Assessing Allowable Take of Migratory Birds

Legal removal of migratory birds from the wild occurs for several reasons, including subsistence, sport harvest, damage control, and the pet trade. We argue that harvest theory provides the basis for assessing the impact of authorized take, advance a simplified rendering of harvest theory known as potential biological removal as a useful starting point for assessing take, and demonstrate this approach with a case study of depredation control of black vultures (Coragyps atratus) in Virginia, USA. Based on data from the North American Breeding Bird Survey and other sources, we estimated that the black vulture population in Virginia was 91,190 (95% credible interval = 44,520–212,100) in 2006. Using a simple population model and available estimates of life-history parameters, we estimated the intrinsic rate of growth (rmax) to be in the range 7–14%, with 10.6% a plausible point estimate. For a take program to seek an equilibrium population size on the conservative side of the yield curve, the rate of take needs to be less than that which achieves a maximum sustained yield (0.5 x rmax). Based on the point estimate for rmax and using the lower 60% credible interval for population size to account for uncertainty, these conditions would be met if the take of black vultures in Virginia in 2006 was rmax. Further, the ratio of desired harvest rate to 0.5 x rmax may be a useful metric for ascertaining the applicability of specific requirements of the National Environmental Protection Act

Enhanced Migratory Waterfowl Distribution Modeling by Inclusion of Depth to Water Table Data

In addition to being used as a tool for ecological understanding, management and conservation of migratory waterfowl rely heavily on distribution models; yet these models have poor accuracy when compared to models of other bird groups. The goal of this study is to offer methods to enhance our ability to accurately model the spatial distributions of six migratory waterfowl species. This goal is accomplished by creating models based on species-specific annual cycles and introducing a depth to water table (DWT) data set. The DWT data set, a wetland proxy, is a simulated long-term measure of the point either at or below the surface where climate and geological/topographic water fluxes balance. For species occurrences, the USGS' banding bird data for six relatively common species was used. Distribution models are constructed using Random Forest and MaxEnt. Random Forest classification of habitat and non-habitat provided a measure of DWT variable importance, which indicated that DWT is as important, and often more important, to model accuracy as temperature, precipitation, elevation, and an alternative wetland measure. MaxEnt models that included DWT in addition to traditional predictor variables had a considerable increase in classification accuracy. Also, MaxEnt models created with DWT often had higher accuracy when compared with models created with an alternative measure of wetland habitat. By comparing maps of predicted probability of occurrence and response curves, it is possible to explore how different species respond to water table depth and how a species responds in different seasons. The results of this analysis also illustrate that, as expected, all waterfowl species are tightly affiliated with shallow water table habitat. However, this study illustrates that the intensity of affiliation is not constant between seasons for a species, nor is it consistent between species

Testing the Accuracy of Aerial Surveys for Large Mammals: An Experiment with African Savanna Elephants (Loxodonta africana)

Accurate counts of animals are critical for prioritizing conservation efforts. Past research, however, suggests that observers on aerial surveys may fail to detect all individuals of the target species present in the survey area. Such errors could bias population estimates low and confound trend estimation. We used two approaches to assess the accuracy of aerial surveys for African savanna elephants (Loxodonta africana) in northern Botswana. First, we used double-observer sampling, in which two observers make observations on the same herds, to estimate detectability of elephants and determine what variables affect it. Second, we compared total counts, a complete survey of the entire study area, against sample counts, in which only a portion of the study area is sampled. Total counts are often considered a complete census, so comparing total counts against sample counts can help to determine if sample counts are underestimating elephant numbers. We estimated that observers detected only 76% ± SE of 2% of elephant herds and 87 ± 1% of individual elephants present in survey strips. Detectability increased strongly with elephant herd size. Out of the four observers used in total, one observer had a lower detection probability than the other three, and detectability was higher in the rear row of seats than the front. The habitat immediately adjacent to animals also affected detectability, with detection more likely in more open habitats. Total counts were not statistically distinguishable from sample counts. Because, however, the double-observer samples revealed that observers missed 13% of elephants, we conclude that total counts may be undercounting elephants as well. These results suggest that elephant population estimates from both sample and total counts are biased low. Because factors such as observer and habitat affected detectability of elephants, comparisons of elephant populations across time or space may be confounded. We encourage survey teams to incorporate detectability analysis in all aerial surveys for mammals

Trends in Duck Breeding Populations, 1955-2005

This report summarizes information about the status of duck populations and wetland habitats during spring 2005, focusing on areas encompassed by the U.S. Fish and Wildlife (USFWS) and Canadian Wildlife Services’ (CWS) Waterfowl Breeding Population and Habitat Survey. The estimates do not include information from surveys conducted by State or Provincial agencies. In the traditional survey area, which includes strata 1-18, 20-50, and 75-77 (Fig. 1), the total duck population estimate (excluding scoters [Melanitta spp.], eiders [Somateria and Polysticta spp.], long-tailed ducks [Clangula hyemalis], mergansers [Mergus and Lophodytes spp.], and wood ducks [Aix sponsa]) was 31.7 ± 0.6 [SE] million birds, similar to last year’s estimate of 32.2 ± 0.6 million birds but 5% below the 1955-2004 long-term averagea. Mallard (Anas platyrhynchos) abundance was 6.8 ± 0.3 million birds, which was 9% below last year’s estimate of 7.4 ± 0.3 million birds and 10% below the long-term average. Blue-winged teal (A. discors) abundance was 4.6 ± 0.2 million birds. This value was similar to last year’s estimate of 4.1 ± 0.2 million birds and the long-term average. Of the other duck species, gadwall (A. strepera; 2.2 ± 0.1 million) was 16% below that of 2004, while estimates of northern pintails (A. acuta; 2.6 ± 0.1 million; +17%) and northern shovelers (A. clypeata; 3.6 ± 0.2 million; +28%) were significantly above 2004 estimates. The estimate for northern shovelers was 67% above the long-term average for this species, as were estimates of gadwall (+30%) and green-winged teal (A. crecca; 2.2 ± 0.1 million; +16%). Northern pintails remained 38% below their long-term average despite this year’s increase in abundance. Estimates of American wigeon (A. americana; 2.2 ± 0.1 million; -15%) and scaup (Aythya affinis and A. marila combined; 3.4 ± 0.2; -35%) also were below their respective long-term averages; the estimate for scaup was a record low

Trends in Duck Breeding Populations, 1955-2008

This report summarizes information about the status of duck populations and wetland habitats during spring 2008, focusing on areas encompassed by the U.S. Fish and Wildlife (USFWS) and Canadian Wildlife Services\u27 (CWS) Waterfowl Breeding Population and Habitat Survey. This report does not include information from surveys conducted by state or provincial agencies. In the traditional survey area, which includes strata 1-18, 20-50, and 75-77 (Figure 1), the total duck population estimate (excluding scoters [Melanitta spp.], eiders [Somateria and Polysticta spp.], long-tailed ducks [Clangula hyemalis], mergansers [Mergus and Lophodytes spp.], and wood ducks [Aix sponsa]) was 37.3 ± 0.6 [SE] million birds. This estimate represents a 9% decline over last year\u27s estimate of 41.2 ± 0.7 million birds, but remains 11% above the 1955-2007 long-term averagea (Table 1). Estimated mallard (Anas platyrhynchos) abundance was 7.7 ± 0.3 million birds, which was similar to last year\u27s estimate of 8.3 ± 0.3 million birds and the long-term average (Table 2). Blue-winged teal (A. discors) abundance was 6.6 ± 0.3 million birds. This value is similar to last year\u27s estimate of 6.7 ± 0.4 million birds and 45% above the long-term average. Estimated abundances of gadwall (A. strepera; 2.7 ± 0.2 million) and Northern shovelers (A. clypeata; 3.5 ± 0.2 million) were below 2007 estimates (-19% and -23%, respectively) but remain well above their long-term averages (+56% and +56%, respectively). Estimated abundances of green-winged teal (A. crecca; 3.0 ± 0.2 million) and redheads (Aythya americana; 1.1 ± 0.1 million) were similar to last year\u27s and were \u3e50% above their long-term averages. Estimates of canvasbacks (A. valisineria; 0.5 ± 0.05 million) were 44% below the 2007 estimate (0.9 ± 0.09 million) and 14% below the long-term average. The estimate for Northern pintails (Anas acuta) was 2.6 ± 0.1 million, which was 22% below the 2007 estimate of 3.3 ± 0.2 million, and 36% below the long-term average. The scaup estimate (Aythya affinis and A. marila combined; 3.7 ± 0.2 million) was similar to 2007, and remained 27% below the long-term average of 5.1 ± 0.2 million

Trends in Duck Breeding Populations, 1955-2010

This report summarizes information about the status of duck populations and wetland habitats during spring 2010, focusing on areas encompassed by the U.S. Fish and Wildlife (USFWS) and Canadian Wildlife Services\u27 (CWS) Waterfowl Breeding Population and Habitat Survey. We do not include information from surveys conducted by state or provincial agencies. In the traditional survey area, which includes strata 1-18, 20-50, and 75-77 (Figure 1), the total duck population estimate (excluding scoters [Melanitta spp.], eiders [Somateria spp. and Polysticta stelleri], long-tailed ducks [Clangula hyemalis], mergansers [Mergus spp. and Lophodytes cucullatus], and wood ducks [Aix sponsa]) was 40.9 ± 0.7 [SE] million birds. This estimate was similar to last year\u27s estimate of 42.0 ± 0.7 million birds and was 21% above the long-term averagea (1955{2009; Table 1). Estimated mallard (Anas platyrhynchos) abundance was 8.4 ± 0.3 million birds, which was similar to the 2009 estimate of 8.5 ± 0.2 million birds and 12% above the long-term average (Table 2). Estimated abundance of gadwall (A. strepera; 3.0 ± 0.2 million) was similar to the 2009 estimate and 67% above the long-term average (Table 3). Estimated abundance of American wigeon (A. americana; 2.4 ± 0.1 million) was similar to 2009 and the long-term average (Table 4). The estimated abundance of green-winged teal (A. crecca) was 3.5 ± 0.2 million, which was similar to the 2009 estimate and 78% above their long-term average of 1.9 ± 0.2 million (Table 5). The estimate of blue-winged teal abundance (A. discors) was 6.3 ± 0.4 million, which was 14% below the 2009 estimate and 36% above their long-term average of 4.7 ± 0.4 million (Table 6). The estimate for northern pintails (A. acuta; 3.5 ± 0.2 million) was similar to the 2009 estimate, and 13% below the long-term average of 4.0 ± 0.4 million (Table 7). Estimates of northern shovelers (A. clypeata; 4.1 ± 0.2 million) and redheads (Aythya americana; 1.1 ± 0.1 million) were similar to their 2009 estimates and were 76% and 63% above their long-term averages of 2.3 ± 0.02 million and 0.7 ± 0.01 million, respectively (Tables 8 and 9). The canvasback estimate (A. valisineria; 0.6 ± 0.05 million) was similar the 2009 estimate and to the long-term average (Table 10). The scaup estimate (A. affinis and A. marila combined; 4.2 ± 0.2 million) was similar to that of 2009 and 16% below the long-term average of 5.1 ± 0.05 million (Table 11)

Assessing Allowable Take of Migratory Birds

Legal removal of migratory birds from the wild occurs for several reasons, including subsistence, sport harvest, damage control, and the pet trade. We argue that harvest theory provides the basis for assessing the impact of authorized take, advance a simplified rendering of harvest theory known as potential biological removal as a useful starting point for assessing take, and demonstrate this approach with a case study of depredation control of black vultures (Coragyps atratus) in Virginia, USA. Based on data from the North American Breeding Bird Survey and other sources, we estimated that the black vulture population in Virginia was 91,190 (95% credible interval = 44,520–212,100) in 2006. Using a simple population model and available estimates of life-history parameters, we estimated the intrinsic rate of growth (rmax) to be in the range 7–14%, with 10.6% a plausible point estimate. For a take program to seek an equilibrium population size on the conservative side of the yield curve, the rate of take needs to be less than that which achieves a maximum sustained yield (0.5 x rmax). Based on the point estimate for rmax and using the lower 60% credible interval for population size to account for uncertainty, these conditions would be met if the take of black vultures in Virginia in 2006 was rmax. Further, the ratio of desired harvest rate to 0.5 x rmax may be a useful metric for ascertaining the applicability of specific requirements of the National Environmental Protection Act