Primarily sand with some silty sand or silt bases. Sizes of islands range from 0.5 acres to over 100 acres. Wildlife islands were created using dredged material. All older islands were colonized naturally. Some additions or newer islands were partially planted.
Most islands are so old that records have been lost.
In Texas, the Fish-eating Bird Survey collected annual data on all colonies, but does not distinguish dredged material or natural islands. Periodic data have been collected in Louisiana, Alabama, Mississippi, and Florida. Extensive Dredged Material Research Program data exists on these bird islands, including vegetation in and out of colonies, feeding information, and nesting populations and relationships.