Assessment of color using human vision (or standards based thereon) is central to tests of many evolutionary hypotheses. Yet fundamental differences in color vision between humans and other animals call this approach into question. Here we use techniques for objectively assessing color patterns that avoid reliance on species‐specific (e.g., human) perception. Reflectance spectra are the invariant features that we expect the animal's color cognition to have evolved to extract. We performed multivariate analyses on principal components derived from >2,600 reflectance spectra (300–720 nm) sampled in a stratified random design from different body regions of male and female starlings in breeding plumage. Starlings possess spatially complex plumage patterns and extensive areas of iridescence. Our study revealed previously unnoticed sex differences in plumage coloration and the nature of iridescent and noniridescent sex differences. Sex differences occurred in some body regions but not others, were mor...

https://www.jstor.org/stable/10.1086/303160

Plumage Reflectance and the Objective Assessment of Avian Sexual Dichromatism.

Seasonal plasticity in the adult brain.

Avian plumage has long been used to test theories of sexual selection, with humans assessing the colors. However, many birds see in the ultraviolet (<400 nm), to which humans are blind. Consequently, it is important to know whether natural variation in UV reflectance from plumage functions in sexual signaling. We show that female starlings rank males differently when UV wavelengths are present or absent. Principal component analysis of ≈1300 reflectance spectra (300–700 nm) taken from sexually dimorphic plumage regions of males predicted preference under the UV+ treatment. Under UV− conditions, females ranked males in a different and nonrandom order, but plumage reflectance in the human visible spectrum did not predict choice. Natural variation in UV reflectance is thus important in avian mate assessment, and the prevailing light environment can have profound effects on observed mating preferences.

https://www.pnas.org/content/pnas/94/16/8618.full.pdf

Ultraviolet plumage colors predict mate preferences in starlings

Many behavioural and physiological functions of organisms are adjusted to the periodic changes in their environment, particularly to those related to the natural day and year. This adjustment is often achieved through the action of endogenous daily (circadian) and annual (circannual) clocks. Studies of the control of avian moult, migration and reproduction have played a major role in understanding how biological clocks function and interact with rhythms in the environment. Investigations on tropical birds such as the East African subspecies of the Stonechat (Saxicola torquata axillaris) and long-distance migrants like the Garden Warbler (Sylvia boriri) have provided the longest records of circannual rhythms, some of them running for more than 12 years, with periods ranging from about 9 to 13 months. Avian circannual rhythms are organized in a characteristic way for a particular species or population, and cross-breeding experiments have shown that some of the differences found among them are genetically determined. In African Stonechats circannual rhythms guarantee that seasonal events occur at the appropriate times of the year and in the characteristic sequence. They also control a “reproductive window” that provides the temporal framework for breeding. The width of this window is rather inflexible but the performance of a bird within this framework (e.g. whether it breeds once or twice per season) is subject to modification by environmental conditions. In migratory birds circannual programs are involved in determining the time course, distance and direction of migration. Circannual rhythms are synchronized with and modified by environmental factors in a complex way, but the endogenous mechanisms usually respond to environmental cues such that an optimal adjustment to season and latitude is guaranteed.

Circannual clocks in avian reproduction and migration

I. Introduction WHILE the effects of hyper- and hypothyroidism1 in female gonadal functions are well established (1, 2), controversy exists regarding the impact of thyroid diseases on male reproduction. This is due to various reasons: 1) the apparent clinical irrelevance of signs and symptoms related to male gonadal function, compared with the systemic effects of hyper- and hypothyroidism; 2) the paucity of well controlled clinical studies, due to the fact that thyroid diseases are more common in females than in males; and 3) the demonstration in the 1950s that the adult male gonad of experimental animals is metabolically unresponsive to thyroid hormone (3). Since then, the concept that the testis is unaffected by iodothyronines became widely accepted. For this reason, the potential of thyroid hormone in the modulation of male reproductive functions was not determined for several years. Indeed, in experimental studies on the effect of thyroid hormone in various tissues, the testis has been used as a negat...

A. R. Goldsmith

Papers

Photorefractoriness in birds and comparison with mammals