Biology Reference
In-Depth Information
Double-crested Cormorants roost,
fish, and congregate in the Central
Bay, but a great place to see them
up close is Oakland's Lake Merritt.
Look for them drying their wings on
the buoy lines that separate the wild-
life sanctuary from the navigable
part of the lake, or swimming and
diving together in groups while fish-
ing. (Max Eissler)
line water, it is far more common in the estuary waters than Brandt's Cor-
morant (
Phalacrocorax penicillatus
), which also fishes in the bay. An or-
ange throat pouch further distinguishes the species from the blue or grey
throat of Brandt's.
Agile swimmers, cormorants may hunt in groups of up to 600. They eat
small invertebrates and fish such as sardines, smelt, the Plainfin Midship-
man, and Yellowfin Goby. They can remain underwater for about 30 sec-
onds, diving from 5 to 60 feet deep.
Unlike many waterbirds, cormorants lack oil glands to waterproof
their feathers. To prevent their plumage from getting waterlogged, the
birds repair to roosts whenever they are not fishing. On rocks and bridge
platforms, they prop themselves up on stubby tail feathers, often perching
with wings akimbo to dry their feathers in the sun.
These cormorants are monogamous and breed from April to August.
Unlike most other birds, Double-crested Cormorant parents wrap their
webbed feet around their three to four eggs during incubation. Young are
ready to begin fishing on their own within 10 weeks.
In recent decades, double-crested cormorants have been enjoying a
major population rebound after nearly a century of human persecution.
First fishermen tried to eradicate them because the birds competed for
their catch; then pesticides took their toll. Laws protecting seabirds and
banning DDT helped the cormorant population to recover. Then the con-
struction of bridges, power lines, and other structures on bay waters gave
the birds new nesting sites. Today, all 12 Double-crested Cormorant colo-
nies in the bay are located on human-built or altered environments. More
than 10,000 Double-crested Cormorants may be counted in the estuary
during midwinter.
