Stress responses in birds

During the last decades more and more attention has been focused on the fun ction of the stress hormone corticosterone in free-living birds. Normally b irds have low circulating levels of corticosterone throughout the year, but when exposed to a stressor they will, however, rapidly increase the cortic osterone secretion, and thereby adjust their phyiology and behaviour to the new situation. To avoid chronic effects of high corticosterone levels the secretion rate returns to basal as soon as the alternate behaviour has been established or when the individual in some other way has succeeded to cope with the stressor. In short, elevated corticosterone levels will redirect behaviour away from reproduction towards survival, for example by stimulati ng dispersal and/or foraging behaviour. However, a bird must be able to mod ulate the adrenocortical stress response as one and the same response will have different biological effects in different ecological situations. So th at at a certain time the bird should react rapidly to a stressor, whereas i n another situation it should have a suppressed response to the same stress or. Thus, it is the adrenocortical stress response at a particular time, an d not the basal corticosterone level, that affects an individuals' fitness and consequently the target for evolutionary forces. This paper reviews stu dies dealing with the adrenocortical stress response in birds. A major hypo thesis is that seasonal modulations of an acute stress response should be f ound basically in species with short breeding seasons and/or in species bre eding in severe environments. Although the patterns differ between species, most studies support this hypothesis. As a stress induced elevation of cor ticosterone levels can disrupt reproductive functions and reduce reproducti ve succees birds not only modulate their adrenocortical stress response on a seasonal basis, but also within or between populations. Among populations in a species breeding in a wide variety of habitats those breeding in a mo re unpredictable and severe habitat normally are less sensitive to environm ental stressors. Consequently individuals in such a population have a suppr essed adrenocortical stress response to an acute stressor. There are, howev er, some interesting exceptions to this rule. The selective forces behind t hese differences are so far unknown. In addition there are normally large i nter-individual variations, as well as sex related differences, in the adre nocortical stress response. It has been suggested that individuals with lar ge fat depots should be the ones to be more resistent to acute stress as th ey are less likely to be affected by, for example, severe weather condition s. Furthermore, it has been hypothesised that the sex investing most in par ental responsibilities should reduce the adrenocortical stress response to avoid induction of a corticosterone mediated escape behaviour. Results from different studies are however not always consistent with these hypotheses, and the bases for individual variations are today unclear. The adrenocorti cal stress response is also discussed in relation to social hierarchies and migratory strategies.