Goose Island's proximity to oyster reefs explains its attraction to waterbirds. The reefs surround the island and offer an abundant menu for many shorebirds. The island might even be an oyster reef itself, washed over with shell and sand, which allowed plants to take hold. The fishing pier is built on an oyster reef extending into the bay.
Oyster reefs form when the mollusks cement themselves to a hard foundation. Oysters then create a hard shell from the calcium in the seawater. Barnacles, mussels and other mollusks stick to the oysters' shells, creating an amplitude of nooks and crannies. The crevices provide hideouts for crabs, snapping shrimp and nematode worms. Blue-green algae grow on the shells.
More oysters and mussels attach themselves at every angle to the group. The reef grows upward and outward, depending on the nutrient flow of the water. Blue crabs dig holes in the mud near clumps of oysters, and tiny fish dart around the convoluted structure.
All this activity attracts predators, one of which is the American oystercatcher. This black-and-white bird has a specialized orange bill shaped like a chisel. The oystercatcher inserts its bill into a partially opened oyster, cuts the muscle and eats it. It pries mussels and clams off the reef for food. Herons, egrets and other shorebirds eat crustaceans and other fauna inhabiting the reef.
Birding around Goose Island State Park offers rewards at any season, but the best scenario for seeing a maximum number of species and peak numbers of individuals occurs with low-tide conditions during spring and fall migration. At low tide and when the wind has pushed water across the tidal flats, mud-dwelling prey are exposed to the surface, a situation that provides a banquet for feasting shorebirds. During spring and fall, the augmented mix of avian species includes year-round residents, winter visitors just arriving or on the verge of departure, and transients pausing to rest and refuel between Latin America wintering grounds and breeding areas to the north.
Fall provides the challenging opportunity of studying birds' field marks after they have lost their distinctive breeding plumage. In spring, some species, like ruddy turnstones, allow South Texas birders to see alternate or breeding plumage before they head north to raise families. Both spring and fall migrations depend on Mother Nature's temperament. With 1999's mild fall, many birds took their time moving down the flyway and appeared much later than usual. In spring, birders watch weather reports for storms, which interrupt north-bound migrants and create impressive "fall-outs" of birds at places like Goose Island.
Winter visitors include many types of ducks, such as pintails, gadwalls, teals and redheads. Sandhill cranes fly overhead while kestrels perch on utility lines. Shorebirds return from the north to spend the mild winter feeding in the shallow waters around Goose Island.
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