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On the surface, migrations might be defined simply as organisms moving from place to place on a seasonal or annual basis, but a more detailed consideration of what does-and what does not-constitute a migration quickly reveals that this is not an easy distinction. In fact, certain nonanimal organisms (eg. plants or fungi) make movements such as seasonal dispersal of seeds via water or air that could be considered as "migration" under some definitions. Even among animals, great variation in the timing, distance, and motivation underlying movements makes general definitions of migration challenging. Early efforts to formally define migration were arguably quite effective for their simplicity, and they may be very relevant for conservation policy. For example. "true migrations" according to Landsborough Thomson, an ornithologist, were defined as "chances of habitat, periodically recurring and alternating in direction, which tend to secure optimal environmental conditions at all times." Such optimal conditions might include milder air, water, or soil temperatures, availability of food or water, lack of predators or diseases. suitable habitats for breeding (e.g., nesting substrates for birds or calmer waters for whale calves), or some combination of the above. Importantly, this definition of migration does not specify or discriminate based on distance traveled or the types of organisms that qualify. Instead, the emphasis in Thomson`s definition is placed on 1) a change in habitat, 2) a seasonal phenology [relationship of biological phenomena with climate] and 3) a "to-and-fro passage." Certainly, many important and familiar migrations such as those made by songbirds, waterfowl, shorebirds, and seabirds that winter farther south and return each spring to breed at higher latitudes would be included in such a definition.

However appealing, though, this definition could exclude many stunning and extensive animal movements that serve equally important ecological functions. In particular, less spatially or temporally predictable migrations such as the seminomadic circular roamings that were once exhibited by American bison, or those that require multiple generations for the return trip as is the case for many species of moths or butterflies, including the classic monarch butterfly journey, could arguably fail to meet these criteria of migration. Conversely, in the open ocean, billions of organisms ranging from plankton to squid to sharks make daily to-and-fro movements through the water column to the surface and back down again in order to exploit optimal conditions. These could qualify as migrations under Thomson`s definition, yet such movements are clearly of a different variety than those that occur only once per year and span continents, especially in the context of conservation policy and management.