Monitoring of Mount Sutro's Birds and Lepidoptera
“Trees, trees, beautiful trees” the high pitched song of the Brown Creeper rings out from a nearby tree trunk. The well-camouflaged little birds creep up the trees, hunting insects in the bark. Brown Creepers don’t breed on Mount Sutro but they often come around to forage in the Spring and Fall. I imagine they wish they could change their little tune when they come to this hill we call a Mountain. For the Brown Creepers, the Eucalyptus that dominates the mountain is not their cup of tea. There are far more insects and spiders on the mature old oaks in Golden Gate Park, or the big Redwoods sprinkled through our parks. On one visit to Mount Sutro, a Creeper was navigating the corrugated sides of a seatrain in a parking lot, giving up on the surrounding trees for some easy pickings of spiders.
Birds adapt. Many of the traits we find beautiful about birds are also the things which make them able to fill new niches and adapt to strange new environments; namely flight, charisma, and intelligence. In the late 1880s Adolf Sutro developed the grassland and dune landscape of San Francisco by densely planting some of it with Eucalyptus and Cypress, with the intention of harvesting it for lumber. Much of it was harvested and later developed, but certain areas like Mount Sutro were left with this tightly packed foreign forest. The now 61 acres was designated an Open Space Reserve by its current owner, UCSF, in 1976.
Despite the main forest being non-native, other native understory plants like ferns, elderberry, and poison oak thrive. And because of decades of hard work from groups like The Sutro Stewards and the Rotary Club, the summit area in particular is a paradise full of the native plant species our local birds love.
In 2018 a new chapter began for the Mount Sutro Open Space Reserve. After many years of planning, UCSF published the 20-year Vegetation Management Plan. Work began by removing the dead and dying trees in target areas of the Mountain. UCSF contracted Golden Gate Audubon Society to perform a bird survey to monitor the impact of the work on bird populations, and starting in 2020, Lepidoptera were added to the survey.
For bird monitoring, UCSF identified 13 plots throughout the Open Space Reserve, representing the different forest types in terms of vegetation health. Twice per month, a surveyor starts one hour after sunrise, visiting each of the 13 plots for a stationary period of 5-20 minutes per plot. Each bird seen or heard is documented, as is any important breeding behavior. We hope to find out how bird life changes in response to the vegetation changes. When trees are removed, how does that impact bird populations? How long does it take after major tree work before birds return for key activities like foraging and breeding? Comparing plots against each other, are there more birds returning to the areas with more native trees planted?
It’s still early to draw conclusions, and 2020 was a crazy year with many impactful factors such as the reduced foot traffic due to the pandemic, weeks of dangerously high smoke levels from nearby fires, and less rain. But one conclusion is already abundantly clear: Mount Sutro is a bird and butterfly oasis. Over the course of the two years, 61 bird species have been observed. Of those, 15 species have been confirmed to be breeding (observed carrying food, carrying nesting material, active nest, or dependent fledglings), and another 9 species observed possibly or probably breeding (observed in suitable nesting habitat, singing in suitable habitat, courtship behavior, breeding displays, or copulation).
In 2020, 20 species of Lepidoptera were observed on the Mountain, that’s out of 34 species found in San Francisco more broadly. Most remarkably, a rare Pale Tiger Swallowtail was observed on the Summit in May. This is the first observation of this species in San Francisco in 120 years! Most likely a vagrant, but future surveys will confirm if that is true.
You can read the full 2020 report below!