Droege, S. 1985. The response of an Adirondack
Mountain bird community to understory defoliation. M.S. Thesis,
SUNY- ESF, Syracuse, 48 pp.
Abstract: Avian community structure and dynamics are directly and indirectly influenced by the structure and dynamics of the surrounding plant community. In North America, increased demand for forest products has resulted in more forest acreage cut each year and an increased intensity of timber management. Recent avian habitat and forest fragmentation studies suggest that such disturbances would strongly affect local bird populations.
I studied an avian community's response to defoliation of the understory of an Adirondack Mountain deciduous forest. I censussed birds using the spot-map technique on 2 20 ha and 1 24.5 ha sites. Vegetation was measured annually using a minimum of 50 0.04 ha circular plots per site. After 2 summer of pre-treatment censussing one site was treated with herbicides over the following fall and winter months. Fosamine was applied aerially with a backpack mistblower and monosodium methane was injected into all trees between 3 and 23 cm dbh. Following treatment there were significant changes in forest structure (e.g. 75% of the trees in the 3-23 cm diameter size classes were killed and understory foliage density decreased by 67%). There were, however, no concurrent changes in avian community structure, diversity, richness, or composition that yearly regional variation could not account for. Similarly, there were no detectable changes in foraging and nesting guilds or in individual species populations.
Several community processes could cause the present bird community to not exhibit any outward signs of response. Simple behavioral plasticity may compensate for lost foraging and nesting site, especially because the treatment did not affect tree canopy cover. Philopatry, population saturation, or aging preferences, and the breeding site selection process could also be influencing mechanisms in this system.
The results of this study suggest that even major changes in habitat structure may not always produce detectable changes in censuses of resident avian populations. Without knowledge of a community's underlying population dynamics census data may inaccurately reflect the impacts of habitat change on avian communities.