Riparian: Bird Habitats in Riparian Corridors
As important as a riparian area is to the quality of water, riparian widths designed only for nutrient filtration and water quality do not satisfy the needs of birds and other wildlife (Gregory and Ashkenas 1990). There is a strong need for wider forested corridors to preserve species' habitats, and most researchers advocate as wide a corridor width as possible for the protection of that wildlife. A table in Wenger (1999) summarizes a number of studies done on the habitat needs of certain types of birds (table 1).

Most of these studies concentrate on differentiating between clear-cut, upland forest, and riparian forest habitats. In developed lands there is less available habitat for birds, while in clear-cut areas, early successional habitats are important for some species.

Riparian corridors contain certain structural differences, which provide the right scale for animal and bird habitats. For example, riparian forests have greater canopy cover, lower live tree density, and a greater amount of tall shrub cover. Spackman and Hughes (1995) concluded that 90% of bird species could be found within a corridor 150 meters (450 feet) wide, and 95% of bird species could be found if the riparian corridor was at least 175 meters (525 feet) in width.

Several other components of bird habitats have also been studied. These components include species density and diversity, divergences in habitat requirements for edge-dwelling and forest-dwelling species and the likelihood of nest depredation and brood parasitism.

Although bird density does increase in riparian areas as more lands are cut, Kinley and Newhouse (1997) concluded that in narrower riparian corridors, the bird population did not fully make up for the lost bird habitat by "packing" more birds into a smaller riparian zone. Kilgo (1993) also noted that as riparian corridor width increased up to 500 meters, bird density declined.

The value of an uncut riparian corridor may still be affected when the adjacent forested area has been timbered. Narrower riparian reserves are of less value than wider ones, even when there is forest on the opposite side of the stream (Darveau 1995, Stauffer and Best 1980, Croonquist and Brooks 1993).

Kilgo (1993) goes even further in his recommendation for riparian forest width. In his study, twelve out of thirteen species were more likely to occur in a forest as its width increased. From his studies in South Carolina, Kilgo believes that only a corridor 500 meters or wider would maintain a complete avian (bird) community.

Keller (1993) found in his studies in Maryland and Delaware that narrow corridors might only provide for forest edge-dwelling species. He looked primarily for certain neo-tropical migrant species, which occur in large areas of forest; Keller found that the number of these neotropical species increased with the forest width. He also found that ten species out of the twenty-two sampled were statistically more likely to occur as the width of the forest increased. Examples of these "area-sensitive" species include the Acadian flycatcher, the Wood thrush and the Kentucky warbler. Keller believes that as the riparian forest width decreased, the twelve species statistically more likely being seen were probably edge-dwelling species. He concludes by saying that in order to maintain a suitable habitat for these types of "area-sensitive" interior dwellers, it is imperative that we conserve riparian forest widths of at least 100 meters.

If there is no suitable riparian corridor in place, Keller recommends creating an area that will provide some habitat for the more sensitive species. Even a narrow riparian forest increases the chances for habitat. For example, a corridor less than 50 meters wide still provides cover for migrating songbirds and edge-dwelling species, even though there is insufficient room for the "area sensitive" interior dwellers.

Nest depredation is another concern of researchers with the narrowing of riparian corridors. As these forests become smaller, the proportion of protected interior habitat decreases with respect to the edge. Edge-nesting species look for the more complex shrub nesting areas to provide both cover and food. Nest depredation is the principle cause of nest failure for most passerine species, often reaching 50 to 80 % and sometimes as high as 90% (Sargent 1997).

According to some studies, edge habitats may exceed 300 – 600 meters into a forest (Wilcove 1985 in Sargent 1997). Since small predators are especially abundant in forest fragments, the highest nest depredation rates occur in fragments adjoined by the highest percentages of clear-cut land. Yet if a forest borders that land, depredation rates decrease considerably (Yahner and Scott 1988 in Sargent 1997).
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