated use and non-use areas from paired sampling points within and outside 95% utilization distributions of radiomarked brood females, respectively, during the first 60 days post-hatch. We measured vegetation cover and invertebrate biomass (Acrididae and “other” invertebrates) at 71 paired points on 2 study sites (Site I=4 broods, Site II = 12 broods). Both Acrididae and other invertebrate biomasses were greater at brood areas than non-use areas on both study sites, suggesting this food source likely had a greater influence on brood habitat use than vegetation type. Vegetation structure described brood-use areas better than vegetation type because brood-use areas had greater visual obstruction readings (VORs) than non-use areas regardless of dominant cover type. We also examined the predictive relationship between vegetation type and invertebrate biomass. Sand sagebrush density was the best linear predictor of Acrididae biomass, with lower densities having the greatest Acrididae biomass. We propose experiments to determine best management practices that produce abundant invertebrate biomasses for lesser prairie-chicken brood habitat, using our study as a baseline.

Density and success of bird nests relative to grazing on western Montana grasslands

Grassland birds are declining at a faster rate than any other group of North American bird species. Livestock grazing is the primary economic use of grasslands in the western United States, but the effects of this use on distribution and productivity of grassland birds are unclear. We examined nest density and success of ground-nesting birds on grazed and ungrazed grasslands in western Montana. In comparison to grazed plots, ungrazed plots had reduced forb cover, increased litter cover, increased litter depth, and increased visual obstruction readings (VOR) of vegetation. Nest density among 10 of 11 common bird species was most strongly correlated with VOR of plots, and greatest nest density for each species occured where mean VOR of the plot was similar to mean VOR at nests. Additionally, all bird species were relatively consistent in their choice of VOR at nests despite substantial differences in VOR among plots. We suggest that birds selected plots based in part on availability of suitable nest sites and that variation in nest density relative to grazing reflected the effect of grazing on availability of nest sites. Nest success was similar between grazed plots and ungrazed plots for two species but was lower for nests on grazed plots than on ungrazed plots for two other species because of increased rates of predation, trampling, or parasitism by brown-headed cowbirds ( Molothrus ater). Other species nested almost exclusively on ungrazed plots (six species) or grazed plots (one species), precluding evaluation of the effects of grazing on nest success. We demonstrate that each species in a diverse suite of ground-nesting birds preferentially used certain habitats for nesting and that grazing altered availability of preferred nesting habitats through changes in vegetation structure and plant species composition. We also show that grazing directly or indirectly predisposed some bird species to increased nesting mortality. Management alternatives that avoid intensive grazing during the breeding season would be expected to benefit many grassland bird species.