Habitat Characteristics of Kirtland’s Warbler

In perhaps one of the best examples of a proactive and integrative program to monitor and maintain habitat, both private and public organizations have come together to manage for Kirtland’s warbler in Michigan’s northern Lower Peninsula. Research conducted in the area found significant correlation among the structural characteristics of jack pine forests and population density of Kirtland’s warbler, a federally endangered species (Zou et al. 1992). Specifically, Kirtland’s warbler prefers dense jack pines stands that range from 6 to 23 years old and are approximately 2 to 5 m tall for initial colonization of an area and for breeding (Probst 1987). Additionally, it was determined that warblers focus on jack pine stands with specific environmental conditions – those found on dry, xeric sites typical of glacial outwash plains (Zou et al. 1992). Once established, warblers tend to use contiguous patches of jack pine rather than a random or systematic pattern of tree distribution. Thus, size and age-class information of local jack pine stands, as well as the juxtaposition of jack pine stands across the landscape, is needed to successfully manage for populations of Kirtland’s warbler.

To demonstrate how an integrated and hierarchical classification system could be used to monitor and maintain viable populations of Kirtland’s warbler, we used existing compositional and structural data provided by the Forest Inventory and Assessment (FIA) database (Hansen et al. 1992).

1. We selected all the FIA inventory plots that were classified as jack pine stands (estimated as 435,700 acres of a total of 14,600,000 acres). This sample represented those stands that meet the compositional habitat requirements of Kirtland’s warbler.
2. We selected for specific structural characteristics, i.e., those jack pine stands between the ages of 6 and 23 years of age. Of the 435,700 acres of jack pine forest in the northern Lower Peninsula of Michigan, 95,500 acres had suitable structural components for Kirtland’s warbler.
3. To demonstrate the importance of an integrated scheme that incorporates environmental information, we constrained our query further by selecting only those stands found on xeric or mesoxeric sites as classified by the FIA team. Thus, out of the total 435,700 acres of jack pine stands in the northern Lower Peninsula, only 71,400 acres that were found on xeric or mesoxeric sites provided the necessary compositional and structural requirements for Kirtland’s warbler.

Additionally, since a viable Kirtland’s warbler population requires contiguous patches of jack pine, input by all parties is necessary for any successful monitoring or management plan. Thus, it is important that basic geographic data such as ownership boundaries and urban centers, be available in the GIS. In the northern Lower Peninsula, the majority of land is owned and managed by the state of Michigan. Additionally, it can clearly be seen that over 75% of the optimum habitat for Kirtland’s warbler is found on federal and state lands. Understanding the mixture of ownership, as well as where the majority of optimum habitat is located, would be critical information to any proactive monitoring and maintenance program. This example illustrates why it is important to have spatially explicit information concerning multiple factors, especially topography, soils, and current forest composition and structure. Such analyses could become more routine and sophisticated in the future if the proper attention is given to developing a coherent set of geographically referenced data.

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