The earliest oak tree fossils, found in what is now the eastern US, are dated from twenty-five million years ago. Since then oaks have diversified around the globe, where they provide a wealth of food and shelter in support of a variety of rich, diverse ecosystems.
Western Bluebird Male
Several types of oaks make the North State home. Valley oaks deck our waterways with the majestic art of massive trunks and branches. Live oaks provide green foliage year round. Black oaks finger into conifers, painting our lower mountains with streaks and fields of autumn golds. But in the bathtub ring around the Central Valley, the foothills below the cooling mountain altitudes but above historic waterways, in the land of summer heat and drought, it is the blue oaks that dominate and define much of the landscape.
Female Lesser Goldfinch Feeding Nestlings
As one of the few trees that can populate this seasonally harsh environment, blue oaks are home to a vast variety of life forms. Their summer leaves, gone bluish and leathery to protect from water loss, are often studded with starbursts of galls, the nests of tiny wasps and flies. Studies on the blue oak’s Midwest cousins have shown as many as five hundred fifty-seven species of caterpillars on a single tree. As larvae and pupae these insects are life-sustaining for the birds that pick them from leaf and twig and trunk, variably to raise their young, fuel migration, and survive the winter–nuthatches, titmice, wrens, kinglets, warblers, vireos, and orioles. When the insects morph into adults with wings, they similarly feed flycatchers, hummingbirds, bluebirds, swallows, and waxwings.
Oak Titmouse Approaches Nest with Insect for Nestlings
Downy woodpeckers join oak tree specialists–the theatrical acorn woodpeckers, beautiful Lewis’s woodpeckers, and trilling Nuttall’s woodpeckers–in picking beetle larvae, ants, and termites from the trees, and in making homes in the long-standing soft wood of dying oak branches–homes that are used for nesting by most of the birds already feeding on the oaks, plus a variety of owls and a falcon.
Great Horned Owl with Owlets Nesting in an Oak Tree
But none of this vitality even mentions the defining characteristic of oaks: acorns! Acorns are packed with proteins and fats and calories in general. Woodpeckers, band-tailed pigeons, scrub-jays, and turkeys are only some of their gourmands. Deer and bears feast on them. Gophers, mice, and ground squirrels chomp on fallen acorns, and in turn are eaten by bobcats, foxes, coyotes, and hawks. Yet more insects infest the acorns, and are eaten by reptiles, amphibians, and ground-feeding birds–sparrows, towhees, and quail.
California Scrub-Jay Atop an Oak with an Acorn
Acorns are also devoured by cattle, who prefer grazing and resting near and under the oaks, and gain weight faster in oak-rich grasslands.
As a foundation for so much life, oaks truly are Giving Trees, and the blue oaks have a particularly special place for us in the North State.
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