Status and Population Size of Breeding Grassland Birds on Rancho Los Fresnos, Northern Sonora, Mexico

Despite persistent declines and widespread threats to grassland-dependant wildlife throughout much of North America, Rancho Los Fresnos offers good prospects for conservation and restoration of grasslands in northern Sonora, Mexico. In August 2007, I estimated abundance of breeding grassland birds throughout Rancho Los Fresnos to establish a baseline for long-term monitoring. I used distance-sampling methods during point counts at 66 stations and detected 1,066 individuals of 60 species and an additional 23 species incidentally. Grasshopper Sparrow (Ammodramus savannarum) and Botteri’s Sparrow (Aimophila botterrii) were the most abundant breeding species on the ranch. I estimate there are 0.84 ± 0.09 (± SE) Grasshopper Sparrow territories per hectare or 3,256 territories overall on Rancho Los Fresnos. Similarly, I estimate there are 0.47 ± 0.07 Botteri’s Sparrow territories per hectare or 1,806 territories overall. Mourning Dove (Zenaida macroura), Rufous-crowned Sparrow (Aimophila ruficeps), Blue Grosbeak (Passerina caerulea), and Eastern Meadowlark (Sturnella magna) were also common with estimates of population size ranging from 257 to 790 territories. These results indicate that Rancho Los Fresnos supports relatively large populations of breeding grassland birds including several species that seem to occur at much lower densities in the surrounding landscape. Although surveys during a single occasion each summer may be adequate to monitor changes in population size of abundant species, sampling stations two times per summer will greatly improve precision for monitoring grassland bird communities.