Regional Analysis of Black-Capped Vireo Breeding Habitats

Structural vegetation components of Black-capped Vireo (Vireo atricapillus) breeding habitats and unoccupied scrubland areas were analyzed at 13 sites from three regions: the Lampasas Cut Plains in central Texas; the Wichita Mountains and adjacent areas in western Oklahoma; and the Edwards Plateau (Kerr County) in central Texas. We evaluated 61 vegetation variables from individual vireo territories and equivalent scrubland plots unoccupied by vireos. Principal components indicated strong vegetation trends for all plots involving the abundance of junipers (from high to low) and openness (from low to high) extending from the Lampasas Cut Plains to the Wichita Mountains and then to the Edwards Plateau. In all regions, vireo territories had relatively high densities of deciduous vegetation (primarily oaks) close to the ground (i.e., up to 1-2 m, and in one region 3 m). In addition, vireo territories occurred where variation in relative density measures of woody vegetation was highest. However, even with the vireo's limited geographic range, differences were found among regions in the vegetational characteristics of areas defended by territorial males. Vireos avoided higher juniper densities on the Lampasas Cut Plains and more open areas on the Edwards Plateau. On the Edwards Plateau, with generally less dense vegetation and the lowest density of junipers, the older (ASY; after-second-year) males selected territories with higher juniper densities at lower height intervals than found on nonvireo plots. Territories of second-year males (SY) and older males (ASY) differed in vegetation characteristics. On the Edwards Plateau, SY males occupied territories with vegetation structurally more like nonvireo plots than like territories of ASY males. In the Wichita Mountains, where a remnant vireo population occurs, territories of SY males were similar to those of ASY males. In both regions, however, SY males were found in more open habitats. Competition with other foliage-gleaning species may have played a role in restricting the Black-capped Vireo to early successional stages of scrubland habitats, where they also may effectively avoid several species of nest predators. Periodic habitat disturbance has beneficial effects for vireos and can be used in management of this endangered species. Attempts should be made to maintain 35 to 55% dispersed scrub cover (primarily deciduous) in spatially heterogeneous configurations. Sometimes inherent site characteristics (e.g., poor soils) may naturally promote the degree of vegetational heterogeneity requisite for Black-capped Vireos. Managers should employ techniques that encourage thick regrowth of deciduous woody vegetation. Typically, juniper cover should be kept well below 10%, which may be possible only through habitat manipulation. However, in areas where amounts of deciduous vegetation are marginal (e.g., Edwards Plateau region and to the southwest), junipers may contribute important cover. Thus, site and regional characteristics need consideration when making decisions about juniper control.