Association of Weather and Nest-Site Structure with Reproductive Success in California Spotted Owls

Abstract

Although the spotted owl (Strix occidentalis) has been intensively studied, factors influencing its reproduction are not well understood. We examined a 9-year demographic study of 51-86 pairs of the California spotted owl (S. o. occidentalis), weather conditions, and forest structure at nest sites in oak (Quercus sp.) woodland and conifer forests to assess if weather or nest-site variables are associated with reproduction. Mean reproduction was highly variable between years, but within a given year was largely synchronous among all owl pairs across forest types (i.e., oak woodlands and conifer) with different prey bases. There was no significant difference in reproduction between owls on National Forest and National Park lands. In oak woodlands and conifer forest, mean fledgling production was negatively correlated with nesting period precipitation, and in conifer forests, positively correlated with April's minimum temperature. For both forest types, live-tree nests were in large (diam at breast height [dbh] > 150 cm), old trees (>225 yr) with large crowns (foliage volume >1,700 m3). Regardless of forest type, all nest sites had similar canopy cover (76%), tree density (312 stems/ha), and foliage volume (45,000 m3/ha). Nests with repeated use produced more young than nests used only 1 year. In oak woodlands, nests with higher reproduction were on shrubby, north-aspect slopes in trees or snags surrounded by a well-developed canopy. In conifer forests, reproductive success was associated with nests overtopped by a canopy with a high foliage volume. Synchronous annual reproduction and the association of nest-period weather and canopy structure with production of young, suggests that reproduction is influenced by both regional weather conditions and nest-site canopy structure, which protects fledglings from detrimental weather.