Non-breeding adults or ‘floaters’ in bird populations

Bird populations can be subdivided into different stages based on age, with the main categories juveniles, sub-adults, and adults (Figure 1). The age at which birds physiologically reach sexual maturity is species-specific. Generally, this age correlates with body size: at one extreme small-sized songbirds start breeding in their first year, whereas large birds, such as albatrosses, may not begin breeding until the age of ten (Newton 1998). However, not all individuals enter the breeding population in the first year they become sexually mature. Birds that have passed the age of sexual maturity but do not attempt breeding are called non-breeding adults (Figure 1). Among a lot of species there is a difference between the average age of sexual maturity and the average age of first breeding (Becker & Bradley 2007). However, the non-breeder fraction of the population encompasses not only young adults that have not started breeding yet, but also older birds that for a certain reason (e.g., they loose their partner, territory or in years of scarcity) skip breeding for one or more breeding seasons. Moreover, adults of some long-lived species (e.g., some large raptors and albatrosses) regularly breed only every second year (Newton 1979). Furthermore, there are even examples of individuals within a population that never breed, and so never contribute to the next generation (Brown 1987). Another term commonly used for adults not breeding or holding a territory, often living a solitary life-style, is “floaters” (Newton 1998).