The black-and-white warbler is a forest-interior specialist that occurs in the
mature deciduous hardwood forests of the eastern United States and Canada
(Kricher 1995). Although not as sensitive to fragmentation as some species
(e.g., cerulean warbler), the black-and-white warbler is typically absent from
small woodlots (<7.5 ha; Galli and others 1976). Hamel (1992) suggested 550 ha
was the minimum tract size this species would reliably occupy in the Southeast.
Few studies have focused exclusively on the habitat ecology of this species;
however, Conner and others (1983) found black-and-white warblers to be
associated with mature forest stands with high densities of large (>32 cm
d.b.h.) trees. Although a ground-nesting species, black-and-white warblers are
associated with high densities of hardwood saplings. Conversely, pine saplings
negatively affect both the presence and abundance of black-and-white warblers.
Birds occupy upland and bottomland forests, but reach higher densities in the
former, with oak-hickory and cove forests considered optimal (Hamel 1992).
Nevertheless, successional age may be the most critical habitat factor
influencing black-and-white warblers. Dettmers and others (2002) validated
Hamel’s (1992) black-and-white warbler habitat suitability model and found it
performed well due to the restriction of black-and-white warblers to older
age-class forests. Thompson and others (1992) and Annand and Thompson (1997),
though, commonly observed black-and-white warblers in sapling and clearcut
stands in Missouri.
Our habitat suitability model for black-and-white warblers contains six variables:
- landform
- landcover type
- successional age class
- forest patch size
- percent forest in a 1-km radius
- 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 028)
. We directly assigned habitat suitability index scores to these
combinations based on vegetation type and age class associations of
black-and-white warblers reported by Hamel (1992). However, we assigned higher
values to shrub-seedling and grass-forb stands based on data from Thompson and
others (1992) and Annand and Thompson (1997).
Forest patch size (SI2) affects this species’ occurrence, with birds notably
absent from small forest blocks. Therefore, we fit a logarithmic function
(Figure 012)
relating forest patch size to habitat suitability index scores derived from
probability of occurrence data reported by Robbins and others (1989;
Table 029
).
The relative value of a forest of a specific patch size is influenced by its
landscape context. In predominantly forested landscapes, small forest patch
sizes that may not be utilized in predominantly non-forested landscapes may
provide habitat due to their proximity to large forest blocks (Rosenberg and
others 1999). Thus we fit a logistic function to hypothetical data that captured
this relationship
(Figure 013)
. We considered landscapes with <30 percent forest to
be non-habitat (suitability index score = 0.000), while landscapes with >90
percent forest were considered excellent habitat (suitability index score ≥
0.900;
Table 030
). We used the maximum value of either SI2 or SI3 to account for
small patches in predominantly forested landscapes and large patches in
predominantly non-forested landscapes.
Canopy cover (SI4) may also affect black-and-white warbler habitat quality. Thus,
we included it as a factor in our overall suitability index model. King and
DeGraaf (2000) and Prather and Smith (2003) reported higher densities of
black-and-white warblers in forests with relatively open canopies, so we used
their data
(Table 031)
to derive a quadratic function
(Figure 014)
and quantify the
relationship between canopy cover and black-and-white warbler habitat
suitability index scores.
We calculated the overall suitability index score as the geometric mean of the
geometric mean of individual suitability index functions related to forest
structure (SI1 and SI4) multiplied by the maximum suitability index score for
either forest patch size or percent forest in the 1-km radius landscape.
Overall SI = ((SI1 * SI4)0.500 * Max(SI2, SI3))0.500
Black-and-white warblers occurred in all but 3 of the 88 subsections within the
CH and WGCP. Not surprisingly, Spearman rank correlations based on all
subsections and only subsections in which black-and-white warblers occurred
produced similar results: significant (P ≤ 0.001 for both analyses) positive
relationships (r = 0.54 and 0.53, respectively) between average HSI score and
mean BBS route abundance. Negative binomial regression indicated HSI scores were
positively related to black-and-white warbler abundance and that the HSI model
was an improvement over a null model. The generalized R2 for this comparison was
0.40. Route-level analysis predicting black-and-white warbler abundance to
average HSI scores within 3-km buffers around the 147 BBS routes in the CH and
WGCP also confirmed the improvement of the HSI model over an intercept-only null
model and elucidated the positive relationship between HSI scores and
black-and-white warbler abundance. However, the strength of the relationship was
weaker (generalized R2 = 0.16). We considered this model both verified and
validated.