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Western Bluebird Incubation Survival Was 11% Lower in the Later Half of a 22-Year Study

Adult Western Bluebird removing a fecal sac from a nest box in Los Osos, California
Research: Reproduction & development, Ecology

Western Bluebirds (Sialia mexicana) nesting in north-central New Mexico had 11% lower survival during incubation in 2007–2018 than in 1997–2006. At the same time, nests higher in the landscape were more successful. These differences emerged from a 22-year study that followed 1,378 Western Bluebird nests and compared them with 271 nests of Ash-throated Flycatchers (Myiarchus cinerascens).

Both birds are secondary cavity nesters: they breed in existing holes instead of excavating their own. Researchers tracked whether nests remained active through incubation and the nestling stage, examining how survival varied with breeding date, elevation and climate. The long study period also covered a major episode of conifer mortality that changed the surrounding habitat.

Higher nests had better survival

Western Bluebird nests in the upper third of the studied elevation range had 15% higher incubation success and overall nesting success than nests in the lower two-thirds. The Ash-throated Flycatcher showed no comparable relationship with elevation, despite using similar nesting habitat. The contrast suggests that the two cavity-nesting birds responded differently to conditions across the same landscape.

For both species, daily nest survival improved as the breeding season progressed, then levelled off. But survival also declined as nests aged during incubation and the nestling stage. These patterns show why nesting success cannot be reduced to a single date or environmental factor.

The decline occurred before hatching

Western Bluebird incubation survival was 11% lower in the later period, while nestling survival showed no corresponding decline. Neither stage showed the same long-term shift in Ash-throated Flycatchers. The difference for bluebirds was therefore concentrated in the period before eggs hatched rather than across the whole breeding cycle.

The researchers suggest that the elevation pattern may be related to changing conditions after widespread tree mortality, but the study did not establish one cause. What the records show is that the two species responded differently to elevation and long-term environmental change, even though both rely on existing nesting cavities in the same region.

Sources and image credit

  1. Fair, J. M., Dinsmore, S. J., Thompson, B. E., Sanchez, A. A., Stanek, J. E., & Hathcock, C. D. (2026). Long-term environmental variation in nest survival for Sialia mexicana (Western Bluebird) and Myiarchus cinerascens (Ash-throated Flycatcher). Ornithology, ukag052.Original journal abstract and lay summary checked: 22 years, 1,649 nests including 1,378 Western Bluebird nests, an 11% difference in incubation survival between periods, and a 15% elevation-related difference. The accepted-manuscript full text was access-restricted at review, so this published article is limited to results supported by the primary abstract and lay summary.

Image: Adult Western Bluebird (Sialia mexicana) removing a fecal sac from a nest box in Los Osos, California, USA, 22 April 2008. Photo: Kevin Cole / Wikimedia Commons. Representative breeding image; not a study bird. · CC BY-SA 2.0 · Source

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