We used point counts and multi-species nest monitoring to assess the effects of forest thinning (commercial and biomass combined) on breeding bird communities in stands of Sierran mixed conifer forest commercially managed by Sierra Pacific Industries.

During three successive seasons of study (1998-2000), we found and monitored 537 active nests of 37 species on ten 36-ha study plots in the northern Sierra Nevada. Five of the study plots had been thinned between 1990 and 1993; the other five plots served as controls.

Point count data indicated that birds were present in much greater densities (approximately 1.6 times as many individual birds counted) on the thinned plots than on the control plots. Shrub-nesting species, such as Dusky Flycatcher and Cassin's Vireo, were dramatically more abundant on the the thinned plots, while species that are generally considered interior forest specialists, such as Golden-crowned Kinglet and Brown Creeper, were detected in similar densities on both sets of plots.

The overwhelming majority of nests we found (74%) were located on the thinned plots. Nest survivorship rates for each of four nesting guilds (ground-nesters, shrub-nesters, canopy-nesters, and cavity-nesters), however, were statistically equivalent between thinned and control plots, though there was a slight, non-significant tendency for nests on the control plots to succeed in greater proportions than nests on thinned plots, particularly among ground- and cavity-nesting species.

A Cassin's Vireo at its nest on one of the study plots.

Given the dramatic preponderance of birds on the thinned plots, and the nearly equivalent nest success rates on the two sets of plots, the thinned plots clearly produced many more fledglings each year than the control plots.

Several ecologically inter-related forest attributes correlated with increased abundance of nesting birds, but the presence of a much more extensive shrub understory on the thinned plots appeared to be the primary factor driving differences in bird communities on the two sets of plots. We surmise that the thinning protocol successfully stimulated vigorous shrub growth, particularly of Deer Brush (Ceanothus integerrimus), and conclude that the presence of this well-developed shrub understory is highly beneficial to the majority of breeding birds in the Sierran mixed conifer community. This type of thinning thus appears to be a useful tool for enhancing habitat value for forest-nesting birds, at least within stands affected by historical fire suppression.

Thinned Sierran mixed conifer forest with a vigorous understory.

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Peer-reviewed publications that have resulted from this work to date:

Siegel, R. B. and D. F. DeSante. 2003. Bird communities in thinned versus unthinned stands of Sierran mixed conifer forest. Wilson Bulletin 115:155-165.

For more information, contact Rodney Siegel.

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