A highly heterogeneous, worldwide order of field, marsh, and aquatic birds that may be closely related to the shorebirds and their allies (see illustration). Diagnosis of this order is difficult because of its great diversity; some taxonomists divide the gruiforms into a number of separate and possibly unrelated orders. Some families, such as the Turnicidae and the Pedionomidae, are considered by some workers to be members of the Charadriiformes. Many of the families contain a small number of species and live in restricted areas.