Avian Productivity and Forest Disturbance

Personnel: Ernest Buford, David Capen, B. K. Williams

Cooperators: Clay Grove, Green Mountain National Forest, Vermont Cooperative Fish and Wildlife Research Unit

Effective conservation of forest songbirds requires knowledge of bird response to landscape pattern, including information about reproductive success. Negative impacts of forest fragmentation on avian productivity have been documented, particularly in the highly fragmented agricultural landscapes of midwestern and mid-Atlantic states. Relatively few studies have been conducted in extensively forested landscapes. Rather than being fragmented, many forests in these landscapes are characterized by a forest matrix perforated by scattered openings.

I conducted a landscape-level study of songbird productivity on the Green Mountain National Forest (GMNF) in central Vermont. My study area is an example of a perforated, rather than fragmented, forest landscape. I compared songbird species composition, abundance, and productivity on disturbed-canopy and closed-canopy sites. The disturbed-canopy sites were characterized by canopy openings accounting for 10% of site area, and were selected from a focal analysis of the study area.

One paper from this study describes the use of line-transect surveys to estimate density of fledgling broods. A second paper reports on the investigation of altered landscape pattern as it relates to songbird productivity.

Distance Sampling to Estimate Fledgling Brood Density of Forest Birds

Ernest W. Buford, David E. Capen, and B. K. Williams. 1996. Canadian Field-Naturalist 110(4):642-648.
Abstract: Research on the status of avian communities often relies on estimates of abundance, but does not always consider demographic factors such as productivity. We introduce the application of a distance-sampling technique for estimating brood density of fledgling birds in forested habitats. During 1993 and 1994, we conducted 60 line-transect surveys on 10 sites in the Green Mountain National Forest in Vermont. Sites were divided into 2 groups, which allowed us to test for differences in fledgling density between groups. We detected 508 broods representing 38 species. Using standard distance-sampling procedures, we estimated densities of 0.576 (C.V. = 11.89) and 0.513 (C.V. = 12.54) broods per hectare. Density was not statistically different between groups. As with other survey methods, distance sampling favors easily detected species; however, line-transect density of fledglings is less obtrusive and less labor-intensive than mist-netting or nest searches, and can contribute important information to studies of avian communities.

Abundance and Productivity of Forest Songbirds in a Managed, Unfragmented Landscape in Vermont

Ernest W. Buford and David E. Capen. 1999. Journal of Wildlife Management 63(1):180-188.
Abstract: Effective conservation of forest songbirds requires knowledge of responses to landscape pattern. We studied a breeding songbird community in a landscape characterized by scattered openings in a forest matrix in Vermont. We measured species composition, abundance, and fledgling brood density in the forested areas of 10 500-ha sites; we compared a group of 5 disturbed-canopy sites (<10% canopy disturbance) with a control group of 5 undisturbed-canopy sites. We conducted 456 point counts to determine species composition and abundance, and we conducted 60 line transect surveys to estimate fledgling brood density as a measure of productivity. Adult densities were not different between groups (P > 0.05) for 18 species listed as priority for conservation, although power was low. Combined brood densities of all species, Neotropical migrants, and ground nesters were not different among groups (P > 0.05). Brood densities of 3 neotropical migrants also were not different (P > 0.05). However, combined brood density of area-sensitive, forest interior species was greater on control sites (P < 0.05). At the scale of this study, a minimal amount of canopy removal in the extensively forested landscape did not affect abundance and overall productivity of songbirds inhabiting the remaining forest, but had begun to suppress productivity of some forest interior species.

       Updated: 28 June 2000