Peak Health home > Wildlife > Birds > Birds Overall
Why Was This Indicator Chosen?
Birds have long been a national conservation priority, and native species are protected by the Migratory Bird Treaty Act. However, many of the birds included in this chapter are both common and widespread in the One Tam area of focus and the surrounding region, and lack any special conservation status, making birds an ideal resource to study here. Exceptions include the federally and state threatened Northern Spotted Owl and several California Species of Special Concern, among them, Northern Harrier, Olive-sided Flycatcher, Purple Martin, Grasshopper Sparrow, and local subspecies of Savannah Sparrow and Common Yellowthroat (Shuford & Gardali, 2008).
The One Tam area of focus supports a rich diversity of birds. Birds engender a great deal of public interest, attracting many bird watchers to the Mt. Tam region and delighting visitors and local residents alike. Monitoring migratory bird populations provides a way to connect the mountain to ecosystems and people well outside of the Bay Area. Several bird-focused, community-science projects generate data and engage the public (e.g., eBird, iNaturalist, Audubon’s Christmas Bird Count, and the current Marin County Breeding Bird Atlas effort). Bird watching can also have a significant positive impact on local to national economies (Carver, 2013).
Birds are recognized indicators of ecological change (Carignan & Villard, 2002) and are relatively easy to monitor. They and provide a wide variety of ecosystem services, including feeding on pests, pollinating flowers, dispersing seeds, scavenging carrion, cycling nutrients, and modifying the environment in ways that benefit other species (Whelan et al., 2015).
What is Healthy?
The desired status of the abundance of birds in the One Tam area of focus (including some of the nearby environs as described previously) is for the combined condition for all birds to be good, the trend to be improving or no change, and confidence to be high. This would represent stable-to-increasing populations, no immediate cause for concern, and high confidence in our understanding of the patterns we are seeing. This status would also apply to the majority of the individual species assessed as well as for the different habitats and other guilds (e.g., Oak Woodland Birds, climate-vulnerable species) we included.
What Are the Biggest Threats?
- Historical impacts from land development and past logging that has altered forest structure and bird communities. Across the continent populations have declined steeply over the past 50 years, and while land protection helps, broader forces driving these losses extend beyond human boundaries.
- Invasive species have changed habitat structure, which may affect some species more than others, plus the potential impacts of invasive species on wildfire dynamics.
- Climate change has several impacts including shifts in vegetation communities which alter the distribution/availability of habitat, extreme weather may affect survival, and seasonal patterns such as migration timing and resource availability may shift.
- Lack of fire has altered vegetation communities and habitats.
- Free-ranging domestic and feral cats have been shown to have a substantial negative impact on native birds and other animals.
What is the Current Condition?
In 2022, the current condition of birds overall is Caution. The overall abundance of birds in the One Tam area of focus (and some surrounding areas) has remained relatively stable since the first assessment in 2016. However, we detected some changes in condition when looking at all birds, and for the condition and trend of some of the specific guilds examined. This warrants concern and a change in condition to caution for this update because quite a few species were either declining, showing mixed trends across datasets, or while we lacked enough data for trend analysis, we had enough information to warrant concern about their overall condition. Even if we excluded those species for which we had limited data (the latter category), the overall condition of all birds in the area of focus would fall in the caution category. At its core, the shift from good to caution means that in 2022, we have greater concern about the trends for more species than we did in 2016.
On a positive note, the Oak Woodland Bird guild trend went from no change to improving. This may be in part because the number of species included increased, with a few species assigned to this habitat in 2022 that were either not previously included or were included but for which we lacked adequate data to analyze a trend.
• Potentially concerning changes since 2016 include the condition of Scrub/Chaparral, Riparian, and Climate-Vulnerable Bird guilds, which went from good to caution, while our confidence for Scrub/Chaparral Birds went from high to moderate. Species included in the Scrub/Chaparral and Riparian Bird guilds changed slightly between 2016 and 2022, but the species included in Climate-Vulnerable Birds remained the same.
• Monitoring of Grassland Birds (a new effort undertaken after a data gap was identified in the 2016 Peak Health report) resulted in a change in their condition from unknown to caution, and the confidence in this assessment from unknown to low.
• Additional years of bird surveys included here have been conducted since 2016 as part of long-term monitoring efforts on lands managed predominantly by Marin Water (where a new analytical approach also allowed us to include more species) and in riparian habitat throughout western Marin County on lands managed predominantly by the National Park Service.
What is the Current Trend?
The current trend is No Change.
How Sure Are We?
Our confidence in the results of this assessment remains High. In 2022, 79 species were assessed, including 26 that were not previously included. Of these, we were able to apply at least one of the parameters of our metric evaluation—condition, trend, and/or confidence—to 67 species. The remaining 12 species were classified as unknown across all parameters.
There is a long history of bird monitoring by One Tam partner agencies and their collaborators, enabling population trend estimates for many species across multiple vegetation communities. For example, the National Park Service (Gardali et al., 2020) and Marin Water (Cormier et al., 2020) both have ongoing landbird monitoring programs co-developed and implemented by Point Blue Conservation Science (Point Blue). The resulting robust datasets helps provide the statistical power needed to identify population changes. By studying the entire bird community, these data can also reveal mechanisms behind observed changes (e.g., changes that may be specific to a particular habitat, foraging guild, or migratory status).
What is This Assessment Based On?
We were able to reevaluate many species due to the following: new bird survey data collected through continued long-term monitoring efforts by Point Blue and collaborating partner agencies in the area of focus and surrounding environs; and new monitoring data collected to fill data gaps identified in the 2016 report, especially in grassland habitat. Additionally, some analyses were refined, which allowed us to include new species.
- Continuation of a long-term landbird-community monitoring collaboration between Point Blue and Marin Water, which involves extensive point counts throughout lands primarily managed by Marin Water within the area of focus.
- Continuation of another a long-term landbird-community monitoring collaboration, this between Point Blue and the National Park Service, Marin County Parks, and California State Parks. This effort involves point counts and constant-effort mist netting throughout riparian habitats in western Marin County conducted by researchers at Point Blue’s Palomarin Field Station both within and outside of the area of focus (within Point Reyes National Seashore, Golden Gate National Recreation Area, Bolinas Lagoon Open Space Preserve, and Mount Tamalpais State Park).
- Continuation of annual long-term Northern Spotted Owl monitoring throughout Marin County, a collaboration between National Park Service, Point Blue, Marin County Parks, Marin Water, and California State Parks.
- Point Blue monitoring at additional sites in 2018–2019 (DiGaudio & Humple, 2019) in collaboration with the Parks Conservancy and One Tam partner agencies to fill data gaps related to land management and particular habitats (most notably grasslands) identified in the 2016 Peak Health report.
What Don’t We Know?
Key information gaps include:
- Most long-term bird monitoring has occurred on Marin Water lands, with smaller efforts in other One Tam partner lands. Additional studies were initiated in response to data gaps identified in 2016, but revisiting historic sites on multiple partner lands over time would help establish more trends. In particular, grassland and coastal scrub bird populations in the One Tam area are difficult to assess due to recent but limited surveys.
- Status of seasonal bird populations including wintering and migrating birds, and how connectivity may impact migrating birds
- Demographic data (e.g., survival, reproductive success) across species and across One Tam partner lands
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