In February, House Sparrows (Passer domesticus) begin busily building nests in roof cavities, old woodpecker holes, and in birdhouses here in Tucson, Arizona. While I was wandering around the Tucson Botanical Gardens in February of 2006, I noticed this industrious male House Sparrow collecting a mouthful of feathers to use for lining his nest.
He then flew off to an old woodpecker hole in a Saguaro (Carnegiea gigantea) cactus where he and a female House Sparrow were constructing a nest. This Saguaro had sprouted a tiny new arm under their nest hole, making a nice perch for the House Sparrows.
In urban areas, like where the Tucson Botanical Gardens are located, House Sparrows are more common than most native birds and many old holes in Saguaros will be occupied by non-native cavity dwellers like European Starlings (Sturnus vulgaris) and of course House Sparrows.