Management Practices: Forest Thinning

Avian Responses to Fuels-reduction in Ponderosa Pine and Oak habitat

A fuels-reduction program was initiated in Sugarite Canyon State Park primarily to prevent the likelihood of a catastrophic crown fire that could threaten the municipal water supply to the City of Raton. The fuels-reduction practices entail the removal of ladder, ground, and small diameter tree fuels (primarily oaks) within stands dominated by ponderosa pine. The fuels-reduction program was initiated in 2005, with additional stands treated in 2006 and 2007. As part of a multidisciplinary monitoring program initiated by New Mexico State Parks and the City of Raton, Rocky Mountain Bird Observatory (RMBO) collected bird-use data in 2006 and 2007 to assist in monitoring the effects of the fuels-reduction program on birds within SCSP. In each year, RMBO completed two point-count visits to each of 33 survey points located on a grid.

In 2006, the most common birds detected were American Robin, Western Wood-Pewee, Hermit Thrush, Dark-eyed Junco, and Spotted Towhee. RMBO also recorded data regarding other species of interest, including Northern Goshawk and five species of owls. No statistically significant differences in numbers of birds counted or species detected were found between the thinned and unthinned forest. Three species were found to be less abundant in thinned forest and one species more abundant in thinned forest. A group of species including woodpeckers, chickadees, and nuthatches showed a trend for reduced abundance in the thinned area. However, sample sizes were limited.

In 2007, the most common birds were also American Robin and Western Wood-Peewee. Again, no differences were found in the overall numbers of birds with regard to the forest treatments. Using simple summaries of birds per point-count survey, we found reduced species richness of birds within the forest thinned prior to the 2006 field season, as compared to forest thinned prior to the 2007 field season and intact forest. This suggests that thinned forest may be suitable for a smaller array of species than intact forest. The lack of this effect in recently thinned forest may indicate that birds had not yet adjusted to the new status of the habitat, inhabiting less than optimal habitat. However, sample sizes were quite low, with only four points fully within the pre-2006 thinned area.

Spotted Towhees were found to be less abundant in the thinned areas in both 2006 and 2007. This effect could be due to their reliance on dense shrubs for foraging and nesting. There was also a trend for Mourning Doves to be less abundant in thinned forest, although sample sizes were low generally (n = 7 detections, all within intact forest). Hermit Thrush was less abundant in the pre-2006 thinned area. All of these species are generally associated with thick understory habitat. In contrast to 2006, we did not find reduced abundances of Pygmy Nuthatch or Northern Flicker. Indeed, we did not find any differences in grouped woodpeckers, chickadees, and nuthatches amongst the forestry treatments in 2007. Thus, the pattern perceived in 2006 was likely related to other aspects of habitat suitability.

We found several species that appeared to be more abundant in thinned forest in 2007, including Yellow-rumped (Audubon’s) Warbler and Hairy Woodpecker. Wild Turkey was apparently more abundant in the recently thinned area, but not the older thinned forest; Plumbeous Vireo was more abundant in the older thinned area. It could be that these species prefer the thinned habitat, although we lack strong ecological predictions for these associations. Alternatively, these patterns could have been driven by an improved ability to detect these species in the cleared habitat. If additional funds become available, analyzing the data in a distance-sampling framework will enable us to determine if probability of detection varied with habitat treatment, and will give us a stronger ability to compare estimated densities of birds with respect to forestry practice.


Spotted Towhee. Photo courtesy of Tony Leukering