Flexibility in annual cycles of birds: implications for endocrine control mechanisms

Abstract

Life cycles of birds and other vertebrates are composed of series of life history stages each with unique combinations of morphological, physiological and behavioral characteristics. For example, in the white-crowned sparrow, Zonotrichia leucophrys, the nonbreeding stage (winter), vernal migration, breeding, moult and autumn migration stages occur in a fixed and repeated sequence where each cycle is 1 year. The sequence of stages cannot be reversed. Transition from one life history stage to the next and the duration of each stage are dependent upon a combination of genetic factors and environmental cues. The latter include the annual change in photoperiod and the former may involve endogenous circannual rhythms. All vertebrates also express the emergency life history stage in response to perturbations of the environment that allow individuals to cope with the unpredictable. Each stage has a unique repertoire of sub-stages (physiological and behavioral, and to a lesser extent morphological), which can be expressed in any sequence or combination to give the state of the individual at any point in its life cycle. This state is presumably maximally adapted to the environmental conditions at that time. Although the sequence of life history stages appears to be innate, the rate of transition from stage to stage, and the expression of sub-stages can be modified by the local environmental factors and, particularly, by social cues. These environmental cues acting on the phenotype result in neuroendocrine and endocrine secretions that regulate development of the life history stage, its onset once mature capability has been attained, and then terminate it at the appropriate times. The environmental cues (from the physical and social environment) impart a strong experiential component. Because, there is a set number of life history stages and their sub-stages, there is a finite number of states that can be expressed in response to the environmental variation experienced by the individual. The more life history stages a phenotype expresses, the less flexibility is there in the overall timing of these stages owing to the time taken to develop one stage and terminate the last (about 1 month). However, many phenotypes have increased flexibility in their life cycles by overlapping some life history stages (i.e., with overlapping mature capability of two or perhaps even more stages). Another potential strategy is to dissociate some components of a life history stage so they are expressed at other times of year thus spreading out potential costs associated with that life history stage. Examples of both overlap and dissociation of life history stages are given including implications for hormonal control mechanisms.