Aerial Counting Experiments to Estimate Pronghorn Density and Herd Structure

Unbiased and precise estimates of pronghorn (Antilocapra americana) density and herd structure are necessary for proper management. We applied sample-based survey methods in Colorado sagebrush (Artemisia spp.) steppe (SBS) and shortgrass prairie (SGP) habitats to estimate pronghorn density and herd structure concurrently during late summer surveys with random quadrat (2.59 km2) and random strip transect (1.6 km wide) sample units. Density estimates were lower (P = 0.015) for strips than for quadrats (2.24 pronghorn/km2± 1.05) in SBS, but not for SGP (P = 0.122, 0.25 pronghorn/km2± 0.27). Strip transect survey estimates of buck:doe ratios were lower than quadrat survey estimates in sagebrush (P = 0.014) and shortgrass (P = 0.003) habitats, which we attribute to observer bias in detecting bucks while searching strip transects. The mean difference in buck:doe ratios between strips and quadrats was 12.8 ± 5.87 in sagebrush and 20.0 ± 5.35 in shortgrass. Fawn:doe ratios did not differ by method in either sagebrush (P = 0.149, 5.2 fawns:100 does ± 6.40) or shortgrass (P = 0.849, 1.3 fawns:100 does ± 14.16). Double sample tests for bias failed to detect a bias in quadrat-based estimates of population size. On average, the population estimate on strips was about 50 and 70% of the estimate on SBS and SGP quadrats, respectively. We conclude there is undercounting bias in wide strip surveys. Population estimates from narrow strip transects (200 m wide) in both areas were consistent with line transect estimates. Line and narrow strip transect estimates were numerically (10-27%) but not statistically (P > 0.05) larger than quadrat estimates, leaving open possibilities that quadrats produce underestimate bias, lines and narrow strips produce overestimate bias, or the numerical differences represent sampling error. Quadrats offer acceptable precision and may be the least biased of any method tested for estimating population size and herd structure concurrently during the same aerial survey in late summer.