|The list of human disturbances impacting California's oak woodlands is textbook--urbanization, agriculture/grazing, fire suppression, and the introduction of invasive exotics. Also textbook is the outcome--habitat loss and degradation. What is unclear is how these disturbances affect the birds that make their livings in California's many species of oak. PRBO is partnering with local, national, and international organizations to study this question and has received funding from the new federal Neotropical Migratory Bird Act. The link uniting all such work is the conservation of high-priority oak habitats and oak-associated birds, with partners cooperating on: protection, restoration, and management of oak woodlands; inventory, monitoring, and bird research; and education and outreach. Locally, our partners are studying bird abundance, community composition, and the occupancy rates and reproductive success of birds using nest boxes in relation to agricultural intensity--"vineyardization." PRBO is currently studying the population health of species breeding in the remnant low-elevation oak woodlands of Sonoma Valley by indexing reproductive success. These are critical first steps in understanding how best to conserve oak woodlands and the birds they support.|
--Tom Gardali, Project Biologist, Terrestrial Ecology Division
The Regional Park is one of two sites where we study oak woodland birds; the other is nearby, behind the Sonoma Developmental Center. This nest monitoring study will teach us which birds are nesting here, at what densities, and how successful they are at producing young. In addition, we are working with The Nature Conservancy and Emily Heaton, a Berkeley graduate student, to study the effects of "vineyardization" on bird populations in oak woodlands of Sonoma and Napa counties and to examine the effectiveness of nest boxes placed in vineyards to attract and support Western Bluebirds and Tree Swallows. Our data will lead to management recommendations for public and private land in California's "wine country" and beyond, and add new information to The Oak Woodland Bird Conservation Plan, co-authored by PRBO and published by California Partners in Flight. But first, we must do the field work.
Throughout the morning I search for nests and map the bird activity I see. The nest of an Ash-throated Flycatcher eludes me, but I am luckier with an Oregon Junco and a Nuttall's Woodpecker. At around 10 AM I decide to do my nest checks, monitoring the status of nests already found: if I don't start now, it will be dreadfully hot by the time I finish.
Between nest checks I am stopped in my tracks by a Blue-gray Gnatcatcher's aggressive call. Nearby, in a blue oak, a pair of Gnatcatchers are mobbing a Western Scrub-Jay, giving their "wheezing" cry as loud as they can while repeatedly buzzing the head of the jay. The jay, being a smart nest predator, likely knows that this behavior is in defense of a nest. It searches for the nest until it can no longer stand the incessant mobbing and then flies off until the coast is clear-when it flies right back to continue searching. Of course, the gnatcatchers notice and quickly resume the raucous mobbing. This scene repeats itself numerous times, and I notice that every time the jay leaves, the gnatcatchers fly to a bunch of trees about 80 meters away.
This is suspicious, so I follow the gnatcatchers, taking care not to clue the jay into what I am doing. I sneak over and, pretending to be disinterested (though nothing is further from the truth), observe the gnatcatchers repeatedly going to a spot about 13 meters up, concealed from my view. Switching vantage points, I see the gnatcatchers carrying food to the nest! It is small and well concealed by leaves and branches of a blue oak.
In the pilot year of our study we have found 30 species of birds nesting in these oak woodlands-species ranging from the common (Oregon Junco, European Starling, and others) to the more threatened (e.g., Blue-gray Gnatcatcher). With few exceptions, most birds have had encouraging success at producing young this year, highlighting the importance of oak woodland habitat not just for one or two threatened species but for a broad, diverse group of birds.