The evolutionary causes of small clutch sizes in tropical and Southern Hemisphere regions are poorly understood. Alexander Skutch proposed 50 years ago that higher nest predation in the south constrains the rate at which parent birds can deliver food to young and thereby constrains clutch size by limiting the number of young that parents can feed. This hypothesis for explaining differences in clutch size and parental behaviors between latitudes has remained untested. Here, a detailed study of bird species in Arizona and Argentina shows that Skutch's hypothesis explains clutch size variation within North and South America. However, neither Skutch's hypothesis nor two major alternatives explain differences between latitudes.

For many bird embryos, periodic cooling occurs when the incubating adult leaves the nest to forage, but the effects of periodic cooling on embryo growth, yolk use, and metabolism are poorly known. To address this question, we conducted incubation experiments on eggs of zebra finches (Taeniopygia guttata) that were frequently cooled and then rewarmed or were allowed to develop at a constant temperature. After 12 d of incubation, embryo mass and yolk reserves were less in eggs that experienced periodic cooling than in controls incubated constantly at 37.5C. Embryos that regularly cooled to 20C had higher mass-specific metabolic rates than embryos incubated constantly at 37.5C. Periodic cooling delayed development and increased metabolic costs, reducing the efficiency with which egg nutrients were converted into embryo tissue. Avian embryos can tolerate periodic cooling, possibly by adjusting their physiology to variable thermal conditions, but at a cost to growth efficiency as well as rate of development. This reduction in embryo growth efficiency adds a new dimension to the fitness consequences of variation in adult nest attentiveness. Disciplines Ornithology | Other Ecology and Evolutionary Biology | Terrestrial and Aquatic Ecology | Zoology CommentsThis article is from Physiological and Biochemical Zoology 79 (2006) ABSTRACTFor many bird embryos, periodic cooling occurs when the incubating adult leaves the nest to forage, but the effects of periodic cooling on embryo growth, yolk use, and metabolism are poorly known. To address this question, we conducted incubation experiments on eggs of zebra finches (Taeniopygia guttata) that were frequently cooled and then rewarmed or were allowed to develop at a constant temperature. After 12 d of incubation, embryo mass and yolk reserves were less in eggs that experienced periodic cooling than in controls incubated constantly at 37.5ЊC. Embryos that regularly cooled to 20ЊC had higher mass-specific metabolic rates than embryos incubated constantly at 37.5ЊC. Periodic cooling delayed development and increased metabolic costs, reducing the efficiency with which egg nutrients were converted into embryo tissue. Avian embryos can tolerate periodic cooling, possibly by adjusting their physiology to variable thermal conditions, but at a cost to growth efficiency as well as rate of development. This reduction in embryo growth efficiency adds a new dimension to the fitness consequences of variation in adult nest attentiveness.

We evaluated wet and dry ventricular and embryo weights, hemodynamic parameters of ventricular function, and myocardial myocyte organelle composition in the developing chick embryo. Phasic and dP/dt ventricular pressure were measured with a servo null pressure system, and phasic, mean, and dV/dt dorsal aortic velocities were measured with a 20-MHz pulsed-Doppler meter. Ventricular and embryo weight increased geometrically with development, but at different rates, so that the ventricle-to-embryo weight ratio decreased from 0.02 to 0.001 just prior to hatching. Ventricular systolic and end-diastolic pressure increased from 1.31 +/- 0.05/0.33 +/- 0.03 mmHg at stage 18 to 3.45 +/- 0.10/0.82 +/- 0.03 mmHg at stage 29, while dP/dt increased from 23.04 +/- 1.32 to 79.55 +/- 3.69 mmHg/s over the same period. Dorsal aortic dV/dt increased from 878 +/- 17 to 2.076 +/- 65 mm/s2 from stage 18 to 29. Myocyte percent volume of myofibrils increased from 16.7 +/- 0.9% at stage 18 to 23.6 +/- 1.1% at stage 27 and diminished to 18.4 +/- 0.8% at stage 29. Mitochondrial percent myocyte volume remained constant at about 11%. These data define the parameters of normal ventricular function and morphology during embryonic development in the chick.

Retinoid acid, the bioactive metabolite of vitamin A, is a potent signaling molecule in the brains of growing and adult animals, regulates numerous gene products, and modulates neurogenesis, neuronal survival and synaptic plasticity. Vitamin A deficiency (VAD) is a global health problem, yet our knowledge of its effects on behavior and learning is still emerging. Here we review studies that have implicated retinoids in learning and memory deficits of post-embryonic and adult rodent and songbird models. Dietary vitamin A supplementation improves learning and memory in VAD rodents and can ameliorate cognitive declines associated with normal aging. Songbird studies examine the effects of retinoid signaling on vocal/auditory learning and are uniquely suited to study the behavioral effects of VAD because the neural circuitry of the song system is discrete and well understood. Similar to human speech acquisition, avian vocal learning proceeds in well-defined stages of template acquisition, rendition and maturation. Local blockade of retinoic acid production in the brain or excess dietary retinoic acid results in the failure of song maturation, yet does not affect prior song acquisition. Together these results yield significant insights into the role of vitamin A in maintaining neuronal plasticity and cognitive function in adulthood.

Temperature affects growth and development, and morphometry can provide a quantitative description of how temperature changes affect the resulting phenotype. We performed a morphometric analysis on zebra finch (Taeniopygia guttata) embryos that were either exposed to periodic cooling to 20 or 30 degrees C throughout incubation over a background temperature of 37.5 degrees C, or were incubated at a constant temperature of 37.5 degrees C. Using a principle components analysis, we found that the relationship between the multivariate size (first principle component) and dry embryo mass depended upon the thermal treatment to which the developing embryos were exposed. Periodic cooling resulted in a smaller embryo mass, but had no effect on the multivariate size of the embryo. This suggests that the growth of phenotypic traits such as the length of long bones and the skull are less affected by temperature than is growth of other soft tissues such as muscle and organs that contribute to body mass.