Comparison of black-headed grosbeaks nesting in riparian and Gambel oak pastures in southwestern Colorado

摘要

From mid-May through July, 1992 to 1998, we searched for black-headed grosbeak (Pheucticus melanocephalus) nests in riparian and Gambel oak (Quercus gambelii) pastures in La Plata County, Colorado. Twenty-six nests (63%) were successful in fledging at least 1 young, and 15 nests (37%) failed because of predation. The mean number of grosbeaks hatched and fledged per nest did not differ between riparian pastures and oak pastures. The proportion of nests depredated in riparian pastures (10/25, 40%) and oak pastures (5/17, 29%) did not differ significantly. Mortality rates did not differ between habitats during the incubation stage (riparian: 0.056 nests/day; oak: 0.046 nests/day) or during the nestling stage (riparian: 0.011 nests/day; oak: 0.0 nests/day). Mean nest height did not differ between depredated nests and successful nests in either riparian pastures or oak pastures.