by Robyn Bailey, NestWatch Project Leader
On October 29, 2022, the North American Bluebird Society convened a virtual conference to assess the status of the least-studied bluebird species—the Mountain Bluebird. The goal was to understand the population status and trend and to identify gaps in our knowledge of how the species is faring. Four citizen-science data experts presented and took questions from an audience made up mostly by nest box stewards. John Sauer from the USGS Eastern Ecological Science Center spoke about North American Breeding Bird Survey insights. Tom Auer and Robyn Bailey from the Cornell Lab of Ornithology shared results from eBird and NestWatch (respectively), and Brooke Bateman from National Audubon Society showed data from ClimateWatch. You can view the entire conference at the bottom of this page, but for those who don’t have four hours to watch it, I’ve summarized some of the findings here.
According to research scientist John Sauer, there are approximately four million Mountain Bluebirds as of 2021, a reduction of around one million individuals since 1970. Breeding Bird Survey data show a declining trend estimate of -1.55% per year on average. According to Dr. Sauer’s analysis, Mountain Bluebirds do not seem to be arriving earlier in the spring, as some species have been shown to do in response to climate change. This was consistent with NestWatch data, which also did not show a trend towards earlier laying dates. More work needs to be done to understand whether other climate-related changes might be happening (e.g., earlier cessation of breeding, heat-induced nest failure).
Tom Auer revealed a new trend map from eBird which showed that Mountain Bluebirds have declined by an estimated 34% from 2007–2021. All models have uncertainty in them, and the range of possible loss could be at worst -49% or at best -15%. To compare, the Breeding Bird Survey data show a decline of 26.4% over this same time period, which is within the range estimated from eBird data. So although these two datasets are completely different, they both tell a similar story.
From NestWatch data, we can see that the largest reported reason for nest failure is eggs not hatching. House Sparrows did not represent a major source of nest mortality, although this is likely because NestWatchers are managing them in their boxes. The data suggest that predator guards are unlikely to significantly improve nest survival (as was also found for Western Bluebirds in Bailey and Bonter 2017). More nesting data on Mountain Bluebirds are needed to improve models.
Dr. Bateman from Audubon’s ClimateWatch project pointed out that the Mountain Bluebird’s winter range is considered stable, but during the summer season, over half of the range could become unsuitable if North American temperatures rise by 3°C over historical norms (a 1°C increase has already occurred). The species might shift its breeding range northward to regain suitable thermal conditions, but this gain would be offset by a much larger loss of breeding range. The Mountain Bluebird is highly vulnerable to climate change, and is more likely to leave breeding sites when thermal suitability worsens with climate change.
All of the panelists were at a loss to explain why these declines are happening specifically for Mountain Bluebirds, largely because a range-wide integrated population model has not been conducted. Without knowing the reason(s) for decline, it is difficult to know what recovery measures would be appropriate.
Future research should focus on:
- Creating a comprehensive integrated population model to elucidate which demographic rates are most in need of improvement (e.g., adult survival, reproductive success, juvenile survival, etc.).
- Comparing reproductive success in areas of decline and areas of increase.
- Exploring the effects of climate change and severe weather on adult survival and egg viability.
- Understanding where we should be placing nest boxes to make the most difference.
As for how NestWatchers can help, it is always true that more data are needed—especially in data-deficient areas (these are usually areas that are further from human settlements). Those at the northernmost edge of the Mountain Bluebird breeding range should also keep an eye out for range expansion. If Mountain Bluebirds are shifting their breeding range north, then nest boxes could be placed for them in suitable habitat (or habitat that will one day be suitable). All of the research and maps presented during this event were made possible by citizen scientists like you. Thank you for your contributions to NestWatch and other conservation efforts!
- Bailey, R. L., and D. N. Bonter. 2017. Predator guards on nest boxes improve nesting success of birds. Wildlife Society Bulletin 41(3):434–441. DOI: 10.1002/wsb.801