A population is a group of organisms of one species that interbreed and is (more or less) isolated from other such groups.
The degree in which a population grows or declines (the population growth rate) depends on several ‘demographic parameters’: the size of the population decreases through emigration and mortality and increases through immigration and births. When the magnitude of these demographic parameters is known, one can calculate how much a population will grow or decrease. Researchers use mathematical models for these calculations, known as population models.
In addition to calculating the population growth rate itself, population models can estimate the change in this growth rate corresponding to a change in one of the demographic parameters, and thus determine which demographic parameter is most influential. Demographic parameters may change through changes in biotic (e.g. food, predators, diseases) and a-biotic (e.g. temperature, rainfall) environmental factors - the driving factors of a population.
With the knowledge of the most influential demographic parameter of a population and of the driving factors behind this parameter, managers can take the right measures to adjust this parameter.
In many long-lived species, mortality of adult birds has a large impact on the population growth rate. Conservation measures such as banning hunting or decreasing traffic collisions can decrease mortality and thereby increase population size. In such species this will probably be more effective than creating new nesting sites.