Birds Are Like Amoebas

I like to think of the range and population densities of bird species as three-dimensional feathered amoebas slowly inching their way across the surface of the globe.

The range, or distribution, of a species is rarely static for very long. Rather, it is continually changing, much like the movements of an amoeba, expanding in some directions and contracting in others in response to changing environmental conditions or other stimuli. The rate of range change is inherently variable from one species to another; some are inherently fast (relatively speaking), others inherently slow (perhaps even static).

Similarly, population density is not uniformly constant across the range of a species, but is constantly changing. Population density may be increasing (rising or thickening) in parts of the range where conditions are favorable for production of young and survivorship, while at the same moment in time it is declining (falling or flattening) in those parts of the range where conditions are less favorable.

For any given species, expansions and contractions of range and increases and declines in population density are all going on simultaneously, so it’s range and population density are constantly in a state of flux, in much the same manner as a shape-shifting amoeba. It is in trying to detect and monitor these amoebic shifts in range and changes in population density that amateur field ornithologists make their most useful (indeed, essential) contributions to science.