Cactus wrens (Campylorhynchus brunneicapillus) build bulky, domed nests with a tunnel-shaped entrance in cactus (especially cholla) or thorn trees. Although conspicuous, their nests are inaccessible to predators because they are surrounded by sharp thorns (hence the other common name for the cactus wren, desert thorn bird).
A cactus wren nest is constructed of coarse grasses and plant fibers. Inside, the 2 to 5 white to pale pink eggs, heavily spotted with brown, are incubated by the female. While the female broods the eggs, the male builds several extra nests. Once the original eggs have hatched, the female chooses another nest in which to lay a second clutch of eggs. The male tends to the newborn chicks while the female incubates the second (or third) batch of eggs.
Why do cactus wrens build several nests, called dummy nests? The answer is not certain, however, having a new nest available as soon as the first clutch of eggs hatches enables the cactus wren to raise more young. Extra nests may fool predators. Adult cactus wrens also use unoccupied nests to roost at night, providing shelter and protection.
Walking along an arroyo near Casa Paloma 2 in Green Valley AZ, where this cactus wren and nest were photographed, almost every cholla seems to harbor a cactus wren nest.