Like other animals, cranes exist as natural populations that are dependent upon particular environmental conditions and that vary in population density between the absolute minimum numbers that have permitted survival to relatively dense populations that may approach or even temporarily exceed the carrying capacity of the habitat. Each species may also have an upper species-typical limit on population density, or "saturation point," which is independent of the carrying capacity of the habitat but which may be determined by such social adaptations as territorial requirements or individual distance characteristics. Within crane populations, individual birds or families remain within home ranges or geographic areas in which their movements are limited and within which they may spend much of their lives. Part of the occupied area may be defended from intrusion by conspecifics for varying periods; these areas of local social dominance range from individual distances to territories and probably play important roles in determining space requirements for crane populations. During periods of the year when breeding or wintering territories are not held, as during migration, dominance hierarchies serve to integrate the activities of the family and flock, and may likewise play important roles in population behavior and ecology. Interspecific differences in morphology and innate behavior patterns may further dictate specific foraging niches for each species, and these too may be of importance in regulating potential population sizes in cranes and in determining competition levels with other species.
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