Floater strategies and dynamics in birds, and their importance in conservation biology: towards an understanding of nonbreeders in avian populations

Over the past 40 years, the study of animal populations has shifted from a relatively simple science that assumed the most crucial regulating mechanisms were the intrinsic properties of breeders, to a more complex and refined discipline that reflects a greater understanding of populations and their dynamics. Part of this shift has been the explicit recognition of the importance of a previously overlooked nonbreeding component of animal populations, the so-called floaters. Here we review and discuss the various effects that pools of floaters can have on the structure, dynamics and persistence of bird populations. Under some circumstances floaters can be both active intruders that can alter the breeding performance and behavioural traits of territory owners, as well as secretive presences reducing the extinction risk of populations. Floaters are crucial elements in conservation biology, and knowledge of their behaviour and dynamics in avian communities can be a key factor in the success of conservation strategies. Firstly, larger pools of floaters are associated with more stable breeding populations, even in cases where breeding output could be impacted through interference by floaters. Secondly, the effects of habitat loss and mortality rates are frequently ignored in areas where floaters settle, and consequently conservation measures tend to overlook these sites, potentially leading to increased risks of mortality among floaters. Thirdly, because an increase in the proportion of juveniles in the breeding component of a population may be because of increased pre-adult and/or adult mortality rates, a change in the age of breeders has the potential to function as a warning of an imminent decline in breeding populations. Most population studies have only considered the breeding components of animal populations, so it is time now to focus attention on the floater component, wherein the solution to many conservation issues may be found.