Nest survival of Western Bluebirds using tree cavities in managed ponderosa pine forests of Central Washington

Abstract

We examined the association of temporal and spatial factors with nest survival of Western Bluebirds (Sialia mexicana) nesting in tree cavities in ponderosa pine (Pinus ponderosa) forests along the east slope of the Cascade Mountains, Washington. All study areas were managed for timber production through planned harvests or postfire salvage logging. Bluebirds laid a mean clutch of 5.3 ± 0.1 (SE) eggs (n = 131), and successful nests fledged an average of 4.5 ± 0.2 young (n = 85). Using a model-selection framework, we found that nest survival was a function of clutch size and treatment and that there was a quadratic effect of nest age. Daily survival rates decreased after the onset of incubation, then increased through the nestling period, and were higher for clutches with ≥5 eggs and in stands that were burned and salvaged. Survivorship over the entire period for clutches (n = 131 nests) with ≤4, 5, and ≥6 eggs was 0.39 (95% CI: 0.11, 0.65), 0.61 (95% CI: 0.34, 0.80), and 0.71 (95% CI: 0.46, 0.85), respectively. Vegetation variables associated with nest sites did not significantly affect nest survival. Predation accounted for the most nest failures (34% of nests). We suggest that parental defense of nests accounts for the quadratic effect of nest age, with adult bluebirds defending nests more aggressively as nestlings approach fledging, and that bluebirds laying larger clutches are older, more experienced birds, resulting in greater nest survival. © The Cooper Ornithological Society 2010.