Hooded Warbler
Ecoregional Scale Conservation Planning

Made possible through a partnership with the National Wetlands Research Center

Louisiana Waterthrush (Seiurus motacilla)
The Louisiana waterthrush is a long-distance Neotropical migrant found throughout the deciduous forests of the eastern and central United States. The small population in the WGCP has remained relatively stable since 1966, while the larger population in the CH has increased 2.6 percent annually (Sauer and others 2005; Table 005 Table 005) . The species is considered a Bird of Conservation Concern in both regions Table 001 (Table 001) . However, PIF differentiates the priority for this species in the CH (planning and responsibility species, regional combined score = 15) and WGCP (management attention species, regional combined score = 18; Table 001 Table 001) .
Relative abundance of Louisiana Waterthrush, derived from Breeding Bird Survey data, 1994 - 2003.
image courtesy of www.whatbird.com

Natural History:

As its name implies, the Louisiana waterthrush is associated with water throughout its range (Robinson 1995). Louisiana waterthrushes reach their highest densities along gravel-bottomed first- and second-order streams flowing through large (>350 ha) tracts of mature deciduous forest (Robbins and others 1989, Robinson 1995). Birds also breed at lower densities along mud-bottomed streams in cypress swamps and bottomland hardwood forests (Hamel 1992, Robinson 1995).

Prosser and Brooks (1998) developed and validated a habitat suitability model for Louisiana waterthrushes in central Pennsylvania. The model contained eight variables: canopy cover (>80 percent considered ideal), shrub cover (<25 percent), ratio of deciduous to conifer cover (30–69 percent, mostly reflecting hemlock dominance along streams in the Northeast), herbaceous cover (<25 percent), stream order (1st or 2nd with well-developed pools and riffles), water clarity and substrate (clear and rocky or sandy), nesting cover (uprooted trees or creviced, steep banks), and forest area (>350 ha).

Model Description:

Our Louisiana waterthrush habitat suitability index model contained eight parameters:

  • landform
  • landcover
  • successional age class
  • distance to stream
  • canopy coverv
  • small (<2.5cm d.b.h.) stem density
  • forest patch size
  • percent forest in a 1-km radius.

The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) defining unique combinations of these classes Table 077 (Table 077) . We directly assigned suitability index scores to these combinations based on the vegetation and successional age class associations of Louisiana waterthrush outlined in Hamel (1992).

We included distance to stream (SI2) as a model variable because waterthrushes use streams and creeks for foraging and nesting. Louisiana waterthrushes restrict their foraging to the stream bed and bank, so we assumed a sharp decline in suitability with increasing distance to a stream Table 078 (Table 078) . We used an inverse logistic function to characterize this relationship Figure 044 (Figure 044) .

We also included canopy cover (SI3) and small stem density (SI4) as model variables, based on the preference of this species for mature forested sites with closed canopies and open understories. We fit logistic Figure 045 (Figure 045) and inverse logistic Figure 046 (Figure 046) functions to data adapted from the habitat suitability model of Prosser and Brooks (1998) for canopy cover Table 079 (Table 079) and small stem density Table 080 (Table 080) , respectively.

Forest patch size (SI5) affects the occupancy of habitats by Louisiana waterthrush. To predict the habitat suitability from forest patch size, we fit a logarithmic function Figure 047 (Figure 047) to data from Hayden and others (1985) and Robbins and others (1989; Table 081 Table 081) on the detection probabilities of Louisiana waterthrush in forest patches of varying size. Forest patch size alone may not be an appropriate measure of a site’s suitability, though. In predominantly forested landscapes, small forest patches not otherwise suitable may be occupied due to their proximity to large forest blocks (Rosenberg and others 1999). We fit a logistic function Figure 048. (Figure 048) to a dataset based on the assumptions that landscapes with <30 percent forest provided poor habitat (suitability index score ≤ 0.100) and landscapes >70 percent forest were excellent habitat (suitability index score ≥ 0.900; Table 082 Table 082) . We used the maximum suitability index score from either S5 or SI6 to ensure small forest blocks in predominantly forested landscapes were assigned an appropriate suitability score.

To calculate the overall suitability index, we determined the geometric mean of SI scores for forest structure (SI1, SI3, and SI4) and landscape composition (Max(SI5, SI6) and SI2) separately and then the geometric mean of these means together.

Overall SI = ((SI1 * SI3 * SI4)0.333 * (Max (SI5, SI6) * SI2)0.500)0.500