AERIAL COUNTING EXPERIMENTS TO ESTIMATE PRONGHORN DENSITY AND HERD STRUCTURE

Unbiased and precise estimates of pronghorn (Antilocapra americana) de nsity and herd structure are necessary for proper management. We appli ed sample-based survey methods in Colorado sagebrush (Artemisia spp.) steppe (SBS) and shortgrass prairie (SGP) habitats to estimate prongho rn density and herd structure concurrently during late summer surveys with random quadrat (2.59 km(2)) and random strip transect (1.6 km wid e) sample units. Density estimates were lower (P = 0.015) for strips t han for quadrats (2.24 pronghorn/km(2) +/- 1.05) in SBS, but not for S GP (P = 0.122, 0.25 pronghorn/km(2) +/- 0.27). Strip transect survey e stimates of buck:doe ratios were lower than quadrat survey estimates i n sagebrush (P = 0.014) and shortgrass (P = 0.003) habitats, which we attribute to observer bias in detecting bucks while searching strip tr ansects. The mean difference in buck:doe ratios between strips and qua drats 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 = 0.14 9, 5.2 fawns:100 does +/- 6.40) or shortgrass (P = 0.849, 1.3 fawns:10 0 does +/- 14.16). Double sample tests for bias failed to detect a bia s in quadrat-based estimates of population size. On average, the popul ation estimate on strips was about 50 and 70% of the estimate on SBS a nd SGP quadrats, respectively. We conclude there is undercounting bias in wide strip surveys. Population estimates from narrow strip transec ts (200 m wide) in both areas were consistent with line transect estim ates. Line and narrow strip transect estimates were numerically (10-27 %) but not statistically (P > 0.05) larger than quadrat estimates, lea ving open possibilities that quadrats produce underestimate bias, line s and narrow strips produce overestimate bias, or the numerical differ ences represent sampling error. Quadrats offer acceptable precision an d may be the least biased of any method tested for estimating populati on size and herd structure concurrently during the same aerial survey in late summer.