Animals are increasingly exposed to novel environmental conditions because of human activities. An organism's ability to cope with these challenges is critical to its survival. The decision of whether or not an animal should investigate environmental changes involves a trade-off between the risk and the potential benefit of a novel resource. Novelty often elicits approach (exploratory) as well as avoidance behaviour in animals, and the extent of each of these behaviours may influence learning about the environment. We formulated and tested several hypotheses to determine how external factors (complexity of a novel stimulus) and internal factors (experience with objects) might influence exploration latencies to touch a novel object based on either cost or benefit considerations. Garden warblers were confronted with a simple and a complex novel object in their familiar cages, and latencies until first touch were measured. Additionally, corticosterone levels before and after the experiment were measured to assess whether differences in baseline levels might explain variations in latency to approach and whether presentation of novel objects is stressful. The complex object was touched significantly later than the simple object. Experienced birds investigated both types of objects later than inexperienced birds. Although birds showed marked approach-avoidance behaviour with the complex object, none of the objects elicited a corticosterone stress response. The results indicate that garden warblers consider both costs and benefits when exploring novel objects, and that the relative influence of costs and benefits varies with external and internal factors.

Male aggressive behavior is generally regulated by testosterone (T). In most temperate breeding males, aggressive behavior is only expressed during the reproductive period. At this time circulating T concentrations, brain steroid receptors, and steroid metabolic enzymes are elevated in many species relative to the nonreproductive period. Many tropical birds, however, display aggressive behavior both during the breeding and the nonbreeding season, but plasma levels of T can remain low throughout the year and show little seasonal fluctuation. Studies on the year-round territorial spotted antbird (Hylophylax n. naevioides) suggest that T nevertheless regulates aggressive behavior in both the breeding and nonbreeding season. We hypothesize that to regulate aggressive behaviors during the nonbreeding season, when T is at its minimum, male spotted antbirds increase brain sensitivity to steroids. This can be achieved by locally up-regulating androgen receptors (ARs), estrogen receptors (ERs), or the enzyme aromatase (AROM) that converts T into estradiol. We therefore compared mRNA expression of AR, ERa, and AROM in freeliving male spotted antbirds across reproductive and nonreproductive seasons in two brain regions known to regulate both reproductive and aggressive behaviors. mRNA expression of ER in the preoptic area and AR in the nucleus taeniae were elevated in male spotted antbirds during the nonbreeding season when circulating T concentrations were low. This unusual seasonal receptor regulation may represent a means for the year-round regulation of vertebrate aggressive behavior via steroids by increasing the brain's sensitivity to sex steroids during the nonbreeding season.

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