Demography of Lewis's Woodpecker, breeding bird densities, and riparian aspen integrity in a grazed landscape
Newlon, Karen Rachel.
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Aspen (Populus tremuloides) riparian woodlands are extremely limited in distribution throughout the western U.S., yet these habitats have a disproportionate value to breeding birds. Aspen habitats are also considered prime sheep and cattle summer range, particularly in the semiarid Intermountain West. Such concentrated use has raised concern about the effects of sheep and cattle grazing on these habitats. We examined the influences of sheep and cattle grazing on aspen riparian woodlands and the associated breeding bird community. We had two objectives: 1) to determine how habitats grazed by sheep and cattle influenced the reproductive biology of Lewis's Woodpecker (Melanerpes lewis), and 2) to determine how vegetation differed between sheep and cattle-grazed aspen riparian woodlands and how these differences influenced breeding birds densities. To examine what factors influenced nest-site selection in Lewis's Woodpecker and whether these factors influenced nest survival, we quantified nest-site characteristics and monitored nests to determine nest fate. Lewis's Woodpecker nest-site selection was positively influenced by nest tree diameter and tree density and negatively influenced by increasing amounts of bare ground and woody stems. However, these characteristics were not strong predictors of nest survival. Instead, nest initiation date and daily average temperature had the strongest influence. We then compared several vegetation characteristics between grazing treatments, conducted point transect surveys and used distance sampling techniques to estimate breeding bird densities. Cattle-grazed sites had more bare ground, less herbaceous cover, lower willow (Salix spp.) densities, and lower aspen densities than sheep-grazed sites. Differences in breeding bird densities between these grazing treatments reflected these vegetation differences, as ground-nesting species and riparian specialists had lower densities in cattle-grazed areas. Data from this study provide landowners and land managers with information on the habitat requirements of sensitive bird species needed to implement compatible grazing strategies in aspen riparian woodlands.