Properties of populations include age and spatial distribution, both of which emerge from actions and properties of individuals and can affect population dynamics, the changes in populations and metapopulations over time and space. The age structure of a population is described and analyzed to determine how it affects the growth of a population. The various aspects of spatial structure of populations, which also arise from characteristics and behaviors of individuals, are examined and used to develop the concept of a metapopulation. Finally, this book discusses how individuals perform behaviors that can lead to other properties observed at the population level, such as birds flocking. The advantages and disadvantages to flying in flocks are evaluated, as are the mechanisms by which flocks of birds are maintained and how they respond to an attack by a predator.