Effects of Alpine Shrub Cover on Territorial Male White-Crown Sparrows Densities.

Abstract

Avian populations at high elevation are well-adapted to extreme and variable abiotic conditions. Species have adapted efficient foraging strategies, as well as well-timed nest incubation patterns. These behavioral traits are essential in order for parents to meet their metabolic needs while simultaneously having a successful reproductive season. Alpine vegetation structure can be seen as a determinant for a species’ success, or overall abundance due to gap structure and efficient foraging opportunities. Alpine environments consist largely of fragmented vegetation due to steep elevation gradients. Yet, alpine bird populations historically have preferred dense and protective vegetative structures. This study investigates the relationship at singing male white-crowned sparrow densities and percent willow (Salix spp.) cover between three alpine sites located across the Elk Mountain Range, Colorado. Post-migratory sparrow males pair with a female, breed, and establish delineated territory borders by singing. For the mountain white-crowned sparrow (Zonotrichia leucophrys oriantha; MWCS), space availability between willow thickets may act as a driving factor in the species’ overall abundance. A negative correlation between number of singing males and percent willow cover indicates alpine populations of MWCS actually prefer fragmented willow cover. Possible explanations for this finding could include (1) greater accessibility to dietary resources between willow thickets, (2) greater solar exposure and thermal warming for a nest not nestled in a dense willow’s canopy cover, or (3) an overall decrease in predatory risk and edge effects within alpine environments. This study holds important climate change implications. By analyzing changes in vegetation structure over time we can determine if vegetative health is ultimately affecting the adaptability of mountain white-crowned sparrow populations. 3

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