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It is well documented that irruptive and nomadic migrants move in response to resources that are distributed unpredictably in space, time or both. Increasing evidence, however, suggests that irruptive and nomadic species may use seasonal timing mechanisms to prepare for migrations, despite the more facultative nature of their movements. Here we use data from free-living and captive Red Crossbills Loxia curvirostra, a typical irruptive nomad, to examine three hypotheses regarding control of facultative migration: (1) the facultative migration hypothesis, which states that both preparation and departure decisions are regulated by resource availability; (2) the seasonal preparation hypothesis, which states that preparation is initiated by seasonal factors (i.e. endogenous rhythms and/or photoperiod) but that departure decisions are dependent on local resource availability; and (3) the seasonal migration hypothesis, which states that both preparation and departure decisions are initiated by seasonal mechanisms and are independent of local food resources. Red Crossbills in North America are thought to make temporally consistent spring migrations in anticipation of conifer cone maturation. In this study, fat deposits of free-living Red Crossbills peaked in May and June, exceeding even winter deposits. In agreement with the field data, captive birds on a natural photoperiod with constant food and temperature showed a peak in fat deposition and activity levels in June. These findings are consistent with the seasonal preparation and the seasonal migration hypotheses and contribute to a growing literature that suggests that facultative migrants may prepare for movements in similar ways to seasonal migrants. [ABSTRACT FROM AUTHOR]

Trade-offs between self-maintenance processes can affect life-history evolution. Integument replacement and the stress response both promote self-maintenance and affect survival in vertebrates. Relationships between the two processes have been studied most extensively in birds, where hormonal stress suppression is down regulated during molt in seasonal species, suggesting a resource-based trade-off between the two processes. The only species found to differ are the rock dove and Eurasian tree sparrow, at least one of which performs a very slow molt that may reduce resource demands during feather growth, permitting investment in the stress response. To test for the presence of a molt-stress response trade-off, we measured hormonal stress responsiveness during and outside molt in two additional species with extended molts, red crossbills (Loxia curvirostra) and zebra finches (Taeniopygia guttata). We found that both species maintain hormonal stress responsiveness during molt. Further, a comparative analysis of all available species revealed a strong relationship between molt duration and degree of hormonal suppression. Though our results support trade-off hypotheses, these data can also be explained by alternative hypotheses that have not been formally addressed in the literature. We found a strong relationship between stress suppression and seasonality of breeding and evidence suggesting that the degree of suppression may be either locally adaptable or plastic and responsive to local environmental conditions. We hypothesize that environmental unpredictability favors extended molt duration, which in turn allows for maintenance of the hormonal stress response, and discuss implications of a possible trade-off for the evolution of molt schedules. [ABSTRACT FROM AUTHOR]

Abstract: How birds use environmental cues to time breeding, migration and molt has been the subject of intensive study for nearly 90years. Most work has focused on seasonal breeders; opportunistic breeders have been presumed to differ fundamentally from seasonal taxa in ways that facilitate coping with unpredictable environments. Understanding patterns and mechanisms of opportunists’ responses to environmental cues can reveal the extent to which different environments require specialized adaptations of cue response systems. In this review we will present our perspective on how patterns and mechanisms of environmental cue response of three groups of opportunists—zebra finches, crossbills and Darwin’s finches—compare with seasonal breeders. Long-standing predictions regarding tonic activity of the hypothalamic gonadotropin-releasing hormone system have been confirmed in at least some opportunists. However, opportunists resemble seasonal breeders in some surprising ways, illustrating basic similarity among taxa facing very different timing challenges. For instance, many opportunists completely regress the gonads outside breeding times, rely on initial predictive cues (both photic and non-photic) to regulate timing and rate of reproductive development, and in some cases even appear to display internal changes in responsiveness to environmental cues (i.e., cycles of reproductive refractoriness and sensitivity). Although advantages of unrestricted temporal flexibility are intuitively clear for animals coping with unpredictable habitats, the available data on these opportunists indicate that in all but the most extremely capricious situations the advantages of flexibility may be at least partly outweighed by contrasting advantages of following a reliable temporal schedule. [Copyright &y& Elsevier]

Abstract: Appropriate timing of transitions between annual cycle stages (reproduction, plumage molt, migration) is critical to fitness for birds living in temporally varying environments. Environmental cue response systems permit birds to orchestrate these transitions. This paper focuses on how photorefractoriness and one neuroendocrine correlate of it (GnRH system plasticity) have evolved to permit appropriate timing of the transition from breeding to plumage molt. Photorefractoriness is defined by two criteria. Criterion 1: photoinduced gonadal regression occurs without any decline in photoperiod. Criterion 2: photoinduced gonadal regression cannot be reversed by increased photoperiod, even continuous light. Through a comparative approach we show that: (1) Loss of Refractoriness Criterion 1 and of GnRH system down-regulation appear to represent adaptive specializations favoring highly temporally flexible or continuous breeding, (2) Refractoriness Criteria 1 and 2 are not always concordant, and Criterion 2 in particular is not well-correlated with degree of temporal reproductive flexibility, (3) occurrence of some cue response traits are better-explained by phylogenetic relationships among taxa than by current reproductive schedules (seasonal, flexible, opportunistic), (4) substantial temporal flexibility can be achieved in a variety of ways besides adaptive modifications of refractoriness, such as relaxation of long-day requirements for reproductive development, and enhancement of non-photic cue responsiveness. These comparisons also highlight fundamental similarities between some of the most opportunistic species and seasonal breeders, such as an autumn reproductive hiatus during molt. Even in the face of substantial environmental unpredictability, selection may often strongly favor regular scheduling of particularly critical life cycle stages such as plumage molt. [Copyright &y& Elsevier]