Bird Netting & Control in San Juan Bautista, CA

Local Context

Droppings Do More Damage to a 19th-Century Roof Than to a New One

Bird droppings are acidic, and what they land on decides how much that costs. San Juan Bautista has an unusual concentration of 19th-century roofs, parapets, cornices and porch structures built from wood, lime mortar and clay tile, materials that stain, soften and erode under uric acid in a way modern asphalt and metal do not. Nesting debris in a valley or a downspout on one of these buildings then holds water against wood that has already stood for well over a century.

The roosting pressure comes from two directions. In town, pigeons (Columba livia) use the ledges, cornices and covered walkways of the Plaza Historic District and the storefront row along Third Street, where there is shade, a flat perch and steady foot traffic dropping food. Out in the San Juan Valley, starlings (Sturnus vulgaris) and house sparrows (Passer domesticus) work the produce and storage buildings, where open bay doors, truss space and the underside of loading canopies supply cover and a food source in the same trip.

Exclusion is the only thing that holds. Birds return to a roost by habit, so cleaning without excluding just resets the clock. On a historic structure the constraint is that hardware has to go up without permanently altering the building, which means tensioned cable, existing structural attachment points and reversible fixings do the work a run of anchors would do on a warehouse. On the agricultural buildings the constraint is different: the exclusion has to survive equipment, dust and daily door cycles.