by David Jaffe
Even if you haven’t spent much time in the kitchen, it is likely you have come across the term "a baker’s dozen" and would know roughly where to look for one of the "dozen". However, would you know where to look for one of the "birder’s dozen" or even what this term refers to? Recently, Audubon Vermont published a list called The Birder’s Dozen, twelve common breeding birds in Vermont that Audubon is working hard to protect.
These are birds, sometimes referred to as “responsibility birds”, which have “…a high proportion of its global population breeding in the region”, implying that conservation efforts should focus on preserving habitat where these birds have been observed. The twelve responsibility birds that make up the "Birder's Dozen" include American Woodcock, Yellow-bellied Sapsucker, Eastern Wood-Pewee, Blue-headed Vireo, Veery, Wood Thrush, Chestnut-sided Warbler, Black-throated Blue Warbler, Black-throated Green Warbler, Canada Warbler, White-throated Sparrow, and Scarlet Tanager.
In June, 2008, I worked with the The Nature Conservancy to study which bird species occurred in forests bordering the Little Chazy River in northeastern New York. Such forests are known as riparian zones or buffer strips and they tend to host a greater diversity of plants and animals than the surrounding landscape. The landscape within the Little Chazy watershed is a mixed-use habitat, typical of the Lake Champlain valley. As one drives through this basin, agricultural fields, small villages, and rivers descending from the nearby Adirondack Mountains are a few of the common sights.
The riparian condition of this habitat such as many trees or no trees, the proximity of agriculture to the river, and whether the forest strip along the river is continuous or highly fragmented, is significant for many reasons, including water quality and bird activity. Traveling downstream from the headwaters of the Little Chazy River, the width of the riparian zone varies from 100+feet to just several feet. Overall, the buffer zone is considerably less in the lower reaches of the river. Thus, I speculated that the species richness, that is the number of different kinds of forest dwelling birds, would decrease significantly as one moved downstream.
Interestingly enough, my data did not support this hypothesis. Each site had a species richness ranging from 6 to 22 per site and there was no evidence for a linear, decreasing trend from upstream to downstream. What was significant was that the number of species observed at sites that were moderately disturbed was greater than the numbers of species observed at either minimally or heavily disturbed sites. At these sites along the river I observed examples of all of the "Birder's Dozen" except American Woodcock and Canada Warbler.
My results agree with the findings of a classic 1978 study by Joseph Connell that described the “intermediate disturbance hypothesis”. Essentially, his idea was that moderately disturbed landscapes have greater species diversity than minimally or heavily disturbed landscapes. Why should this be true? Think about an intact forest with no gaps, clearings, standing dead trees or coarse woody debris on the ground and no difference in the height of the canopy. This is an example of minimally disturbed habitat and species diversity typically is low. Compare this to a forest that has just gone through a large windstorm. Some trees have been knocked down, providing gaps and space for sun-dependent herbs and saplings to sprout. There may be standing dead trees or dead trees on the ground (i.e. coarse woody debris). This moderately disturbed forest provides greater diversity of habitat than is true in minimally disturbed forests and therefore provides habitat for a greater variety of species.
The management implications of my observations are that perhaps in some areas of urban development, there may be benefit to maintaining moderate levels of disturbance. Maintaining open spaces around houses, selective harvesting of trees to diversify the canopy structure and provide gaps, allowing standing dead trees and coarse woody debris to stay on the ground for brush nesting birds and other animals are examples of management practices that might promote species diversity. Even creating a small buffer strip along a river can improve water quality tremendously whereas leaving no buffer strip at all provides habitat for few, if any, birds.
One of the first steps in any land management plan is to recognize what is present compared to what is desired. Becoming familiar with the Birder’s Dozen then finding these birds in the public land in your community is one step toward becoming a proactive land manager supporting community-based efforts that maintain water quality and increase local biodiversity.
David Jaffe is a graduate student in the Field Naturalist program at the University of Vermont. He will receive his M.S. degree in May, 2009. Thanks to Jim Shallow from Audubon Vermont for providing the photograph of the Black-throated Blue Warbler family.