Even here, in the scorching summer heat of Altadena, Seriina Covarrubias’ front yard feels cool and inviting under the dappled shade of a magnificent elm tree. “I thought it was going to take longer for a natural habitat to materialize,” Covarrubias says of her two-year-old garden, which is filled with fragrant coastal scrub. “The birds feel so comfortable here they made a nest on the ground,” she adds, reaching down to reveal a black phoebe’s nest beneath a foothill sedge (Carex tumulicola). Sages and ceonothuses thrive under the canopy of an elm tree. (Mariah Tauger / Los Angeles Times) More than thirsty birds have flocked to her garden since she…