Characteristics of Bird Assemblages along Linear Riparian Zones in Western Montana

Abstract

Bird populations occupying two narrow, linear riparian zones in Mon- tana were studied in 1968 and 1980. The two areas differed in several vegetative parameters such as vertical layering, riparian width, and canopy cover. Bird densities ranged from 355 to 475 pairs per 40 ha. Riparian width, height of vegetation layers 1 and 2, and cover in layer 2 were the most significant variables in male territory location for some bird species. Some species such as the Winter Wren and American Redstart were absent in 1980 from one study area, whereas the populations of most other species were similar between years. Although there have been many studies dealing with the relationships between bird communities and vegetation structure, few have been conducted in narrow, linear ri- parian zones in coniferous forests. Such riparian zones usually have a more diverse bird community, significantly higher total bird density, and more breeding bird species than are found in the surrounding forest (Stevens et al. 1977). During an earlier study (Manuwal 1968) I was impressed with the observation that although the two riparian zones reported in this study were very similar in the general types of vegetation and structural features, there were substantial differences in breed- ing bird species composition. This study was conducted in an attempt to determine why such differences in the avifauna existed. To accomplish this, I tested the hypothesis that bird species richness and density are proportional to the complexity and extent of riparian vegetation. STUDY AREA AND METHODS I conducted this study at Elk Creek and its tributary (the North Fork of Elk Creek) which are located in the Lubrecht Experimental Forest, approximately 64 km NE of Missoula, Montana. The surrounding coniferous forest was a typical mixture of ponderosa pine (Pinus ponderosa), lodgepole pine (P. contorta), Douglas-fir (Pseudotsuga menziesii), and western larch (Larix occidentalis) (Manuwal 1983). I sampled vegetative cover by placing 1-m2 quadrats made of plastic PVC tubing every 30 m along a transect paralleling each side of the creek. At each sample point I recorded percent cover and height of vegetation layers in a vertical column above the quadrat, the species of trees and shrubs (> 1 cm dbh) nearest the quadrat in each cardinal direction, the height of taller trees inter- cepting the sampling column, and width of the riparian strip. I measured the latter two variables with a range finder. I used the spot-map method (Williams 1936, Hall 1964) to determine territorial male density and delineate territory boundaries. I marked the riparian strips either with numbered lath stakes (1968) or numbered plastic flagging (1980) to facilitate the mapping of the exact location of each bird on a map of the study areas.