Fall bird migration is underway across the San Francisco Bay Area and, as if on cue, the native plants that support traveling flocks are setting fruit. As the season changes and food sources become scarce, insect-eating birds, as well as our local gray fox, coyote, deer and chipmunks, supplement their diet with a variety of berries.
Native plants have co-evolved with birds and mammals in many intricate ways for pollination and dispersion. Whether it be aided by gusty winds, flowing waterways or on the wings and coats of migrating animals, seed dispersal is a process that enables the immobile plant to undergo their own migration across generations.
Often showy or brightly colored, the fleshy pericarp (outer most layer covering the seed) catches the eye of migrating birds. Once consumed, this outer layer is digested, and the seed is successfully excreted in a new location. Seed physiology and chemistry evolved to become compatible with the digestive tracks of dominant dispersers in the area and vice versa. For example, native leafy mistletoes, a group of obligate hemiparasites (plants that receive a portion of their nutrients from other plants) sport seeds that are wrapped in an adhesive-like coating called viscin. After quickly passing through the bird’s digestive track, the seeds are effectively glued to the host plant and, following germination, able to initiate parasitism.
Although nutrition fluctuates across species, winter berries tend to be low in carbohydrates and protein but high in energy sustaining fats needed for birds to continue their long journey. When clusters of ripe berries are spotted, birds move fast as competition can be fierce, sometimes stripping the cluster clean before moving along. American Robins and Cedar Waxwings can consume up to their body weight in berries, such as Toyon, each day.
In studying these plant-animal interactions and understanding their role in the local ecology, Midpen staff are able to make informed restoration and invasive plant mitigation decisions. And by supporting the health of native plants on Midpen lands, we are able to support the migrating populations that rely on them.
The following plants are commonly found in Midpen preserves and support resident and migrating birds with their berries.
Toyon: is a prominent member of California’s chaparral and woodland ecosystems and easily spotted by its showy red berries that form in October through December. Commonly referred to as Christmas Berry, this plant is a favorite of cedar waxwings, purple finches and American robins.
Snowberry: is high in saponins and can be toxic to animals, including humans. However, various thrushes and robins tolerate this chemical and will eat the berries when food is scarce.
Poison Oak: although loved by few humans, poison oak plays an important role in our ecosystem. The California Towhee is known to not only eat poison oak’s white berries, but they also build their low-lying nests in poison oak patches.
Coffeeberry: is a member of the buckthorn family and, as the name hints, has seeds that resemble coffee beans. California thrasher and western tanager eat these red &black berries once ripe in early fall.