Nest-site selection by Cassia Crossbills and management implications

Cassia Crossbills (Loxia sinesciuris) are endemic to two small mountain ranges in southern Idaho, the South Hills and Albion Mountains. Although previous studies have shown the importance of mature Rocky Mountain lodgepole pines (Pinus contorta latifolia) that have accumulated a large canopy cone bank as a food resource for Cassia Crossbills, information about nest-site selection is lacking. We located 49 Cassia Crossbill nests in the South Hills in 2001–2002, characterized their locations and surrounding habitat, and characterized the habitat in 26 nearby random locations for comparison. We also include information on tree species and height for 57 nests located from 2003 to 2006. Cassia Crossbills built nests where foliage was most extensive in the upper canopy of large lodgepole pines in relatively dense and mature stands dominated by lodgepole pine. Nests were usually on lateral branches on the south side of trees, but the slope and aspect of nest locations were not distinctive. Other species of crossbills generally build nests in dense foliage, but, unlike Cassia Crossbills, they usually nest in relatively open stands, presumably to reduce the risk of nest predation by tree squirrels. Because tree squirrels do not co-occur with Cassia Crossbills, their preference to nest in dense forests may reduce exposure to wind and inclement weather and reduce the likelihood of detection by nest predators such as Common Ravens (Corvus corax). We believe that nest sites will not be limiting if there is an adequate seed supply. However, this requires a large canopy cone bank throughout the range of Cassia Crossbills, an outcome that is uncertain in the face of increasing temperatures and increasing fire frequency and intensity.