Summary1. In wild birds, incubation period shortens and the general pace of life quickens with distance from the equator. Temperature and various biotic factors, including adult behaviours, cannot fully account for longer incubation periods of equatorial birds and only explain some of the variation between tropical and temperate life histories. Here we consider the role of differences in light in driving variation in incubation period. In poultry, incubation periods can be experimentally shortened by exposing eggs to light. The positive influence of light on embryonic growth, called photoacceleration, can begin within hours after an egg is laid. 2. We artificially incubated house sparrow (Passer domesticus) eggs under photoperiods similar to those found at temperate (18Light : 6Dark) and tropical (12L : 12D) latitudes. We also measured embryonic metabolic rate during light and dark phases. 3. Eggs of house sparrows collected from the wild developed more rapidly under 'temperate' than 'tropical' photoperiods and had higher metabolic rates during phases of light exposure than during phases of darkness. Metabolic rates during light phases were high enough to account for a 1 day difference in incubation periods between temperate and tropical birds. 4. Based on a synthesis of photoacceleration studies on domesticated galliformes and our experimental results on a wild passerine, we provide the first support for the testable hypothesis that differences in photoperiod may influence variation in the rate of embryonic development across latitudes in birds.

Stress is a well-known cause of reproductive dysfunction in many species, including birds, rodents, and humans, though males and females may respond differently. A powerful way to investigate how stress affects reproduction is by examining its effects on a biological system essential for regulating reproduction, the hypothalamic-pituitary-gonadal (HPG) axis. Often this is done by observing how a stressor affects the amount of glucocorticoids, such as cortisol or corticosterone, circulating in the blood and their relationship with a handful of known HPG-producing reproductive hormones, like testosterone and estradiol. Until now, we have lacked a full understanding of how stress affects all genomic activity of the HPG axis and how this might differ between the sexes. We leveraged a highly replicated and sex-balanced experimental approach to test how male and female rock doves (Columba livia) respond to restraint stress at the level of their transcriptome. Females exhibit increased genomic responsiveness to stress at all levels of their HPG axis as compared to males, and these responsive genes are mostly unique to females. Reasons for this may be due to fluctuations in the female endocrine environment over the reproductive cycle and/or their evolutionary history, including parental investment and the potential for maternal effects. Direct links between genome to phenome cause and effect cannot be ascertained at this stage; however, the data we report provide a vital genomic foundation on which sex-specific reproductive dysfunction and adaptation in the face of stress can be further experimentally studied, as well as novel gene targets for genetic intervention and therapy investigations.

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Summary 1.We compare biases in estimates of the rate constant of the logistic and Gompertz functions applied to avian growth using three methods of parameter estimation. The methods differ in how the asymptote is determined, and consist of either floating asymptotes estimated from the full data (floating A) or data truncated at 70% of the adult mass (70% A), or fixing the asymptote at the adult mass (fixed A). 2. First, using data for two passerine species exhibiting different growth patterns, we truncated mass measurements at different ages to explore how each method of growth rate estimation responded to simulated differences in nestling period. We bootstrapped growth rates and error measures from these data to produce unbiased error terms, which we used to compare the different models. 3. We tested the effects of age truncation on both real and simulated data, and found that the fixed A method produced less bias and better fits than using estimates of floating asymptotes from the full growth curve (floating A) or truncated at 70% of the adult mass (70% A). Logistic and Gompertz models with a floating asymptote generally provided poorer fits than those with a fixed asymptote, although fits were improved by including a variable accounting for the ratio of asymptotic to adult mass. 4. To evaluate the performance of the methods across an array of species, we applied the three methods to growth data for 45 species of New World songbirds spanning diverse rates of growth, developmental periods and geographic origins. To determine whether choice of method influenced interpretation of regional and life-history differences in avian growth, we used model selection to estimate the influence of three independent variables (adult mass, nestling period and tropical vs. temperate region) with known effects on nestling growth rate calculated by each method. The coefficients of determination in these analyses suggest that the most appropriate method to estimate growth rates, regardless of the growth function (i.e. logistic, Gompertz), is the fixed A method, taking care to use accurate and appropriate estimates of adult mass.

Stress is a well-known cause of reproductive dysfunction in many species, including birds, 10 rodents, and humans. However, little is known of the genomic basis for this dysfunction and how were mostly unique to females. This result may be due to 1) fluctuations in the female endocrine 17 environment to facilitate ovulation and follicle maturation, and 2) their evolutionary history. We 18 offer a vital genomic foundation on which sex-specific reproductive dysfunction can be studied,

In a study of almost 16 000 nest records from seven swallow species across the entire Western Hemisphere, clutch sizes decline with relative laying date in each population, but the slope of this decline grows steeper with increasing distance from the equator. Late-laying birds at all latitudes lay clutches of similar sizes, suggesting that latitudinal differences may be driven primarily by earlier-laying birds. Focused comparisons of site-years in North America with qualitatively different food availability indicate that food supply significantly affects mean clutch size but not the clutch size-lay date regression. Other studies on the seasonality of swallow food also indicate that steeper clutch size-lay date declines in the North are not caused by steeper earlier food peaks there. The distribution of lay dates grows increasingly right-skewed with increasing latitude. This variation in lay-date distributions could be due to the predominance of higher quality, early-laying (and large-clutched) individuals among populations at higher latitudes, resulting from latitudinal variation in mortality rates and the intensity of sexual selection. Our results underscore the importance of studying clutch size and lay date in tandem and suggest new research into the causes of their joint geographic variation.

BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.

Maternally derived yolk antibodies provide neonates with immune protection in early life at negligible cost to mothers. However, developmental effects on the neonate's future immunity are potentially costly and thus could limit yolk antibody deposition. The benefits to neonatal immunity must be balanced against costs, which may depend on neonate vulnerability to pathogens, developmental trajectories and the immunological strategies best suited to a species' pace of life. We measured yolk antibodies and life-history features of 23 species of small Neotropical birds and assessed the evidence for each of several hypotheses for life history and ecological effects on the evolution of yolk antibody levels. Developmental period and yolk antibodies are negatively related, which possibly reflect the importance of humoral immune priming through antigen exposure, and selection to avoid autoimmunity, in species with a slower pace of life. There is also a strong relationship between body size and yolk antibody concentration, suggesting that larger species are architecturally equipped to produce and transfer higher concentrations of antibodies. These results suggest that developmental effects of maternally derived antibodies, such as imprinting effects on B-cell diversity or autoimmune effects, are important and deserve more consideration in future research.