Breeding at higher latitude is associated with higher photoperiodic threshold and delayed reproductive development in a songbird.

Many seasonally breeding animals exhibit a threshold day length (critical photoperiod; CPP) for gonadal growth, and populations breeding at higher latitudes typically have a higher CPP. Much less is known about latitudinal variation in CPP in migratory population that winter away from their breeding range and must time their reproduction to match favorable conditions at their destination. To address the relationship between migration, breeding latitude, and CPP, we held two closely related songbird populations in a common environment. One population is resident (Junco hyemalis carolinensis), the other winters in sympatry with the residents but migrates north to breed (Junco hyemalis hyemalis). We gradually increased photoperiod and measured indices of readiness to migrate (fat score, body mass) and breed (cloacal protuberance volume, baseline testosterone, and gonadotropin releasing hormone challenged testosterone). To estimate breeding latitude, we measured hydrogen isotopes in feathers grown the preceding year. As we predicted, we found a higher CPP in migrants than residents, and a higher CPP among migrants deriving from higher as opposed to lower latitudes. Migrants also terminated breeding earlier than residents, indicating a shorter breeding season. To our knowledge, this is a first demonstration of latitudinal variation in CPP-dependent reproductive timing in bird populations that co-exist in the non-breeding season but breed at different latitudes. We conclude that bird populations appear to exhibit local adaptation in reproductive timing by relying on differential CPP response that is predictive of future conditions on the breeding ground.

Local adaptation from afar: migratory bird populations diverge in the initiation of reproductive timing while wintering in sympatry.

The initiation of reproduction in many seasonally breeding animals is controlled by photoperiod and tends to be clinal: populations at higher latitudes breed later than those at lower latitudes, often reflecting a higher photoperiodic threshold. Migratory animals presumably time reproduction to match conditions at their breeding grounds, at least in part, by cues perceived on their wintering grounds. We asked how closely related dark-eyed junco (Junco hyemalis) populations that overwinter in sympatry but breed in allopatry respond to their shared winter environment by comparing early spring indices of readiness to migrate (fat and muscle condition) and breed (baseline and elevated testosterone). We measured stable hydrogen isotopes from feathers grown the preceding year and claws grown during winter to estimate breeding and wintering latitudes, respectively. We predicted that if reproductive initiation is adapted to the emergence of resources at their respective breeding destinations, then birds migrating to higher latitudes (slate-coloured junco; J. h. hyemalis) should delay breeding as compared with those migrating to lower latitudes (pink-sided junco; J. h. mearnsi) despite a common overwinter environment. We found higher testosterone in pink-sided juncos, consistent with earlier reproductive initiation, suggesting local adaptation in reproductive phenology is achieved through differential responses to predictive environmental cues.

Neural steroid sensitivity and aggression: comparing individuals of two songbird subspecies.

Hormones coordinate the expression of complex phenotypes and thus may play important roles in evolutionary processes. When populations diverge in hormone-mediated phenotypes, differences may arise via changes in circulating hormones, sensitivity to hormones or both. Determining the relative importance of signal and sensitivity requires consideration of both inter- and intrapopulation variation in hormone levels, hormone sensitivity and phenotype, but such studies are rare, particularly among closely related taxa. We compared males of two subspecies of the dark-eyed junco (Junco hyemalis) for territorial aggression and associations among behaviour, circulating testosterone (T), and gene expression of androgen receptor (AR), aromatase (AROM) and oestrogen receptor α in three behaviourally relevant brain regions. Thus, we examined the degree to which evolution may shape behaviour via changes in plasma T as compared with key sex steroid binding/converting molecules. We found that the white-winged junco (J. h. aikeni) was more aggressive than the smaller, less ornamented Carolina junco (J. h. carolinensis). The subspecies did not differ in circulating testosterone, but did differ significantly in the abundance of AR and AROM mRNA in key areas of the brain. Within populations, both gene expression and circulating T co-varied significantly with individual differences in aggression. Notably, the differences identified between populations were opposite to those predicted by the patterns among individuals within populations. These findings suggest that hormone-phenotype relationships may evolve via multiple pathways, and that changes that have occurred over evolutionary time do not necessarily reflect standing physiological variation on which current evolutionary processes may act.

Physiological impacts of chronic and experimental Plasmodium infection on breeding-condition male songbirds.

While Plasmodium parasitism is common in songbirds, its impact on avian reproduction is unclear owing to conflicting reports in the existing literature. Particularly understudied is the impact of phase of infection on variation in host reproductive physiology in wild, breeding-condition birds. However, assessing the full impact of Plasmodium on reproductive success in the wild can be difficult because individuals experiencing severe effects of parasitism may not enter the breeding population and may be less likely to be captured during field studies. To address these factors, we quantified metrics of health and reproductive physiology in wild-caught, breeding-condition male dark-eyed juncos (Junco hyemalis hyemalis) before and after experimental Plasmodium inoculation in a captive setting. Metrics of health and reproductive physiology included activity rate, hematocrit, scaled body mass, testosterone and sperm production. Individuals already infected at capture (i.e., chronically infected) had higher levels of hematocrit than males without chronic infections. Experimentally infected males showed a larger reduction in hematocrit and activity rate as compared to controls. However, chronic infection status did not influence the extent of metric decline. Testosterone production did not vary by treatment and most birds produced sperm following inoculation. Broadly, our results suggest that male juncos exposed to Plasmodium during the breeding season likely experience declines in general health, but Plasmodium infections do not negatively impact reproductive physiology. We conclude that physiological tradeoffs in males may favor maintenance of reproductive function despite infection.

Neural sensitivity to sex steroids predicts individual differences in aggression: implications for behavioural evolution.

Testosterone (T) regulates many traits related to fitness, including aggression. However, individual variation in aggressiveness does not always relate to circulating T, suggesting that behavioural variation may be more closely related to neural sensitivity to steroids, though this issue remains unresolved. To assess the relative importance of circulating T and neural steroid sensitivity in predicting behaviour, we measured aggressiveness during staged intrusions in free-living male and female dark-eyed juncos (Junco hyemalis). We compared aggressiveness to plasma T levels and to the abundance of androgen receptor (AR), aromatase (AROM) and oestrogen receptor alpha (ORα) mRNA in behaviourally relevant brain areas (avian medial amygdala, hypothalamus and song control regions). We also asked whether patterns of covariation among behaviour and endocrine parameters differed in males and females, anticipating that circulating T may be a better predictor of behaviour in males than in females. We found that circulating T related to aggressiveness only in males, but that gene expression for ORα, AR and AROM covaried with individual differences in aggressiveness in both sexes. These findings are among the first to show that individual variation in neural gene expression for three major sex steroid-processing molecules predicts individual variation in aggressiveness in both sexes in nature. The results have broad implications for our understanding of the mechanisms by which aggressive behaviour may evolve.

Songbird preen oil odour reflects haemosporidian parasite load.

Investigating the impact of parasitism on host phenotype is key to understanding parasite transmission ecology, host behavioural ecology and host-parasite coevolution. Previous studies have provided evidence that avian odour is one such phenotypic trait, as mosquitoes that vector the haemosporidian blood parasite Plasmodium tend to prefer birds that are already infected. Preen oil is a major source of avian odour, yet studies to date have not identified differences in preen oil odour based on the presence or absence of haemosporidian infection. Because preen oil can vary with physiological dynamics, we predicted that the composition of preen oil odours might vary according to parasite load, rather than solely by the presence or absence of infection. We used gas chromatography-mass spectrometry to characterize the composition of volatile compounds in preen oil taken from female dark-eyed juncos, Junco hyemalis carolinensis, and asked whether their composition varied with relative haemosporidian parasite load, which we assessed using quantitative PCR. We identified a subset of volatile compounds (a 'blend') and two specific compounds that varied with increasing parasite load. Importantly, the quantity of these compounds did not vary based on parasite presence or absence, suggesting that birds with low parasite loads might be phenotypically indistinguishable from uninfected birds. The volatile blend associated with parasite load also varied with sampling date, suggesting a possible seasonal relapse of chronic infections triggered by shifts in junco host reproductive state. Furthermore, we found a positive relationship between parasite load and a volatile blend shown in a previous study to predict reproductive success in juncos. This is the first study to demonstrate quantitative differences in avian host odour based on haemosporidian parasite load. Our findings highlight the importance of focusing on parasite load, rather than solely presence or absence, in investigating host-parasite interactions.

Exposure to artificial light at night accelerates but does not override latitude-dependent seasonal reproductive response in a North American songbird.

In the modern era of industrialization, illuminated nights have become a common defining feature of human-occupied environments, particularly cities. Artificial light at night (ALAN) imposes several known negative impacts on the neuroendocrine system, metabolism, and seasonal reproduction of species living in the wild. However, we know little about the impact of ALAN on populations of birds that either live year-round in the same location or move to different latitudes across seasons. To test whether ALAN has a differing impact on the reproductive timing of bird populations that winter in sympatry but breed at different latitudes, we monitored sedentary and migratory male dark-eyed juncos that were or were not exposed to low intensity (∼2.5 ± 0.5 lux) ALAN. All groups were held in common conditions and day length was gradually increased to mimic natural day length changes (NDL). We assessed seasonal reproductive response from initiation to termination of the breeding cycle. As expected based on earlier research, the sedentary birds exhibited earlier gonadal recrudescence and terminated breeding later than the migratory birds. In addition, resident and migrant birds exposed to ALAN initiated gonadal recrudescence earlier and terminated reproductive events sooner as compared to their conspecifics experiencing NDL. Importantly, the difference in the reproductive timing of sedentary and migratory populations was maintained even when exposed to ALAN. This variation in the seasonal reproductive timing may likely have a genetic basis or be the result of early developmental effects imposed due to different light regimes related to the latitude of origin. This study reveals first that ALAN accelerated reproductive development across both migrants and residents and second that latitude-dependent variation in reproductive timing is maintained despite exposure to ALAN. These results corroborate a relationship between latitude, population, and reproductive timing while also revealing ALAN's impact on seasonal reproductive timing. This study reveals that, ALAN accelerated reproductive development but maintained latitude-dependent variation in reproductive timing across both migrant and resident bird populations.

Hormones and honest signals: males with larger ornaments elevate testosterone more when challenged.

When male investment in mating varies with quality, reliable sexual signals may evolve. In many songbirds, testosterone mediates mating investment, suggesting that signals should be linked to testosterone production. However, because testosterone may change rapidly during behaviour such as territorial aggression and courtship, efforts to establish such a relationship have proved challenging. In a population of dark-eyed juncos, we measured individual variation in the production of short-term testosterone increases by injecting gonadotropin-releasing hormone (GnRH). We found a positive correlation between the magnitude of these increases and the size of a plumage ornament ('tail white') previously shown to be important for female choice and male-male competition. We then measured naturally elevated testosterone levels produced during male-male competition and found that they covaried with those induced by GnRH. We suggest that the association between tail white and testosterone increases may allow conspecifics to assess potential mates and competitors reliably using tail white.

Physiological effects on demography: a long-term experimental study of testosterone's effects on fitness.

Understanding physiological and behavioral mechanisms underlying the diversity of observed life-history strategies is challenging because of difficulties in obtaining long-term measures of fitness and in relating fitness to these mechanisms. We evaluated effects of experimentally elevated testosterone on male fitness in a population of dark-eyed juncos studied over nine breeding seasons using a demographic modeling approach. Elevated levels of testosterone decreased survival rates but increased success of producing extra-pair offspring. Higher overall fitness for testosterone-treated males was unexpected and led us to consider indirect effects of testosterone on offspring and females. Nest success was similar for testosterone-treated and control males, but testosterone-treated males produced smaller offspring, and smaller offspring had lower postfledging survival. Older, more experienced females preferred to mate with older males and realized higher reproductive success when they did so. Treatment of young males increased their ability to attract older females yet resulted in poor reproductive performance. The higher fitness of testosterone-treated males in the absence of a comparable natural phenotype suggests that the natural phenotype may be constrained. If this phenotype were to arise, the negative social effects on offspring and mates suggest that these effects might prevent high-testosterone phenotypes from spreading in the population.

Testosterone in females: mediator of adaptive traits, constraint on sexual dimorphism, or both?

When selection on males and females differs, the sexes may diverge in phenotype. Hormones serve as a proximate regulator of sex differences by mediating sex-biased trait expression. To integrate these perspectives, we consider how suites of traits mediated by the same hormone in both sexes might respond to selection. In male birds, plasma testosterone (T) varies seasonally and among species according to mating system. When elevated experimentally, it is known to enhance some components of fitness and to decrease others. We report that female T also varies seasonally and co-varies with male T. Female T is higher in relation to male T in sexually monomorphic species and is higher absolutely in females of species with socially monogamous mating systems, which suggests adaptation. We also consider the effect of experimentally elevated T on females and whether traits are sensitive to altered T. We hypothesize that sensitive traits could become subject to selection after a natural change in T and that traits with opposing fitness consequences in males and females could constrain dimorphism. Results from birds, including the dark-eyed junco (Junco hyemalis), reveal many sensitive traits, some of which appear costly and may help to account for observed levels of sexual dimorphism.

Steroid hormones and immune function: experimental studies in wild and captive dark-eyed juncos (Junco hyemalis).

Monogamous and polygynous male songbirds generally differ in their breeding season profiles of circulating testosterone. Testosterone level spikes early in the breeding season of monogamists and then declines, but it remains high in polygynists. Male dark-eyed juncos (Junco hyemalis) are socially monogamous and exhibit the usual pattern, but experimental maintenance of high testosterone throughout the breeding season alters normal behavior and physiology and affects various components of annual reproductive success but not overall annual success. Because stabilizing selection predicts that alteration of naturally existing phenotypes should reduce lifetime reproductive success, we asked whether prolonged testosterone exposure might impair immune function and perhaps thereby reduce life span. We assessed immune function in captive and wild male juncos that we treated with either testosterone-filled or empty Silastic implants. Results indicate that prolonged elevation of testosterone suppresses antibody production in captive males and cell-mediated immunity in wild males. Together these results suggest that testosterone-treated males may be more susceptible to disease or parasitic infection. As earlier studies have shown, levels of corticosterone as well as testosterone are higher in testosterone-treated males, so it is unclear whether the immune suppression we observed is due to testosterone's direct effects on immunity or testosterone's influence on glucocorticoid production. We discuss results in the context of recent hypotheses regarding life-history theory and potential endocrine-immune interactions.

Maternally derived yolk testosterone enhances the development of the hatching muscle in the red-winged blackbird Agelaius phoeniceus.

Hatching asynchrony in avian species often leads to the formation of a size hierarchy that places last-hatched nestlings at a significant disadvantage. The hatching muscle (musculus complexus) is responsible for breaking the shell during hatching and for dorsal flexion of the neck during begging. An increase in its strength in last-hatched nestlings could mitigate the effects of hatching asynchrony by reducing the time required for hatching or enhancing the effectiveness of begging for parentally delivered food or both. We have previously found that yolk testosterone concentration increases with laying order in the red-winged blackbird Agelaius phoeniceus. In this study, we investigated the hypothesis that yolk testosterone has anabolic effects on the development of the complexus, thereby influencing competition among asynchronously hatched nestlings. We found that both yolk testosterone concentration and relative complexus mass (complexus mass/nestling body mass) increased with laying order and that these two variables were positively correlated in both newly hatched nestlings and in two-day-old broods. Moreover, direct injections of testosterone into egg yolks resulted in an increase in relative complexus mass, while injections of flutamide, a testosterone antagonist, resulted in a decrease in relative complexus mass. Neither yolk testosterone concentration nor relative complexus mass differed between male and female nestlings.

Winter fattening in the dark-eyed junco: plasticity and possible interaction with migration trade-offs.

Although fat often supplies the major source of metabolic fuel during winter fasts of birds, this critical life-history trait is little studied by ecologists. In the dark-eyed junco Junco hyemalis, we have in a series of studies investigated the extent of plasticity in the winter fat reserve. Earlier (Rogers et al. 1993), we reported (1) a highly variable pattern of geographic variation in the winter fat reserve of junco populations in eastern North America, (2) disappearance of statistically significant interpopulation variation after experimental displacement to a common latitude, and (3) post-displacement temporal variation in the fat reserve. In analyses reported here, recent temperature, recent snowfall (a measure of short-term predictability of resources), season (perhaps reflecting continued exposure to unpredictable resources) and daylength explained spatial variation in the fat store. Recent temperature explained temporal variation in the fat reserves of groups of displaced juncos. These results suggest that platticity in a life-history trait has evolved in an uncertain winter environment. Through environment-dependent fattening, the costs of fat can be avoided during warm periods and at locations where fat confers little benefit, whereas benefits of fat can be quickly gained if weather conditions become harsh and snowfall might restrict food. Three types of winter fatteners probably exist among birds: responders (fatten in response to the proximate environment), predictors (fatten in anticipation of long-term environmental conditions), and responder-predictors (combination of both types of regulation). Because dark-eyed juncos select different winter latitudes as they age, we hypothesize that the nonbreeding component of the life-history of juncos includes the co-adapted plastic traits of winter fattening and post-breeding migration. Life-history theory can apparently explain important traits related to fitness in the nonbreeding period.

Two sides of the same coin? Consistency in aggression to conspecifics and predators in a female songbird.

Different forms of aggression have traditionally been treated separately according to function or context (e.g. aggression towards a conspecific versus a predator). However, recent work on individual consistency in behavior predicts that different forms of aggression may be correlated across contexts, suggesting a lack of independence. For nesting birds, aggression towards both conspecifics and nest predators can affect reproductive success, yet the relationship between these behaviors, especially in females, is not known. Here we examine free-living female dark-eyed juncos (Junco hyemalis) and compare their aggressive responses towards three types of simulated intruders near the nest: a same-sex conspecific, an opposite-sex conspecific, and a nest predator. We also examine differences in the strength of response that might relate to the immediacy of the perceived threat the intruder poses for the female or her offspring. We found greater aggression directed towards a predator than a same-sex intruder, and towards a same-sex than an opposite-sex intruder, consistent with a predator being a more immediate threat than a same-sex intruder, followed by an opposite-sex intruder. We also found positive relationships across individuals between responses to a same-sex intruder and a simulated predator, and between responses to a same-sex and an opposite-sex intruder, indicating that individual females are consistent in their relative level of aggression across contexts. If correlated behaviors are mediated by related mechanisms, then different forms of aggression may be expressions of the same behavioral tendency and constrained from evolving independently.

High testosterone prior to song crystallization inhibits singing behavior in captive yearling dark-eyed juncos (Junco hyemalis).

In passerine birds, song is considered crucial in advertising reproductive and territorial status to conspecifics. Variation in the quality and frequency of song may be influenced by hormonal effects during the individual's development. This variation in turn may affect the function and potency of song. We studied the influence of testosterone on vocal production in first-year male dark-eyed juncos (Junco hyemalis), using subcutaneous silastic implants filled with testosterone. Subjects were visually but not acoustically isolated from one another and after capture had no exposure to female or adult male models. Implants were administered when subjects were in the plastic song phase (i.e., after they had begun to sing but before song was fully crystallized). Control males (C males) received empty implants. Experimental males were of two classes: TI males received one dose of testosterone (a single 10-mm implant), and TII males received two doses. Testosterone implants kept plasma levels high well into the breeding season, whereas in nature, levels normally drop after territorial acquisition and pair formation. Control males sang at higher rates than testosterone-treated males of both classes and had the greatest number of song types. This inhibitory effect of testosterone on vocal production suggests that disturbance of seasonal profiles of testosterone in birds may interfere with the production of species-typical song.

Effects of experimentally elevated testosterone on plasma corticosterone and corticosteroid-binding globulin in dark-eyed juncos (Junco hyemalis).

An earlier study of free-living male dark-eyed juncos found an increase in plasma corticosterone (B) in response to experimental elevation of plasma testosterone (T) (E. D. Ketterson et al., 1991, Horm. Behav. 25, 489-503). To investigate whether the increase was caused by enhanced secretion of corticosterone or by a slower clearance rate, or both, we exposed 52 captive yearling male dark-eyed juncos (Junco hyemalis) to day lengths corresponding to those of spring and implanted them with one or two testosterone-filled or sham implants (10 T-I, 22 T-II, and 20 C-males). We then examined the effect of experimentally elevated testosterone on plasma corticosterone and on corticosteroid-binding globulin (CBG), as measured by the ability of steroid-stripped plasma to bind labeled corticosterone. Plasma samples were taken five times, 2 weeks before experimental prolongation of day length and approximately every 3 weeks thereafter. Treatment with testosterone increased both plasma testosterone and plasma corticosterone two to three times above control levels, and the degree of elevation was dose-dependent. Only when all treatment groups were pooled, however, were plasma testosterone and corticosterone significantly correlated. The relationship between plasma corticosterone and time required to bleed the birds was similar for all three treatment groups, suggesting that there was no effect of treatment on the stress response. Testosterone significantly increased the capacity of the plasma to bind corticosterone, presumably because it contained more CBG, when compared to the plasma of controls. However, treatment with testosterone did not affect the affinity of the plasma for corticosterone. It seems likely that exogenous testosterone elevated corticosterone by slowing the corticosterone clearance rate via an increase in CBG. It is not clear what the net effect of chronic elevation of testosterone would be on the availability of corticosterone to target tissues.

Effects of captivity and testosterone on the volumes of four brain regions in the dark-eyed junco (Junco hyemalis).

This study investigates the effects of captivity and testosterone treatment on the volumes of brain regions involved in processing visual and spatial information in adult dark-eyed juncos (Junco hyemalis). We treated captive and free-living male juncos with either testosterone-filled or empty implants. Captive juncos had a smaller hippocampal formation (HF) (both in absolute volume and relative to telencephalon) than free-living birds, regardless of hormone treatment. Testosterone-treated males (both captive and free-living) had a smaller telencephalon and nucleus rotundus, but not a smaller HF or ectostriatum, than controls. We found that free-living testosterone-treated males had larger home ranges than free-living controls in agreement with earlier experiments, but we found no corresponding difference in HF volume. We discuss the implications of the effect of captivity on HF volume for past and future laboratory experiments.

Egg yolk layers vary in the concentration of steroid hormones in two avian species.

Maternally derived steroid hormones are known to be present in the yolks of avian eggs; however, the physiological mechanisms involved in their deposition remain largely unexplored. Investigations of steroid production by avian follicles have demonstrated temporal differences in the concentrations of progesterone, 17beta-estradiol, and testosterone during yolk formation. Because yolk is deposited peripherally in concentric spheres as the oocyte develops, differences in the production of follicular hormones during yolk formation should be manifested in differences in the localization of steroids within layers of the yolk. To investigate this hypothesis we analyzed steroid hormone concentrations in layers of individual eggs of the dark-eyed junco (Junco hyemalis) and the red-winged blackbird (Agelaius phoeniceus). We found that in the dark-eyed junco the concentration of progesterone is significantly greater at the periphery of the yolk, while the concentration of 17beta-estradiol is significantly greater near the center of the yolk. We also found in both the dark-eyed junco and the red-winged blackbird that the concentration of testosterone remains constant from the interior to the intermediate layers of the yolk and then drops sharply between the intermediate and exterior layers. The patterns of hormone localization that we found agree with those predicted by studies of temporal changes in steroidogenesis in the maturing follicle of the chicken, thus suggesting that within-yolk variation in yolk steroid concentrations in the dark-eyed junco and the red-winged blackbird reflects temporal differences in the pattern of follicular steroidogenesis. Variation in the concentration of hormones among yolk layers presents a methodological concern for studies that involve the removal of yolk samples from viable eggs for subsequent hormonal analysis. This variation also has implications for the timing of embryonic exposure to steroid hormones.

The effect of exogenous testosterone on parental behavior, plasma prolactin, and prolactin binding sites in dark-eyed juncos.

Numerous studies have shown that parental behaviors are mediated by prolactin (PRL), while testosterone (T) interferes with their full expression. The limited data available suggest that reduced parental behavior induced by T is not mediated by reduced concentrations of plasma PRL. We hypothesized that T reduces parental behaviors by reducing PRL receptor binding activity at central neural sites that promote the expression of parental behaviors. To test this hypothesis we implanted male dark-eyed juncos (Junco hyemalis) with testosterone-filled or empty implants and measured T and PRL levels, paternal behavior, and specific binding of radio-labeled PRL at selected brain regions that have been implicated in the mediation of parental behaviors. Our findings concurred with previous studies in that T-treated males reduced their parental contributions, had higher levels of T, and had equivalent levels of PRL compared with controls. We found no differences in the capacity to bind 125I-oPRL in three brain regions previously implicated in the mediation of parental care in birds, i.e., the preoptic area, ventromedial nucleus of the hypothalamus, and paraventricular nucleus of the hypothalamus. Thus our findings do not support the hypothesis that T interferes with the expression of parental behavior by reducing PRL receptor binding activity at central sites.

Testosterone and avian life histories: the effect of experimentally elevated testosterone on corticosterone and body mass in dark-eyed juncos.

To assess whether alterations in the normal pattern of testosterone (T) secretion might be beneficial or detrimental, we studied a breeding population of dark-eyed juncos in which we elevated T experimentally and measured its effect on potential correlates of fitness. We treated both free-living and captive males with implants that were either empty (C-males, controls) or packed with T (T-males, experimentals). Timing of implant varied and was designed to mimic natural peak breeding levels except that peaks were either prolonged or premature. We bled the birds at recapture and analyzed their plasma, and that of their female mates, for T and corticosterone (B). We also measured body mass and fat score in free-living T- and C-males. In the field, T-implants elevated T and kept it elevated for at least a month. Experimental males also had higher B than controls. In captives, the effect of the implants on plasma T was detectable within 24 hr. B in captive T-males was again higher than in captive C-males. In females, neither T nor B differed between mates of T- and C-males. T-males implanted in early spring lost more mass between implant and recapture in late spring than did controls and also had lower fat scores when recaptured. When implants were inserted in summer, treatment did not influence mass. Elevated T in early spring apparently hastened the transition from the winter to the breeding mode of fat storage. We suggest that prolonged elevation of testosterone might be selected against because of the association between T and B. Premature elevation of T might be costly because of the resultant loss of mass and fat reserves, which could lead to mortality when spring snowstorms prevent access to food.