Browsing by Subject "Vireo atricapilla"
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Item Brown-headed cowbird parasitism on endangered species: relationships with neighboring avian species(2009-05-15) Farrell, Shannon LeighBrown-headed cowbird (Molothrus ater) parasitism on the endangered black-capped vireo (Vireo atricapilla) is a management concern. Management for black-capped vireos in east-central Texas has focused on habitat restoration through vegetation modification. Loss of productivity due to parasitism, however, may limit the potential habitat restoration in restoring black-capped vireo populations. I investigated the relationships between frequency of parasitism on black-capped and white-eyed (V. griseus) vireos and characteristics of the neighboring avian assemblage to identify avian characteristics useful for predicting expected frequency of parasitism. I located and monitored vireo nests in March?July 2006 to determine frequency of cowbird parasitism on the 2 vireo species on 6 study sites on private land in east-central Texas. I conducted 100-m-radius circular-plot point count surveys on study sites from 1 April? 31 June 2006 to identify neighboring avian assemblages. All black-capped vireo nests (n = 20) and 81% of white-eyed vireo nests (n = 27) were parasitized. Species richness, species evenness, cumulative bird abundance, and cowbird host abundance did not differ significantly among study sites. Parasitism frequency was positively correlated with the number of cowbirds detected (r2 = 0.673, P= 0.045). Parasitism frequency was negatively correlated with abundance (r2 = 0.687, P = 0.042) and proportion of (r2 = 0.692, P = 0.040) of black-and-white warblers (Mniotilta varia), known cowbird hosts. I did not find a relationship between parasitism frequency and abundance of other cowbird hosts. The relationship between black-andwhite warbler abundance and parasitism frequency may reflect an interaction between parasitism and the vegetation characteristics associated with black-and-white warblers, suggesting the need for future study of the relationships between parasitism frequency and vegetation characteristics in the study region. The frequency of parasitism I observed suggests that cowbird control may be an important management tool if management objectives in the study region continue to include recovery of black-capped vireo populations. Number of cowbirds may be a reasonable predictor of expected parasitism frequency in an area, which may be useful to land managers in selecting areas for implementation of cowbird control.Item Ecology of Wintering Black-capped Vireos in Mexico(2013-11-06) Powell, Robert AndrewThe black-capped vireo (Vireo atricapilla) is an endangered Neotropical migratory songbird that has received considerable attention in its breeding range, but relatively little attention in its winter range in Mexico. To address information needs regarding winter ecology of the black-capped vireo, I conducted research in Mexico focused on investigations of the winter distribution, habitat use, and migratory linkages between breeding and wintering sites. Over 3 winter periods in 2002?2004, I identified and described the geographic distribution for the black-capped vireo at study sites across 8 states in western Mexico and determined if differential migration occurs among different classes of individuals. I documented winter occupancy in the 5 most northern Mexican states surveyed (Sinaloa, Durango, Nayarit, Jalisco, and Colima), and identified a strong association between sex/age class and winter latitude with adult males occupying habitat at more northern latitudes relative to females and juveniles. During two winters in 2003-2004, I conducted field research at study sites in Mexico to evaluate patterns of winter habitat use by black-capped vireos and determine which habitat characteristics may influence vireo use of winter patches. Winter habitat use by black-capped vireos was best predicted by increasing values of slope and foliage cover, and by decreasing values of canopy cover and tree diameter. Vireo use plots characterized as thorn forest had greater foliage density, greater shrub density, less canopy cover, and smaller tree diameter than plots classified as tropical deciduous or semi-deciduous forest, suggesting that thorn forests may be most suitable for vireo occupancy during the winter months. I also used stable carbon, hydrogen, and nitrogen isotopes in black-capped vireo feathers obtained across 3 states in the breeding range and 8 states in the wintering range to determine if vireo feathers collected on the wintering grounds could be used to assess breeding origins and to determine if a relationship exists between breeding and wintering latitudes such as chain or leapfrog migration. Feathers collected at both breeding and wintering sites displayed considerable variability in isotopic composition for all 3 isotopes analyzed, and thus did not provide sufficient information to establish migratory linkages between breeding and wintering sites.Item Grazing Impact on Brood Parasitism(2014-04-16) Locatelli, AnthonyAnthropogenic land use changes can have tremendous direct and indirect effects on biota. A prevalent land use change in Texas is conversion of land to grazing. Grazing facilitates foraging opportunities for brown-headed cowbirds (Molothrus ater), a brood parasite. Cowbirds can reduce productivity of their hosts, causing some host species to decline in abundance. Thus, grazing indirectly influences productivity of some songbirds. The black-capped vireo (Vireo atricapilla) is an endangered songbird with most of its breeding range occurring in areas of livestock and other ungulate grazing. A contributing factor to its endangered status is brown-headed cowbird parasitism. I monitored 382 black-capped vireo nests from 2012-2013 in Real, Kerr, Bandera and Edwards counties, Texas and described parasitism. I investigated how grazing system related to parasitism; I defined grazed systems by the presence of enclosed ungulates and wild ungulate only systems by the absence of enclosed ungulates. I also examined how grazing intensity (represented by distance from nest to nearest ungulate water source), nest concealment, and grazing in the landscape (represented by proportion of grassland within 3 km of a nest), related to parasitism. Overall parasitism was 30% (n = 166) in 2012 and 31% (n = 216) in 2013, moderate compared to other research, but above a proposed threshold of concern. Grazing system and grazing in the landscape interacted in predicting probability of parasitism. Grazing in the landscape was not important in predicting brood parasitism in wild ungulate only grazing systems, but was important in predicting brood parasitism in grazed systems. In grazed systems, there was low probability of brood parasitism with low grazing in the landscape and high probability of parasitism with high grazing in the landscape. Nest concealment and grazing intensity were not good predictors of brood parasitism. Land managers could use this information to prioritize cowbird management or preservation efforts.