Reproductive Behavior In Birds Research Articles

Manipulation of laying effort reveals habitat-specific variation in egg production constraints in Great Tits (Parus major)

It has been suggested that a bird's clutch size is not limited by the amount of resources available at the time of laying but that differences in the availability of food for nestlings is the ultimate underlying factor determining spatio-temporal variations in clutch size. However, habitat-related variations in egg production ability has yet to be investigated explicitly. We studied the breeding of Great Tits Parus major in deciduous and coniferous forests in the same area. The sizes of both the clutches and the eggs were, on average, larger in the former habitat than in the latter. A number of females were induced to lay more eggs than usual by removing four eggs from designated experimental clutches early in the laying period. These manipulated females laid approximately one egg more than control females, with the number of additional eggs laid not differing between the habitats. However, in both study years the relative size of the extra eggs – relative to the mean size of earlier laid eggs of the same clutch – was smaller in the coniferous habitat than in the deciduous habitat, while there was no habitat-related difference in the relative size of the last-laid eggs of control clutches. This result indicates that some form of proximate limitation during egg-laying period can contribute to the relatively small clutches and eggs in the coniferous habitat. Our results emphasize the need to take egg production costs into account when attempting to account for spatial variation in the reproductive behaviour of birds.

Widespread accumulation of [(3)H]testosterone in the spinal cord of a wild bird with an elaborate courtship display.

Elaborate courtship displays are relatively common features of the masculine reproductive behavior in birds. However, little is known about their neural and hormonal control. One bird that performs such a display is the golden-collared manakin (Manacus vitellinus) of Panamanian forests. Adult males, but not females, perform a physically intense display requiring substantial neuromuscular control of the wings and legs. We tested the hypothesis that steroid sensitivity is a property of neurons in the manakin spinal cord. Males and females were captured from active courtship leks, treated with drugs to block steroidogenesis, injected with (3)H-labeled testosterone, and the spinal cords were removed and processed for autoradiography. Sex steroid-accumulating cells were widely distributed in the spinal cords in each of six males and in one of five females. Cells, including presumptive motoneurons, reached their highest density in the ventral horns of the cervical and lumbosacral enlargements, regions associated with motor control of the wings and legs. These results suggest that neurons in the adult manakin spinal cord can express sex-steroid receptors, but do so less in females than in males. This evidence for androgen sensitivity and sexual dimorphism in the adult avian spinal cord suggests that sex steroids may control diverse behaviors in male birds in part by acting directly on the spinal neural circuits.

Effects of Exogenous Androgen and Antiandrogen on Territorial and Nonterritorial Red‐winged Blackbirds (Aves: Icterinae)

AbstractWe investigated the role of testosterone (T) in territory establishment and maintenance in male red‐winged blackbirds (Agelaius phoeniceus) by implanting exogenous T or pharmacological agents that block the action of T in nonterritorial floaters and territory owners. Floaters with artificially elevated plasma T levels were unable to obtain territories. Territory owners implanted with T did not expand their territories, although they engaged in more aggressive behavior than did control males. Flutamide, which binds to T receptors in target areas, had no effect on territoriality. However, most territory owners given a combination of flutamide and ATD, an aromatization blocker, lost portions of their territories even though they actively defended them. Our results 1) suggest that, although T influences aggressive behavior, elevated plasma levels alone are insufficient to overcome previously‐established social relationships between territory owners or between owners and floaters; 2) indicate that impairing the action of T leads to reduced abilities of territory owners to maintain territories against vigorous challenges: and 3) support recent findings that T acts on reproductive behavior in birds through both androgenic and estrogenic metabolites.

Symposium: Neurohumoral and Environmental Interactions Determining Avian Reproductive Behavior: Introduction

The increase in our knowledge of nuerohumoral, hormonal, and environmental interactions regulating reproductive behavior in birds has been extraordinary in the past few years. With the emergence of modern analytical tools and new research approaches, research on the physiological basis of reproductive behavior in this class of vertebrates can be expected to continue at an accelerated pace. Improved knowledge of the causal factors underlying reproductive behavior and the consequences of this behavior should lead to improvements in reproductive efficiency of commerical poultry flocks and to development of new criteria for scientific assessment of poultry welfare.This symposium considers some of the factors that influence reproductive behavior and the reproductive cycle in both male and female birds and illustrates the merit of employing a diversity of species as well as of investigative strategies in behavioral research. Cheryl Harding reviewed her data in the male Zebra finch, showing that metabolism of androgens .