Hooded Warbler
Ecoregional Scale Conservation Planning
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Made possible through a partnership with the National Wetlands Research Center


Cerulean Warbler (Dendroica cerulea)
Status:

The cerulean warbler is a long-distance migrant to the eastern United States that reaches its highest densities in the Ohio River Valley and along the Cumberland Plateau. The species has declined across most of its range, including the CH and WGCP (6.3 and 9.5 percent per year from 1966-2004, respectively; Table 005 Table 005 ). The species is classified as a Bird of Conservation Concern requiring critical recovery and immediate management activities in the WGCP (regional combined score = 19) and CH (regional combined score = 19), respectively Table 001 (Table 001) . Concern for the species culminated in a petition to the USFWS to list the cerulean warbler as threatened, but the action was deemed unwarranted based on current scientific information (Federal Register 71:234 [6 December 2006] p. 70717).

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Relative abundance of Cerulean Warbler, derived from Breeding Bird Survey data, 1994 - 2003.
 
image courtesy of www.whatbird.com
 

Natural History:

The cerulean warbler is a forest interior specialist that has experienced some of the most dramatic declines of all songbirds over the last 30 years (Hamel 2000). The species has a fairly broad geographic range, but is only locally abundant. The species may nest semi-colonially, with territories in good habitat highly clumped. Cerulean warblers appear to be extremely sensitive to forest fragmentation. Robbins and others (1989) found a 50 percent reduction in detection of this species as forest patch size declined from 3000 to 700 ha. No birds were detected on forest patches <138 ha. Estimates from other researchers suggest forest tracts as large as 8000 ha may be required to ensure sustainable cerulean warbler populations in the Mississippi Alluvial Valley (summarized in Hamel [2000]).

Although they require large forest tracts, cerulean warblers establish territories near interior forest gaps. Weakland and Wood (2005) observed a positive association between cerulean warblers and forest roads or snags that created small canopy openings. Aside from canopy gaps (a measure of horizontal canopy structure), cerulean warblers may also respond to the vertical canopy profile. Canopy cover between 6–12 m and >24 m was preferred in West Virginia (Weakland and Wood 2005). In Ontario, canopy cover between 12–18 m and >18 m was preferred (Jones and Robertson 2001). The difference in preferred canopy heights between these studies likely reflects differences in local vegetation structure rather than a true absolute difference in canopy height preference. The key habitat feature in both is the multi-layered character of the overstory canopy.

Closed-canopied stands containing large trees (both tall and large d.b.h.) are commonly associated with cerulean warblers, but likely only serve as a crude proxy for the aforementioned canopy features that provide the true selection criteria for this species (Hamel 2000). Cerulean warblers are associated with bottomland hardwoods in the Southeast and ridges in West Virginia (Hamel 2000, Weakland and Wood 2005). Again, moisture index is probably not directly selected by cerulean warblers but is correlated with the location of large tracts of deciduous forest containing big trees and favorable canopy conditions in the landscape.

Quoting Hamel’s (2000) Birds of North America account: “Important habitat elements for this species thus appear to be large tracts with big deciduous trees in mature to old-growth forest with horizontal heterogeneity of the canopy. The pattern of vertical distribution of foliage in the canopy is also important.”

Model Description:

Our cerulean warbler habitat suitability model contains seven parameters:

  • landform
  • landcover
  • successional age class
  • forest patch size
  • percent forest in a 1-km radius
  • dominant tree density
  • canopy cover

The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) defining unique combinations of these classes Table 050 (Table 050) . We directly assigned suitability index scores to these combinations based on cerulean warbler habitat associations outlined in Hamel (1992).

We derived the forest patch size suitability function (SI2) by fitting a logistic curve Figure 025 (Figure 026) to data from Robbins and others (1989) and Rosenberg and others (2000), who observed ~95 percent of all birds in USFWS Region 4 on tracts ≥400 ha Table 051 (Table 051) . Recognizing the suitability of any particular forest patch is affected by its landscape context (Rosenberg and others 1999), we fit a second logistic function Figure 027 (Figure 027) to hypothetical data Table 052 (Table 052) where landscapes with <30 percent forest were considered non-habitat (suitability index score = 0.000) and landscapes with 100 percent forest were considered optimal (suitability index score = 1.000). We used the maximum value from SI2 or SI3 to account for the suitability of small patches in predominantly forested landscapes.

We used dominant tree density (SI4) in the habitat suitability model and assumed trees with a d.b.h. >76.2 cm (30”) would produce the heterogeneous vertical canopy structure preferred by cerulean warblers. Based on qualitative habitat descriptions by Rosenberg and others (2000), we assumed cerulean warblers reached their highest density in stands containing at least 1 dominant tree/ha. Because cerulean warblers nest almost exclusively in these trees (Weakland and Wood 2005), we also assumed cerulean warblers would be absent from stands with a uniform canopy height (i.e., 0 dominant trees). We fit a logistic function Figure 028 (Figure 028) to these data points and assumed stands with ≥14 dominant trees/ha (the maximum number observed in the WGCP during the 1992 FIA survey) were associated with maximum cerulean warbler habitat suitability Table 053 (Table 053) .

We used data from Rosenberg and others (2000), Jones and others (2001), and Weakland and Wood (2005) to derive an inverse quadratic function Figure 029 (Figure 029) that predicted cerulean warbler habitat suitability from canopy cover (SI5; Table 054 Table 054 ).

To calculate overall suitability index scores for cerulean warbler habitat, we calculated the geometric mean of the three suitability indices related to forest structure (SI1, SI4, and SI5) and the maximum value for the two suitability indices related to landscape composition (SI2 and SI3) separately and then the geometric mean of these values together.

Overall SI = ((SI1 * SI4 * SI5)0.333 * Max(SI2, SI3))0.500

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