To structure the life history of the Black Harrier, we defined three age classes fledglings - individuals that have left the nest up to 1 year old , sub-adults for individuals that are 1-2 years old, and adults - for individuals 2 years and older. Adult individuals are the only birds capable of breeding. In addition to an age structure, we defined a survival rate, which is the probability of survival in a given year, and a fertility, which is the number of offspring per adult bird per year (note that fertility usually refers to offspring per female, but we have redefined this to simplify notation later).