the island hosts the san francisco bay’s largest nesting colony of caspian terns. the terns are relatively recent arrivals, first recorded in the south bay in 1922 and nesting on brooks island since around 1980. they now occupy much of the man-made sandspit that stretches west from the north side of the island. the terns feed on fish from areas around the golden gate and san pablo bay. however, the tern population has been decreasing, apparently due to reduced nesting area. the sandspit is being reduced by erosion from the bay waters, and also by the encroachment of invasive plants such as ice plant and marguerite daisies. the terns face strong competition at brooks island from a growing population of california gulls. the gulls "compete with the terns for nesting sites, steal fish from them, take their eggs, even chicks if they're small enough".