- May 13, 2011
Life at the Edge
Where two habitat types meet, there is often overlap of the bird communities from the two adjacent vegetation communities, plus a few species drawn specifically by the presence of the edge itself. For this reason, bird diversity can be higher at the boundary of two habitat types than in either type individually. Our modern landscapes offer many examples of human-induced edges (such as a forest with an adjacent agricultural field, a well-watered golf course in a desert ecosystem, or even a clearcut within a natural forest), while natural edges are more subtle and may escape our attention (as in the tree line on the side of a mountain, or the intergrade between a meadow and shrubland). These special “ecotone” habitat types can frequently have just the right mix of structural layers, food diversity, and foraging places to attract a wide variety of birds. As more people are drawn to the rural-suburban interface, many of whom are former urbanites, there is rising interest in managing these areas for biodiversity, an attractive feature which people value in their neighborhoods.
Life for birds in the transition zone is not all bugs and berries; edges also have their pitfalls. The same attractants that lure birds to edges also work against them. Small omnivores, such as raccoons, skunks, or chipmunks, eat the eggs and nestlings of ground nesting birds and are common in these areas. Browsing mammals, like deer and rabbits, are also attracted by the juxtaposition of food and cover, and can over-browse the vegetation and negatively impact forest regeneration. The Brown-headed Cowbird, a brood parasite that lays its eggs in the nests of other birds, may disproportionately parasitize edge-nesting birds due to its preference for woodland edge habitat. Some bird species have evolved with cowbirds and will reject their young; however, the expansion of “edge” habitats has brought the cowbird into new areas, where naive birds have evolved no defenses against the cowbird’s strategy.
Managing Your Margins
As the proprietor of a woodland edge community, there are some things you can do to encourage biodiversity in these areas while keeping potential nuisances at moderate numbers. First, maximize the vegetation levels available to birds. For instance, if a forest abuts your agricultural field, don’t mow right up to the forest edge. Allow a gradient zone of increasingly taller vegetation to naturally regenerate from the edge of the field; in one narrow strip, habitat then exists for ground, shrub, and canopy-favoring birds! Prickly brambles (i.e., blackberry or raspberry) should survive browsing pressure, and would provide safe hiding places for birds and other creatures, so avoid mowing these species if you can. You can obtain a list of deer-resistant plants suitable for your area from a local native plant nursery or local cooperative extension). And while there isn’t much you can do to prevent Brown-headed Cowbirds from parasitizing nests, simple changes in the types of bird food in your feeders can discourage them from lingering in your area. Furthermore, maximizing the amount of cover through effective use of conifers, vines, and foliage-dense shrubs could prevent cowbirds from finding some nests. Remember, the greater the amount of leaf area in your woodland edge, the greater the amount of insect food available to birds! Don’t subsidize potential nest predators: keep pet food and garbage secure, and install predator guards on any nest boxes you may have.