At some sites, many nests were spaced closely together, similar to the colonies or rookeries associated with certain modern sea birds. Evidence shows that certain nesting sites were used over and over again by various dinosaurs. In addition, not only were some dinosaur eggs arranged in a spiral pattern in the nest, but they had a particular vertical orientation, perhaps to minimize breakage. Other nests contained the fossilized bones of young dinosaurs in a wide range of sizes, indicating that the parents cared for the juveniles for an extended period before the young left the nest.