For Erika Zavaleta and Bernie Tershy, biologists and conservation scientists at the University of California in Santa Cruz, building a bird-friendly house meant using nontoxic building materials like reclaimed wood and straw-bale insulation. (Birds, which have fast metabolisms, are highly sensitive to toxins.) The family and their friends spent a day packing 200 bales into the walls of the 2,650-square-foot house—to the birds’ delight.