Browsing by Subject "Molothrus ater"
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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 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.Item Minimum patch size thresholds of reproductive success of songbirds(2009-05-15) Butcher, Jerrod AnthonyPreservation of large tracts of habitat is often recommended for long-term population viability of area-sensitive species. Large tracts may not always be available. Smaller patches, though not able to contain a viable population individually, may contribute to overall regional population viability if within the small patches pairs could successfully reproduce. By definition, area-sensitive species should have a minimum patch size threshold of habitat below which they will not likely reproduce. Two potential causes for positive relationships between patch size and production are inverse relationships between patch size and brood parasitism and patch size and food availability. My objectives were (1) to determine the minimum patch size thresholds of reproductive success for golden-cheeked warblers (Dendroica chrysoparia), black-and-white warblers (Mniotilta varia), and white-eyed vireos (Vireo griseus); (2) to determine whether thresholds for occupancy, territory establishment by males, or pairing success were indicative of thresholds of reproduction; (3) to determine whether the proportion of pairs fledging brown-headed cowbird (Molothrus ater) young was related to patch size, and (4) to determine the affects of patch size on food availability (i.e., arthropod abundance). The Vickery index of reproductive activity was used to determine reproductive activity of each male or pair and to quantify parasitism occurrences. I collected arthropods using branch clipping to assess the relationship between patch size and arthropod abundance. I found minimum patch size thresholds of reproductive success for golden-cheeked and black-and-white warblers, but not for white-eyed vireos. Minimum patch size of reproductive success was between 15 and 20.1 ha. Minimum patch size thresholds for occupancy, territory establishment by males, and pair formation were not consistent with thresholds for reproductive success. I found no relationships between patch size and cowbird parasitism or patch size and arthropod biomass. Conservation practices for target species based on thresholds of occupancy, territory establishment, or pair formation may not address issues of reproduction. The ability to identify thresholds of reproductive success for target species could be useful in conservation and management in multiple ways including setting goals for retention and restoration of a target species? habitat patch size.