Hooded Warbler
Ecoregional Scale Conservation Planning

Made possible through a partnership with the National Wetlands Research Center

Orchard Oriole (Icterus spurius )
Orchard orioles are Neotropical migrants that occur throughout most of the United States east of the Rocky Mountains with the exception of New England and the northern Great Lakes. Although the species has experienced increases along the edges of its distribution, in the core of its range, where densities are highest, numbers have declined. In the WGCP, orchard oriole populations have declined 3.0 percent per year since 1967. Populations in the adjacent Mississippi Alluvial Valley have declined 4.1 percent Table 005 (Table 005) . The species is a Bird of Conservation Concern in the WGCP and is identified as a species needing management attention in both the CH and WGCP (regional combined score = 17 and 18, respectively; Table 001 Table 001) .
Relative abundance of Orchard Oriole, derived from Breeding Bird Survey data, 1994 - 2003.
image courtesy of www.whatbird.com

Natural History:
The orchard oriole breeds in wooded riparian zones, floodplains, marshes, and shorelines (Scharf and Kren 1996) but also occur in open shrublands and low-density human-dominated areas (e.g., farms and parklands). Orchard orioles are semi-colonial in optimal habitat but are relatively solitary in marginal areas. This species is a common brown-headed cowbird host.

Model Description:

The orchard oriole habitat suitability model contains five parameters:

  • landform
  • landcover
  • successional age class
  • forest within a 1-km radius
  • basal area

The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) that defines unique combinations of these classes Table 096 (Table 096) . We directly assigned habitat suitability scores to these combinations based on orchard oriole vegetation and successional age class associations in Hamel (1992). However, we adjusted Hamel’s values to account for the preference of orchard orioles for mesic habitats (e.g., riparian zones, floodplains, and marshes; Scharf and Kren 1996).

Orchard orioles are not area-sensitive but are generally restricted to forested landscapes. Therefore, we included only local forest composition (SI2) in our model to discount forest patches that were isolated within a matrix of non-forest landcover. Conversely, the species is an edge species whose abundance declines in heavily forested regions (Scharf and Kren 1996). Therefore, we assumed landscapes with 70–80 percent forest provided optimal habitat suitability (1.000) and reduced suitability symmetrically as landscape composition shifted from these optima Table 097 (Table 097 , Figure 056 Figure 056) .

Orchard orioles are most abundant in areas with scattered trees. Therefore, we modeled the basal area (SI3)–habitat suitability relationship as a quadratic function Figure 057 (Figure 057) that maximized suitability index scores at intermediate basal area values Table 098 (Table 098) .

To calculate the overall suitability index score, we determined the geometric mean of SI scores for forest structure indices (SI1 and SI3) and then determined the geometric mean of this value and landscape composition (SI2).

Overall SI = ((SI1 * SI3)0.500 * SI2)0.500