Masters Thesis

Effects of livestock grazing on foraging ecology of Western Wood-Pewees in the southern Sierra Nevada Mountains

Montane meadows represent one of the most critical habitats in the Sierra Nevada Mountains for breeding birds. Livestock grazing in and around montane meadows can produce deleterious ecological effects including changes in the structure and complexity of the herbaceous and shrub layer, declining water tables and increased tree densities. Western Wood-Pewees (Contopus sordidulus) breed in high densities along edges of meadows and exhibit mixed, but mostly negative, responses to cattle grazing in western riparian habitats. I observed the foraging ecology of pewees to investigate the reasons why a species that forages and nests in the canopy may respond negatively to cattle grazing. I estimated foraging attack rate, aerial arthropod abundance, territory density and foraging habitat selection of breeding Western Wood-Pewees on the edges of seven montane meadows during 2007 and 2008 and compared habitat variables between grazed and ungrazed meadows to investigate structural differences. Foraging rates (n = 144 observations) and aerial insect biomass (n = 27 insect traps) were similar between meadow types but territory density was consistently higher in ungrazed meadows. Western Wood-Pewees foraged from large, dead or dying trees and used areas that were closer to the meadow edge and with lower canopy cover but higher variation in tree diameter at breast height than paired random sites. Grazed meadows had higher tree densities and lower mean diameter at breast height than ungrazed meadows. Because foraging rates and insect biomass did not differ between grazed and ungrazed meadows, lower territory densities in grazed meadows likely indicate a lack of foraging habitat rather than limitations related to prey abundance. In songbird foraging studies, data on monochromatic males and females are frequently pooled. While this may increase sample size, it can also obscure important differences in foraging behavior and habitat use. To investigate intersexual variation in foraging habitat use and behavior in relation to effects of livestock grazing, I compared foraging rates, use of foraging substrates and behavior between male and female Western Wood-Pewees. I located 42 nests during the study and compared nest locations with foraging locations to examine whether females foraged closer to nests. I also examined changes in foraging rates by males and females over the course of the breeding season. Females foraged on average 2.83 ± 1.8 (n = 69) times per minute with elevated rates during incubation while males foraged 1.09 ± 0.1 times per minute (n = 75) and rates did not vary through the breeding season. Males perched higher in the canopy, flew longer distances to capture prey and foraged from larger trees than females. Both sexes foraged at similar distances from their nest while males tended to perch higher and foraged from snags and sugar pines while females foraged lower in the understory from logs, hardwoods and snags. Increased densities of small trees in forests surrounding grazed meadows may make foraging in the understory less suitable for female pewees.

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