Swallows spend their days mostly in the air, swooping around in search insects. The biggest concentrations of swallows occur where there are swarms of flying insects, especially over water. Therefore, swallows are less numerous in the desert than in many other habitats. Typical nesting sites include cliff faces, buildings, and bridges. Their gourd-shaped nest is made of mud, feathers and vegetable matter. The Cliff Swallow’s highly colonial life style has led to the evolution of some complex behavioral traits. For instance, they parasitize neighboring nests both by laying parasitic eggs and by moving eggs from their own nest into others; they have a sophisticated vocal system for distinguishing their own young; and they observe each other’s foraging success and learn from other colony residents the locations of food.