San Francisco Environment Department

Springtime Stirrings


Although we are almost a month away from the beginning of spring, a number of resident bird species have already started nesting, and "summer" birds have started to arrive. I've already seen resident hummingbirds on nests in the Presidio, and this past weekend I watched a female Anna's Hummingbird painstakingly taking apart a spider's web to use as nesting material in the Botanic Garden in Golden Gate Park.

The two Great Horned Owl chicks in the western half of Golden Gate Park are now more than a month old, and don't seem to mind the human paparazzi that show up every day to take pictures of them. Great Horned Owls are fairly widespread in San Francisco, but are easily overlooked since they are not active during daylight hours.

The Great Horned Owl is a large bird, standing almost two feet tall, with a wingspan of almost four feet, yet it only weighs about 3 pounds. Although adult humans generally weigh at least 40 times as much as this owl, the owl has bigger eyes than we do. This helps it hunt in low light. Another adaptation that helps owls hunt is the placement of their ears – one ear is located slightly higher than the other, which gives it the acoustical equivalent of stereoscopic vision allowing accurate pinpointing of prey.

The pair of Great Horned Owls nesting on Strawberry Hill in Stowe Lake in Golden Gate Park last year raised four young – the nest in western Golden Gate Park this year has two chicks. The chicks were born almost a week apart, so one chick is currently much bigger than the other.

Meanwhile, the majestic Great Blue Heron has also started to nest at the top of the tall trees near the northern end of Stowe Lake. The largest of the North American herons, standing up to four and a half feet tall, with a wing span that can exceed six feet, this huge bird still seems quite at home in an urban setting. For the next six to eight weeks, they will be easily visible from the ground in front of the Stowe Lake Boathouse as they sit on their large nests made out of sticks.

Great Blue Herons have a varied diet, including fish, amphibians, small mammals, reptiles, insects and other birds. They often catch prey that seem way too big for their slender s-shaped necks. Despite their large size, Great Blue Herons only weigh about six pounds, allowing them to perform such feats as sitting on floating kelp in the ocean waiting for fish to swim by.

Great Blue Herons also nest in trees on the western shores of north Lake Merced. Earlier this month, there were already eight Great Blue Heron nests in place, prompting the Double-Crested Cormorants to also begin nesting. As of mid-February, there were 22 active Double-Crested Cormorant nests at Lake Merced.

The most common North American cormorant, this fish-eating bird is almost entirely black, with a bright orange-yellow face and throat, and whitish plumes during breeding season. Their nests can contain an odd collection of material – including such items as rope, plastic debris, deflated balloons, fishnet and even parts of dead birds.

Double-Crested Cormorants used to be considered primitive birds because, unlike ducks and geese, their feathers lacked water-repelling oils. After swimming, these birds often need to stand with their wings spread out to dry. Recent research has shown that the lack of oil is actually a clever adaptation – without the oil on their wings, cormorants are less buoyant and are able to dive deeper for fish.

Water birds aren't the only resident nesting birds at Lake Merced. The diminutive Bushtit has also started to nest – at Lake Merced in an oak tree (I watched them raise a family in a palm tree in Fort Mason last summer). This tiny bird builds an extended hanging nest that looks like a big sock, made from moss, spider webs and grasses, sometimes augmented by feathers and fur, and often camouflaged with plant matter.

Nesting pairs often get the assistance of other Bushtits to attend the nest and raise the young. These helpers are often adult males, which is unusual for birds nesting cooperatively.

When not on the nest, this gregarious songbird hangs out in flocks that usually include about 20 birds flitting from tree to tree. They often hang upside down on leaves, gleaning insects and spiders.

As some of our year-round resident birds start to nest, other birds begin to arrive from their wintering grounds. The tiny Allen's Hummingbird, which spends its winters in the highlands of Mexico, is the first "summer" visitor in San Francisco, usually showing up in late January.

Following close behind are Tree Swallows, which start showing up in early February. Last weekend I watched more than 30 of these hyper-active birds glide over Lake Merced, catching insects on the wing.

While we take it for granted that birds migrate, earlier generations of humans haven't always realized the reality of migration. Aristotle, while recognizing that many bird species migrate, postulated that swifts and swallows hibernate. This myth was perpetuated for more than a thousand years, with Europeans believing until the 19th Century that swallows spent their winters at the bottom of rivers.

Aristotle wasn't completely wrong about hibernation. In 1946, an American naturalist named Edmund Jaeger discovered that there was a species of bird that effectively hibernated in the winter. Research showed that this bird, the Common Poorwill, spends winters in a state of torpor in rock crevices, earning the name "the sleeping one" given to it by Native Americans of the Hopi Tribe. But out of the approximately 10,000 species of birds found worldwide, the Common Poorwill is the only one found to hibernate.

Tree Swallows spend their winters in Central America, the Caribbean and Mexico, with a very small number wintering in southern and central California. Their summer range goes further north than any other swallow, extending to northern Canada and Alaska. Males are bluish green on top, with white on the bottom. Females are more brownish green on top.

Tree Swallows weren't the only swallow species to show up this month. I saw my first Violet Green Swallows of the year this past weekend as well, back from Central America and Mexico, catching insects in Buena Vista Park. While some of the Tree Swallows I saw are likely to stay in San Francisco for the summer, a smaller number of the Violet Green Swallows will likely remain, since we have more Tree Swallows nesting here than Violet Green Swallows.

Adult male Violet Green Swallows show a green and violet lower back, with obvious white rump patches in flight. They like to nest in open woodlands. They are cavity nesters like Tree Swallows. A pair of Violet Green Swallows was once observed helping Western Bluebirds raise their young. The swallows guarded the nest and helped tend the bluebird nestlings until they fledged. Once the baby bluebirds had left the nest, the Violet Green Swallows used the nest to raise their own young.

© All pictures were taken by David Assmann, Deputy Director of the San Francisco Department of the Environment. Do not reproduce pictures without permission.

Bird List

Anna's Hummingbird
Great Horned Owl
Double-Crested Cormorants
Allen's Hummingbird
Tree Swallows

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