D. Shallin Busch

Territoriality and testosterone in an equatorial population of rufous-collared sparrows, Zonotrichia capensis

Territorial aggression, displayed by male vertebrates in a reproductive context, is generally thought to be mediated by testosterone. The challenge hypothesis predicts that in socially monogamous species, territorial challenges should induce an increase in plasma testosterone concentrations, which will enhance aggressive behaviour and territory defence. This hypothesis is based on northern latitude birds and needs to be tested in tropical birds before it can be universally accepted. We tested the challenge hypothesis in an equatorial population of rufous-collared sparrows in Papallacta, Ecuador. This population shows an extended breeding period during which males aggressively guard territories. During the early breeding season, males were challenged with conspecific or heterospecific simulated territorial intrusions (STIs) lasting 10 min. Conspecific-challenged males responded more aggressively than heterospecific-challenged males. However, there was no increase in plasma testosterone in response to the conspecific STI. During the breeding season, males were challenged with conspecific STIs lasting 0, 10 or 30 min. Males behaved aggressively regardless of STI duration, and did not differ in plasma testosterone or luteinizing hormone concentrations. During the breeding season, males were implanted with testosterone-filled or empty silastic tubes and subsequently challenged with a conspecific STI. Testosterone implants significantly raised plasma testosterone concentrations, but testosterone-implanted males were not more aggressive than blank-implanted controls. Combined, these findings suggest that testosterone concentrations above breeding baseline are not related to territorial aggression in this population and therefore do not support the challenge hypothesis.

Impacts of frequent, acute pulses of corticosterone on condition and behavior of Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii)

Little is known about how frequent, acute stressors affect wild animals. We present two experiments conducted on captive, Gambels white-crowned sparrow (Zonotrichia leucophrys gambelii) that explore how frequent, acute doses of corticosterone (CORT) affect condition and behavior. CORT was administered either once or three times a day to birds in pre-breeding, early-breeding, or late-breeding life-history stages. Two additional groups were included to control for the CORT delivery vehicle, DMSO, and the treatment process. Our results indicate that CORT treatment decreases condition, but that its effects are dependent on frequency and life stage. Specifically, CORT-treated birds delayed the onset of molt and had reduced body mass, flight muscle, and food consumption. CORT treatment did not affect fat stores, bile retention in the gallbladder, or the expression of migratory restlessness behavior. These results increase our understanding of the effects of frequent, acute stressors and the development of chronic stress states.

Effects of repeated, short-term, corticosterone administration on the hypothalamo-pituitary-adrenal axis of the white-crowned sparrow (Zonotrichia leucophrys gambelii).

Our knowledge of glucocorticoid actions in vertebrates comes primarily from laboratory studies, which are often conducted with little consideration of how animals experience changes in glucocorticoid secretion in natural contexts. Typically, free-living animals are exposed to acute perturbations of the environment, ranging from a few minutes to a few hours duration, with varying frequency. The cumulative effects of these perturbations and their resultant glucocorticoid surges are not well known. To investigate the possible cumulative effects of repeated, acute surges in glucocorticoid secretion, we developed an ecologically relevant methodology for treating captive white-crowned sparrows (Zonotrichia leucophrys gambelii) with corticosterone (CORT). We dissolved CORT in dimethylsulfoxide (DMSO) and administered this cocktail directly on the skin. Treatments resulted in small elevations of CORT within the physiological range. In our first experiment at the end of the breeding life stage, birds were treated three times a day (3x). Two control groups were used: one treated with DMSO 3x and one not handled nor treated. In a second study at the beginning of the breeding life stage, one group was treated once a day and a second group 3x. A DMSO-control group was used for each dosage regime. Repeated, acute administration of CORT resulted in higher baseline CORT levels and a down-regulation of the endogenous adrenocortical response to a standardized stress. Maximum CORT and plasma corticosterone binding globulin levels increased in response to the CORT treatments only at the end of the breeding season. CORT treatment did not alter adrenal size, adrenal response to ACTH, or hepatic CORT metabolism.

Seasonal and social modulation of testosterone in Costa Rican rufous-collared sparrows (Zonotrichia capensis costaricensis)

Previous work shows that most birds breeding in northern temperate regions adjust production of testosterone in response to stage of the breeding cycle and in some cases following social interactions. In contrast, prior research suggests that tropical breeding birds are less likely to modulate testosterone in response to social interactions (the propensity to increase testosterone in response to social instability is known as the challenge hypothesis). To further test the challenge hypothesis in tropical birds, we investigated whether variation in season affects reproductive condition, aggressive behavior, and social modulation of testosterone in two populations of Costa Rican rufous-collared sparrow, Zonotrichia capensis costaricensis. We conducted our study at three distinct times of year: the dry season (March and May); the veranillo, a hiatus in the rainy season (July); and the late rainy season (November). Significantly more birds were in breeding condition in the dry season than in the rainy season or veranillo. In each time period, we collected baseline testosterone samples and conducted simulated territorial intrusions (STIs). Our study shows that testosterone is modulated with season independent of breeding condition, as testosterone levels were affected by season, breeding condition, and an interaction of the two factors. Males breeding in the dry season had higher plasma testosterone levels than non-breeding males in the dry season and both breeding and non-breeding males in the veranillo and rainy season. Males did not socially modulate testosterone in any season. Aggressive behaviors expressed during STIs did not differ among seasons with the exception that males sang fewer songs during the rainy season.

Sex Hormones in the Song Wren: Variation with Time of Year, Molt, Gonadotropin Releasing Hormone, and Social Challenge

Abstract Little is known about the role that hormones play in controlling the life cycles and behaviors of resident, tropical-forest passerines. In this study, we document levels of the sex hormones dihydroepiandosterone (DHEA), estradiol, luteinizing hormone (LH), progesterone, and testosterone in male and female Song Wrens (Cyphorhinus phaeocephalus) during the breeding season. In males, we found no significant seasonal changes in sex hormones and no effect of molt or experimental social challenge on hormone levels. The lack of population-level variation in sex hormone levels throughout the reproductive season highlights the asynchronous nature of reproduction in this species. Stimulation of the hypothalamo-pituitary-gonadal axis through injection of gonadotropin-releasing hormone (GnRH) did not affect sex steroid levels in males but did cause a significant increase in LH levels. Low basal testosterone levels in males and the failure of males to respond to GnRH or social challenge with an increase in testosterone indicates either a limited role for systemic testosterone or a system highly sensitive to small changes in hormone titers. Levels of LH, progesterone, and testosterone were similar in females with and without active brood patches, though DHEA levels were higher in females with them. As in males, reproductive hormone levels in females did not change with body or wing molt. However, although wing molt was not significantly related to brood patch status, body molt was negatively associated with the presence of active brood patches in females. This result may suggest an inability of females to engage in body molt and breeding simultaneously.

Disentangling the Effects of Environment and Life‐History Stage on Corticosterone Modulation in Costa Rican Rufous‐Collared Sparrows, Zonotrichia capensis costaricensis

This study explores the factors that influence modulation of baseline corticosterone levels and the hypothalamo‐pituitary‐adrenal cortex (HPA) axis response to stress in Costa Rican rufous‐collared sparrows (Zonotrichia capensis costaricensis). Individuals in our study population vary in their timing of breeding and molt. Thus, at multiple times of the year, we were able to investigate how the HPA axis changes with life‐history stage (breeding, molt) and the interaction of season and life‐history stage. Season explained most of the variation in corticosterone data. During the dry season, males had higher baseline corticosterone levels and both females and males had a higher stress response, regardless of breeding or molting condition. Breeding did not significantly affect corticosterone levels in either males or females. There was no direct effect of molt on corticosterone levels in males or in females, but there was a significant interaction between season and molt for male baseline corticosterone levels and HPA axis responsiveness. Baseline corticosterone levels were not correlated with body condition, suggesting that body condition is not an explanatory factor for HPA axis change. Our data indicate that environmental conditions may affect how the HPA axis in birds is modulated during different life‐history stages. They also emphasize the importance of studying species with diverse life cycles and seasonality.

TERRITORIAL AGGRESSION OF A TROPICAL PASSERINE, ZONOTRICHIA CAPENSIS, IN RESPONSE TO A VARIETY OF CONSPECIFIC INTRUDERS

[The expression of territorial aggression by reproductively active, resident birds varies between the sexes and in response to different intruder types. Previous studies have predicted that individuals should be more aggressive towards conspecific intruders of the same sex and tolerate intruders of the opposite sex and immature individuals. In this study, we investigated the behavioural responses of a tropical population of rufous-collared sparrows (Zonotrichia capensis) to a variety of caged intruder types: Singing Males, Silent Males, Females, and Juveniles. In this species, territories are used by the resident male and female and their young, and are also used by floaters — mature individuals that do not hold territories. Resident males responded similarly and aggressively to all adult intruders in terms of song number, closest approach to the intruder, time within 5 m of the intruder, and a composite aggression score. There was no significant variation in the response of resident females to the different intruder types, although the strongest responses of the resident female were to female intruders. Neither resident males nor females behaved aggressively towards juvenile intruders. These results fail to support the observational predictions for males and females that individuals should be most aggressive towards members of the same sex, who pose the greatest threat in terms of cuckoldry and territorial takeover., The expression of territorial aggression by reproductively active, resident birds varies between the sexes and in response to different intruder types. Previous studies have predicted that individuals should be more aggressive towards conspecific intruders of the same sex and tolerate intruders of the opposite sex and immature individuals. In this study, we investigated the behavioural responses of a tropical population of rufous-collared sparrows (Zonotrichia capensis) to a variety of caged intruder types: Singing Males, Silent Males, Females, and Juveniles. In this species, territories are used by the resident male and female and their young, and are also used by floaters — mature individuals that do not hold territories. Resident males responded similarly and aggressively to all adult intruders in terms of song number, closest approach to the intruder, time within 5 m of the intruder, and a composite aggression score. There was no significant variation in the response of resident females to the different intruder types, although the strongest responses of the resident female were to female intruders. Neither resident males nor females behaved aggressively towards juvenile intruders. These results fail to support the observational predictions for males and females that individuals should be most aggressive towards members of the same sex, who pose the greatest threat in terms of cuckoldry and territorial takeover.]