Wildlife Spotlight: Forster's Tern (Sterna forsteri)

By Peter Winch
Published: June 2007

Tern in flight: Credit: Ron Story

The English used to call terns “sea swallows”, and as you watch these graceful white birds fly deftly in the blustery summer winds of the Farallones Marine Sanctuary, its easy to see how they got this name.  Forster’s Tern is one of the many tern species that frequent the sanctuary in late spring. 

Terns are small white seabirds with pointed wings and bills.  In the summer, Forster’s Tern is distinguished by its medium-sized, black-tipped, orange bill, a black cap that comes down to and includes the eyes, long orange legs, and silver primaries that are longer than the inner wing.

In winter the head lacks the black cap, but has a black patch on the side of the head covering the eye and ear. The tern’s bill is black in the winter.  Also, immature birds share the same plumage as winter birds.

If you are listening for terns, they have two distinct calls: a low-pitched “tzaap” and a short “keer.”

Distribution

Forster’s Terns are found mainly in the wetland and estuary areas of the Sanctuary such as Tomales Bay and Bolinas Lagoon.  Seeing them on the open ocean is rare, with only one recorded sighting at the Farallon Islands in 1986.  However, there is a large breeding population in the San Francisco Bay.  Although this colony is technically not in the Sanctuary, the population base travels in and out of Sanctuary waters.  They can be seen year round in the Bay, though many winter in Southern California and Mexican estuaries.

Feeding

Forster’s terns feed by skimming food off the surface and by plunge diving into the water column.   They eat small fish, amphibians, invertebrates, and even some carrion.

Reproduction

Courtship and nesting in The Bay starts in mid May.  Forster’s Terns nest in simple scrapes on isolated islands and discontinued dykes in the bay, but also like to build floating nests out of vegetation (this is more a characteristic of East Coast and Great Lake populations).   Peak Laying occurs at the beginning of June and the average clutch size is 2.6.

Incubation in Forster’s Terns lasts 25-32 days and the chicks are born with their eyes open and usually stay in the nest for the first day or two.  Their parents feed them until they can fly which takes about 27 to 30 days.  If the chicks do not survive the adults will often re-lay eggs.

Behavior

Forster’s Terns are very social within their own species, but are known to be very aggressive towards other species.  Yet because they nest with other terns, they have been observed feeding the chicks of different tern species by mistake.  Forster’s terns are most vulnerable during nesting, when rats, cats, coyotes, skunks, raccoons, crows and hawks can take the eggs, chicks and occasionally the adults.  Eastern populations have also been witnessed having their floating nests destroyed by spawning carp.

A great place to see Forster’s and other tern species this summer is right by FMSA’s Visitor Center at Crissy Field.  A small colony has been roosting on an old pier by our building and delighting the staff with their aerobatics and tenacious social behavior.