Matthew Costa stepped gingerly into a little pocket wetland near the Del Mar Fairgrounds. The squishy salt marsh is more than just a patch of habitat in the intertidal zone. “Just watch out,” said Costa, a postdoctoral researcher at the Scripps Institution of Oceanography, as he warned a helper. “Watch out for birds.” Endangered ridgeway rails like hiding in the pickleweed that covers the soft, moist ground nestled between train tracks and a busy Del Mar street.