Search Results

This chapter first reviews the evidence for interactions between life-history stages, centering this around reproduction itself: the intention is to put reproduction in the context of the complete life-cycle. It considers how the wintering and pre-breeding period, including spring migration, can influence reproductive decisions, and in turn how reproductive decisions can influence subsequent post-breeding life stages such as molt, fall migration, and over-winter survival. It looks at the costs of reproduction, simply as a more specific example of general carry-over effects, especially from a mechanistic point of view, with potentially common underlying mechanisms. The chapter argues that short-term energy or nutrient “debts” and resource-allocation trade-offs provide unsatisfactory models for long-term carry-over effects or costs of reproduction; instead, it emphasizes potential “non-resource based” mechanisms.

This introductory chapter provides an overview of the book's main themes. This book is primarily about physiological mechanisms, but it also addresses the specific question of what we know about the physiological, metabolic, energetic, and hormonal mechanisms that regulate, and potentially determine, individual, or phenotypic, variation in key reproductive life-history traits, trade-offs between these traits, and trade-offs and carry-over effects between different life-history stages. Initially, it focuses on the avian reproductive cycle (from seasonal gonadal development, through egg-laying and incubation, to chick-rearing), and then it expands this view to consider reproduction in the broader context of the annual cycle and over an individual's entire lifetime. Throughout the book develops two major themes: that we need to consider reproductive physiology and ecology from a female perspective and that we need to consider the causes and consequences of individual (phenotypic) variation in reproductive life-history traits.

Clutch size is generally considered to be one of the most important determinants of reproductive success and lifetime fitness in birds. It is also perhaps the most well-studied life-history trait in birds, though many reviews have focused on interspecific variation. Clutch size sets an upper limit on brood size, and in single-brooded species, clutch size will therefore determine the maximum annual fledgling productivity. This chapter identifies the physiological mechanisms underlying individual, phenotypic variation in clutch size. Topics discussed include individual variations in clutch size and clutch number; why clutch size varies among individuals; selection on clutch size; and physiological mechanisms of clutch-size determination.

This chapter focuses on the regulation of female reproduction and on the physiological regulation of ovary and oviduct function during egg production. Since female reproduction is highly seasonal and occurs in response to seasonal environmental variability, it considers what we know about how information encoded in photoperiodic and non-photoperiodic cues (food, temperature, and social factors) is integrated by the neuroendocrine system and transduced into an (hormonal) output that ultimately determines ovarian function and egg formation. It briefly reviews important aspects of the neuroendocrine system that regulates gonadal function (i.e., the hypothalamic–pituitary–gonadal [HPG] axis). It compares data for males and females in terms of HPG-axis function and reproductive development to provide evidence that males cannot be considered good models for females, and highlights some discrepancies, inconsistencies, and gaps in our knowledge with regard to control of female reproduction. Throughout it attempts to highlight and describe mechanisms that might generate or constrain individual variability in female reproductive traits.

This concluding chapter summarizes key themes and identifies four important, overriding concepts that have emerged from this book. First, a common conclusion of many ecological and evolutionary studies is that there are intrinsic differences between individuals such that certain high-quality females lay many, large eggs, with a relatively early laying date, rear large broods, and recruit more offspring, while also having higher future fecundity and survival, thus apparently “avoiding” underlying the trade-offs that would predict negative correlations between these traits. Second, most individuals do not reproduce successfully; rather, the majority of individuals do not reproduce at all, while a few individuals contribute a large proportion of the offspring making up the next generation. Third, at the physiological level, different individuals are likely to adopt very flexible, “individually variable” strategies to deal with the costs and benefits of investment in self-maintenance or offspring when faced with increased workload or increased parental effort. Fourth, reproduction needs to be viewed as only one part of an individual's integrated annual cycle and its complete life-history.

This chapter argues that the timing of the initiation of a single breeding event, or the initiation of the first of multiple breeding events within the same breeding season, is completely dependent on the female-specific reproductive process of timing of egg production and egg-laying. It discusses how early-season events are critical in determining timing of breeding; fitness consequences of timing decisions; selection on timing of breeding; sex-specific response mechanisms for timing of breeding; physiological mechanisms associated with photoperiod (day length) as a proximate factor; physiological mechanisms associated with temperature as a proximate factor; and physiological mechanisms associated with food availability as a “proximate” factor.

This chapter focuses on individual variation in parental care, the relationship between individual variation and maternal fitness, and the physiological mechanisms underlying this variation. For example, does individual variation in incubation behavior (e.g., the timing of onset, intensity, constancy) determine the number and quality of chicks at hatching? Or, does individual variation in chick-provisioning or other components of parental investment (e.g., brooding, guarding, feeding) explain the number and quality of chicks that fledge and ultimately are recruited to the breeding population? If there are fitness consequences of variation in parental care, then what are the physiological mechanisms that underpin individual variation in parental effort?

This chapter deals with variations in egg size and egg quality. Egg size is a highly variable life-history trait, with up to twofold differences in egg mass among individual females within a population. Larger eggs contain absolutely more major egg components (shell, albumen, yolk) and absolutely more of several minor egg components (maternally derived antibodies and antioxidants), and, in this regard, egg size is a good proxy for egg quality. Many studies assume that high-quality females produce large, high-quality eggs, but it is equally plausible that individual females produce eggs of the optimum size and quality for their phenotype or genotype. Either way, the aspects of a female's phenotype that would determine maximum or optimum egg size are unknown, although these are not primarily factors such as age, experience, body condition, or mate quality that continue to be the focus of much current work.

In this chapter the diverse mating systems exhibited by birds are described and explained. The chapter begins with a discussion of anisogamy and resulting behavioural differences exhibited by male and female birds. Sperm competition, sperm storage, and delayed fertilization are discussed and their consequences in terms of reproductive behaviours and systems are explained. Courtship systems and behaviours are discussed. Social monogamy, polygamy, and lekking behaviour are examined and examples of field research are given to support offered hypotheses. Bird song is considered in some detail through discussion of the function of song and of the genetic, neurological, and physiological control of singing. Particular attention is given to the impact of noise pollution on singing behaviour. The chapter concludes with a discussion of chick rearing including brood size management.

This chapter considers the egg from its conception, through laying and incubation, to hatching; and chicks. It begins with the genetic determination of sex in birds, the resultant secondary sexual characteristics, and associated reproductive behaviours. The chapter then considers the structure and function of the egg. The evolution of clutch size and the trade-offs related to food availability and predator risk that birds make when optimizing clutch size are discussed. Egg shell colouration, camouflage, and mimicry are explored as are the impacts of urban living and of anthropogenic pollution upon egg shell thickness and strength. Nest building and the function of nests are described and incubation behaviours are discussed. Embryonic development is considered as are hatching and post-hatching behaviours.