Sunlight glistens on the wings of a huge flock of sandpipers as they wheel over the mudflats of San Francisco Bay. The Bay is an important wintering area for these and other shorebirds such as the willet, western sandpiper, and plovers that breed in a variety of arctic and temperate habitats. More than 900,000 shorebirds use the Bay sometime during the year, some as a way station along the Pacific Flyway, one of the four major north-south route of travel for migratory birds in the Americas, extending from Alaska to Patagonia.
Every year, migratory birds travel some or all of this route in the spring and fall, following food sources, heading to breeding grounds, or en-route to overwintering sites. Some species such as dunlin, willet, and marbled godwit winter on San Francisco Bay, while other species rest briefly and then continue their migrations. The pectoral sandpiper is one of the long-distance migrants–it makes an 18,000-mile round-trip journey on its annual circuit from its Siberian breeding grounds to its wintering grounds in South America. Shorebirds are thought to use a variety of methods to guide their migration, including the position of stars, the earth’s magnetic field, wind, photoperiod, and even olfactory cues.
While the spring migration is a rush to the breeding grounds to make maximum use of the short Arctic summers, fall migrations are generally more relaxed. Usually the failed breeders are the first to head south, followed by the breeding adults and finally the juveniles depart. The later start gives the juveniles a little more time to build up reserves for the long flight south. In the Bay Area, shorebirds start appearing around July, and those that are continuing their journeys move through by November. So September and October are prime times for bridwatching: lots of birds have arrived, but the weather is still often sunny and warm.
Shorebirds are a diverse group that includes sandpipers, plovers, stilts, avocets, snipes, oystercatchers, turnstones, and phalaropes. They generally have small bodies, lack webbing on their feet, and have long, thin legs which are good for wading and foraging in shallow water. Their bills come in a variety shapes and sizes specifically adapted to feed on the types of invertebrates found in mudflats and marshes.
Overall, the numbers of shorebirds have been falling over several decades. The planned extensive restoration of former salt-ponds in south San Francisco Bay should provide improved habitat and prey to fuel their migrations.