Although it is axiomatic that males and females differ in relation to many aspects of reproduction related to physiology, morphology and behaviour, relatively little is known about possible sex differences in the response to cues from the environment that control the timing of seasonal breeding. This review concerns the environmental regulation of seasonal reproduction in birds and how this process might differ between males and females. From an evolutionary perspective, the sexes can be expected to differ in the cues they use to time reproduction. Female reproductive fitness typically varies more as a function of fecundity selection, while male reproductive fitness varies more as a function sexual selection. Consequently, variation in the precision of the timing of egg laying is likely to have more serious fitness consequences for females than for males, while variation in the timing of recrudescence of the male testes and accompanying territory establishment and courtship are likely to have more serious fitness consequences for males. From the proximate perspective, sex differences in the control of reproduction could be regulated via the response to photoperiod or in the relative importance and action of supplementary factors (such as temperature, food supply, nesting sites and behavioural interactions) that adjust the timing of reproduction so that it is in step with local conditions. For example, there is clear evidence in several temperate zone avian species that females require both supplementary factors and long photoperiods in order for follicles to develop, while males can attain full gonadal size based on photoperiodic stimulation alone. The neuroendocrine basis of these sex differences is not well understood, though there are many candidate mechanisms in the brain as well as throughout the entire hypothalamo–pituitary–gonadal axis that might be important.

/pdf/sex-differences-in-the-response-to-environmental-cues-rawbf0u23w.pdf

Sex differences in the response to environmental cues regulating seasonal reproduction in birds

Appropriately timed integration of breeding into avian annual cycles is critical to both reproductive success and survival. The mechanisms by which birds regulate timing of breeding depend on environmental cue response systems that regulate both when birds do and do not breed. Despite there being multiple possible explanations for birds' abilities to time breeding appropriately in different environments, and for the distribution of different cue response system characteristics among taxa, many studies infer that adaptive specialization of cue response systems has occurred without explicitly considering the alternatives. In this paper, we make explicit three hypotheses concerning the timing of reproduction and distribution of cue response characteristics among taxa: adaptive specialization; conditional plasticity; and phylogenetic history. We emphasize in particular that although conditional plasticity built into avian cue response systems (e.g. differing rates of gonadal development and differing latencies until onset of photorefractoriness) may lead to maladaptive annual cycles in some novel circumstances, this plasticity also can lead to what appear to be adaptively specialized cue response systems if not viewed in a comparative context. We use a comparative approach to account for the distribution of one important feature of avian reproductive cue response systems, photorefractoriness. Analysis of the distribution within songbirds of one criterion for absolute photorefractoriness, the spontaneous regression of the gonads without any decline in photoperiod, reveals that a failure to display this trait probably represents an adaptive specialization to facilitate a flexible reproductive schedule. More finely resolved analysis of both criteria for absolute photorefractoriness (the second being total lack of a reproductive response even to constant light after gonadal regression has occurred) within the cardueline finches not only provides further confirmation of this interpretation, but also indicates that these two criteria for photorefractoriness can be, and have been, uncoupled in some taxa. We suggest that careful comparative studies at different phylogenetic scales will be extremely valuable for distinguishing between adaptive specialization and non-adaptive explanations, such as phylogenetic history as explanations of cue response traits in particular taxa. We also suggest that particular focus on taxa in which individuals may breed on very different photoperiods (latitudes or times of year) in different years should be particularly valuable in identifying the range of environmental conditions across which conditionally plastic cue responses can be adaptive.

/pdf/adaptive-specialization-conditional-plasticity-and-u0il94mwer.pdf

Adaptive specialization, conditional plasticity and phylogenetic history in the reproductive cue response systems of birds

Photorefractoriness in birds—photoperiodic and non-photoperiodic control

Recent advances in behavioral neuroendocrinology: Insights from studies on birds

At the crossroads of physiology and ecology: Food supply and the timing of avian reproduction

Introduction Food supply is widely accepted to be the primary ultimate determinant of the timing of breeding seasons of birds in seasonal environments (3, 21, 33, 40, 41). Food is unusual among ultimate factors in that it can also act as an important proximate regulator of reproductive physiology and behavior. Though this has been known for years, the quantity of information regarding proximate effects of food availability on avian reproductive systems lags far behind that for some other proximate factors, particularly photoperiod. In this paper, we discuss general issues regarding the effects of food on avian reproduction, with particular focus on wild taxa for which tracking changes in food availability is critical to reproductive success. We distinguish among several features of food availability: food amount, food type, food quality, and food predictability. We also discuss how proximate effects of food may be direct, through nutritional and energetic physiology (i.e., availability of fuel and building block molecules), or indirect, via a perceptual pathway (e.g., by seeing or physically handling the food). We also present a selective review of recent field and laboratory studies of wild taxa, including new data on several taxa of cardueline finches. Finally, we highlight avenues for future research in this important area of avian evolutionary physiology.

Effects of food availability on the reproductive system

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Effects of food availability on the reproductive system