Mouth Color Signals Thermal State of Nestling Dark-Eyed Juncos (Junco hyemalis)

In many species of birds, nestlings have brightly colored mouths. Some studies have found that mouth color is related to hunger, and may serve to solicit feedings from parents. We devised two experiments to test the hypothesis that mouth color is an indicator of hunger in nestling dark‐eyed juncos (Junco hyemalis), and neither experiment produced results to support the hypothesis. We did find, however, that mouth redness saturation increased for the duration of our experiments (60 min). We devised a third experiment to investigate the effect of a different stressor, temperature. In the third experiment, mouth redness decreased in saturation when microenvironment temperature increased following a period of cooling. These findings suggest that mouth color indicates thermal state of nestling dark‐eyed juncos and may function as a signal to the female to brood them.     

Seasonal and age-related changes in blood parasite prevalence in Dark-eyed Juncos (Junco hyemalis, Aves, Passeriformes)

We determined seasonal changes in blood parasite infections in a free-living population of Dark-eyed Juncos (Junco hyemalis) breeding in interior Alaska (65 degrees N; 148 degrees W). The common parasites found in blood smears were Leucocytozoon fringillinarum (56%), Trypanosoma avium (33%), and Haemoproteus fringillae (9%). In males, parasite prevalences were relatively high at arrival on breeding grounds and increased during the breeding season. Intensity of infection with Leucocytozoon also increased between spring and summer, and then decreased at the time of migration (September). This decrease did not occur in adult females. Elevated prevalences during the breeding season probably reflected the addition of new cases via vector activity to positive status resulting from spring relapse. We observed neither an association between parasite species nor a consistent relationship between parasite intensity and body condition. To further study relationships between reproductive system activity and parasite infections, we compared prevalences in adult males that were undergoing their first cycle of gonadal development and regression (males in their second calendar year, or SY) with those of older males (males in their third or more calendar year, i.e., after-second-year males or ASY). Circulating testosterone concentrations declined in both groups between arrival on breeding grounds (end of April-early May) and the end of the reproductive period (July), and they were higher in May in ASY than in SY males. At the peak of the breeding season (June), ASY males also had a higher parasite prevalence than SY males. This difference may have resulted from immunosuppressive effects of gonadal hormones and/or from behavioral differences between SY and ASY males such that older males were exposed to more insect vectors than younger males. .     

Song Variation in a Recently Founded Population of the Dark-Eyed Junco (Junco hyemalis)

During natural colonization events, songs are expected to change as a result of both selection and drift-like processes. We studied song variation within a small isolated population of dark-eyed juncos, Junco hyemalis , which became established in an unusual environment on the University of California at San Diego (UCSD) campus in the early 1980's, and compared this variation with the native range. At UCSD, the average repertoire size is 4.2 song types/male, which is similar to that in the native range. There is a low level of song-type sharing across the population. Typically a male shares at least one song type with on average 40% of his neighbors, and song-type sharing decreases rapidly with distance. Thus song-type diversity is high, and in this respect appears similar to that in ancestral mountain populations. These results suggest a moderate to large number of males founded and/or subsequently immigrated into the UCSD population, but several selection mechanisms, if coupled with a high cultural mutation rate, may also favor high song-type diversity. When combined with results from a previous study, songs are significantly shorter at UCSD than in the mountain population we studied. It appears that founder effects have not influenced the evolution of songs in this population. More generally, if song-type diversity and other features of song result from various social and natural selection pressures, many of these pressures may have remained similar between the native and the recently established population.     

Inferring performance in the songs of dark-eyed juncos (Junco hyemalis)

Within bird species, songs differ in their attractiveness tofemales or effectiveness in male–male interactions. Somesongs are more difficult to sing than others, and receiversmay use a singer's performance of difficult songs as a meansfor evaluating the quality of the singer. The concept of songperformance aims at quantifying how physiologically demandingare different songs. Using variation between song types of dark-eyedjuncos, Junco hyemalis, we show that some song traits tradeoff with costly aspects of song output—short intervalsbetween syllables or loud sound amplitude—suggesting thatthose traits are difficult to sing. First, after controllingfor other traits, long syllables require longer intervals forrecovery. This supports the idea that a measure of "respiratoryperformance" could be based on the relative lengths of syllablesand intervals. Second, some syllable traits trade off stronglywith sound amplitude, suggesting that these traits may be difficultto sing at high amplitudes. The ratio of frequency bandwidthand trill rate has been used to infer performance in other birdspecies, but we found no evidence that frequency bandwidth tradesoff with any aspect of song output in the junco. The negativeassociation of bandwidth with trill rate may instead be a passiveconsequence of syllable length, with longer syllables randomlyaccumulating frequency modulation. We conclude that bird receiversmay best evaluate how well a song is performed if they integratemultiple cues and discuss how researchers may similarly devisemeasures of song performance.     

Experimentally-elevated testosterone, female parental care, and reproductive success in a songbird, the Dark-eyed Junco (Junco hyemalis)

In male dark-eyed juncos (Junco hyemalis), experimentally elevated testosterone (T) decreases male parental care and offspring survival, but results in higher overall fitness through greater mating success. To help address the ensuing question of what prevents selection from favoring higher levels of T in male juncos, we manipulated T in female juncos. A previous study demonstrated no effect of experimentally elevated T on female incubation behavior, suggesting that female parental behavior might be insensitive to T. In this study we asked whether experimentally elevated T mediates other female parental behaviors and whether variation in T-mediated parental behavior might influence reproductive success. We videotaped free-living control- and T-females during nesting to quantify brooding behavior when young were 3 days old and provisioning behavior when young were 6 days old. Nest defense was measured by quantifying responses to a mounted predator placed near the nest. Reproductive success was assessed via fecundity, nestling quality, and nest survival. T-females spent less time than control females brooding but did not differ in provisioning rate. T-females performed fewer dives at the predator mount and, unlike controls, failed to increase defense as nesting progressed. T-females also had lower daily nest survival and lower nest success (odds of producing at least one fledgling). We conclude that some aspects of female parental behavior are sensitive to experimentally elevated T while others are not and consider the implications for the evolution of T-mediated characters in both sexes.     

The distribution and turnover of 35S methionine as influenced by diet in the dark-eyed Junco (Junco hyemalis)

1. Dark-eyed Juncos (Junco hyemalis) were maintained on one of three semi-synthetic diets, differing only in their sulphur amino acid content. 2. 35S-methionine was administered intraperitoneally after 3 days on their respective diets and the animals were killed 24 or 96 hr post-injection. 3. Faecal loss of 35S-methionine was significantly lower in the low methionine/high cysteine (HC) group after 24 hr when compared with other two dietary groups (4.69 vs 5.53 and 5.47%) indicating a whole-body conservation of the limiting amino acid. 4. Incorporation of 35S-methionine in hepatic and intestinal tissues was also greater in the HC group, even as the animals were losing body weight.     

The Effects of Experimentally Elevated Testosterone and Food Deprivation on Food Consumption and Prey Size Preferences in Male Dark-Eyed Juncos (Junco hyemalis, Emberizidae: Passeriformes)

Numerous studies have shown that the experimental elevation of circulating levels of testosterone reduces parental behaviour in male birds, particularly the provisioning of young. The mechanisms responsible for this change in behaviour are not fully understood. In this study, we examine the effects of elevated testosterone on food consumption and prey selection, both of which have potential consequences for nestling provisioning behaviour. We manipulated testosterone and performed two experiments on a captive, non-breeding population of male dark-eyed juncos ( Junco hyemalis ) on long day-lengths. In the first experiment, we subjected juncos to 3 h of food deprivation and compared food consumption and prey size selection by males with elevated testosterone (testosterone males) to that of control males. Testosterone males consumed more food than control males and showed a preference for larger prey. In a second experiment in which small prey were more abundant than large prey, food consumption and prey size preferences did not differ between testosterone and control males. We also manipulated the duration of food deprivation in the second experiment. Males of both treatments consumed more small prey under conditions of mild (1 h) or moderate (5 h) food deprivation and consumed more large prey under conditions of intermediate (3 h) food deprivation. We discuss our results and the effects that testosterone has on self-maintenance behaviour and male parental effort.     

The role of corticosterone in supporting migratory lipogenesis in the dark-eyed Junco, Junco hyemalis: a model for central and peripheral regulation

The functional role of corticosterone (CORT) in regulating migratory hyperphagia and lipogenesis was investigated in an annual migrant, the dark-eyed junco (Junco hyemalis). Intraperitoneal injections of either dexamethasone (9 microg DXM/500 microL of 5% EtOH in saline, n=10) to inhibit an increase in baseline CORT or saline (5% EtOH, n=9) were given every 48 h for 15 d after transfer from short (10.5L:13.5D) to long (15.5L:8.5D) days. Food intake, body mass, furcular fat deposition scores, and nocturnal migratory activity were recorded for 29 d after photostimulation. Both groups showed the same increase in daily food intake over the study period (DXM=52%, control=41%). Controls began to increase baseline CORT and mass about 2 wk after photostimulation. DXM-treated birds maintained low CORT and did not increase mass or CORT until injections ceased, at which time they gained mass at the same rate shown earlier by controls. DXM-treated birds did not show greater levels of migratory activity despite experiencing an increase in energy intake during the CORT-inhibited period. Collectively, the results support the migration modulation hypothesis, illustrating how an increase in baseline CORT is needed to support the development of migratory condition. We address the apparent conflict with earlier studies on CORT and migratory food intake and propose a model in which migratory hyperphagia is supported by changes in centrally regulated responses to CORT that can occur even if CORT remains low and lipogenesis is regulated predominantly by peripheral mechanisms that require an increase in baseline CORT.     

Social status does not affect resting metabolic rate in wintering dark-eyed junco (Junco hyemalis)

Studies of wintering birds have demonstrated a correlation between social rank and energy expenditures. It is assumed that dominance is energetically costly because of increased activity, possibly caused by elevated androgen levels. As winter acclimatization leads to an increase in metabolic rate, maintaining dominance status in a cold climate can be a substantial challenge. We measured resting metabolic rates in dominant and subordinate dark-eyed juncos (Junco hyemalis) living in small groups in a controlled winter environment. We found no significant effect of social rank when controlling for body size. It has been shown previously that high testosterone levels during the nonbreeding season can lead to higher body conductance, fat loss, and higher nocturnal body temperature. A hypothesis explaining our result is that for juncos it is preferable to maintain low androgen levels during winter and to maintain social rank using a mechanism other than higher agonistic activity.     

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.     

Behavioral and physiological responses to experimentally elevated testosterone in female dark-eyed juncos (Junco hyemalis carolinensis)

Testosterone mediates the expression of many fitness-related traits in male vertebrates and is thought to account for numerous sex differences in trait expression. Testosterone is also secreted by females; however, far less is known regarding its effects on female physiology and behavior. Using a bird species in which the effects of testosterone on males are well characterized, the dark-eyed junco (Junco hyemalis), we tested whether an increase in exogenous testosterone in females would alter the phenotypic expression of a suite of behavioral and physiological traits. We found that increased testosterone levels in female dark-eyed juncos led to decreased cell-mediated immune function and increased intrasexual aggression, hypothalamo-pituitary-adrenal (HPA) axis responsiveness, baseline corticosterone and corticosterone-binding globulin (CBG) levels. Furthermore, immunosuppression following testosterone implantation was negatively correlated with total and free testosterone but did not appear to be related to either total or free corticosterone. These results demonstrate that the phenotypic impact of elevated testosterone is not confined to males in dark-eyed juncos, and that the impact in adults can be similar in males and females. We discuss these results in the context of potential endocrine-immune interactions and the evolution 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.     

Androgen control of vocal control region volumes in a wild migratory songbird (Junco hyemalis) is region and possibly age dependent

Previous laboratory studies have shown that photoperiodic adult songbirds experience seasonal variations in singing frequency that correlate with plasma androgen levels, as well as changes in the brain regions that control singing (vocal control regions). The present study investigates naturally occurring seasonal changes in the sizes of these regions in a wild migratory species (dark-eyed junco, Junco hyemalis), with samples from adolescence to post-breeding fall migration. In adult males, the volumes of the vocal control regions area X and the higher vocal center (HVC) were large during the breeding season when birds were singing and androgen levels were high, and decreased in size after the breeding season when singing had stopped and androgen levels were low. HVC volume in adolescent males caught in the fall (no singing), when plasma androgen levels were low, was smaller than in breeding adults, thereby following the seasonal pattern of change in plasma androgen levels. In adolescent males, however, area X volume was the same as in breeding adults. Thus, area X size in adolescent male juncos may be testosterone independent. The seasonal pattern of robust nucleus of the archistriatum volume was similar to that of the HVC. The volumes of neither the magnocellular nucleus of the anterior neostriatum nor the nucleus rotundus, a control region, differed seasonally. Castration of breeding adult males caused both area X and HVC volumes to decrease compared to castrated controls with testosterone replacement, indicating that maintenance of these two region volumes is testosterone dependent in adults. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 391–402, 1997.     

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. © 2000 John Wiley & Sons, Inc. J Neurobiol 43: 244–253, 2000     

Effects of testosterone and photoperiodic condition on song production and vocal control region volumes in adult male dark-eyed juncos (Junco hyemalis)

In seasonally breeding male oscines, song learning and expression are controlled by brain regions (vocal control regions, VCRs) that exhibit seasonal neural plasticity in adulthood. Several VCRs contain androgen receptors, and gonadal androgens play important roles in the control of seasonal structural and functional changes of VCRs. Recent studies also found that adult VCRs are influenced by factors other than gonadal hormones, including photoperiod, but the relative importance of these factors and their mechanisms of action are poorly understood. To address this issue, we investigated the contributions of photoperiod and testicular androgens to the regulation of VCR volumes and to the control of song expression in adult dark-eyed juncos, Junco hyemalis. Exposing castrated (CX) photosensitive males to long days (LD) enhanced their high vocal center (HVc) volumes compared to those of males held on short days (SD). These volumes were not further increased by concurrent testosterone (T) treatment, revealing a marked and gonadal androgen-independent stimulatory influence of photoperiod on the size of this brain region. HVc sizes were smaller in LD-exposed photorefractory than photosensitive males irrespective of whether birds were intact or had been castrated before photoperiodic manipulations, but HVc sizes increased in response to T treatment in intact photorefractory males. Thus, LD exposure can increase HVc volumes in the absence of gonadal T, but large volume induction in photorefractory males requires elevated plasma T levels. Testosterone treatment of SD-exposed photosensitive males increased HVc, but not Area X, MAN, or RA volumes. Only T-treated males sang and this treatment given to castrates was equally effective behaviorally when administered to photosensitive, photostimulated, or photorefractory juncos. This result indicates that the stimulating influence of LD exposure on HVc volumes is insufficient to induce song in the absence of elevated plasma T levels.     

Two Novel Vocalizations Are Used by Veeries (Catharus fuscescens) during Agonistic Interactions

Avian vocalizations are common examples of the complex signals used by animals to negotiate during agonistic interactions. In this study, we used two playback experiments to identify agonistic signals in a songbird species with several acoustically complex songs and calls, the veery. In the first experiment, we compared veery singing behavior in response to simulated territorial intrusions including playback of three variations of veery song: 1) song alone as a control, 2) songs with added whisper calls, and 3) songs with introductory notes removed. In the second experiment, we used multimodal stimuli including songs, whisper calls and songs with introductory notes removed, along with a robotic veery mount. Focal males readily responded to all of the playback stimuli, approached the speaker and/or robotic mount, and vocalized. Male veeries gave more whisper calls, and sang more songs without the introductory note in response to all types of playback. However, veeries responded similarly to all types of stimuli presented, and they failed to physically attack the robotic mount. These results indicate that rival veeries use two different types of novel vocalizations: whisper calls and songs lacking the introductory note as agonistic signals, but do not allow us to discern the specific functions of these two vocalizations.     

Agonistic Interactions and Matrifocal Dominance Rank of Wild Bonobos (Pan paniscus) at Wamba

I studied dominance relations in a wild group of bonobos at Wamba, Democratic Republic of Congo. Although agonistic interactions between males occurred frequently, most of them consisted only of display, and physical attacks were infrequent. Dominance rank order seemed to exist among males, but its linearity is unclear. Dominant males rarely disturbed copulatory behavior by subordinate males. However, high-ranking males usually stayed in the central position of the mixed party and, so, would have more chance of access to estrous females. Among females, older individuals tended to be dominant over younger individuals. However, agonistic interactions between females occurred rather infrequently, and most consisted of displacement without any overt aggressive behavior. Dominance between males and females is unclear, but females tended to have priority of access to food. The close social status between males and females may be related to the prolonged estrus of females and their close aggregation during ranging. Existence of a male's mother in the group and her dominance status among females seemed to influence his dominance rank among males. Young adult males whose mothers were alive in the group tended to have high status. In some cases, change in dominance between high-ranking males was preceded by a corresponding change in dominance between their mothers. As the dominance status of females is similar to that of males, mothers may be able to support their sons to achieve high status, stay in the center of the mixed party, and so have greater access to females, which may maximize the number of descendants of the mothers.     

Relative abundance of males to females affects behaviour, condition and immune function in a captive population of dark-eyed juncos Junco hyemalis

Relative numbers of males and females in breeding groups may vary from expected values owing to a variety of factors. To determine how sex ratio might influence individual phenotypes in a captive population of dark-eyed juncos Junco hyemalis during the breeding season, we established three treatment groups: a male-biased (2:1), equal (1:1), and female-biased group (1:2). Within-group density (birds/m²) was constant across groups. We assessed the frequency of flight chases (a proxy for social instability), measured changes in body mass and pectoral muscle condition, assayed plasma levels of testosterone (T) and compared cell-mediated immunity of individuals. We found significantly more chases in the male-biased group than in the female-biased group. Birds in the male-biased group lost more mass and displayed poorer pectoral-muscle condition than birds in the equal group. Cell-mediated immune responses were reduced in individuals in the male-biased group in comparison to the female-biased group. Plasma T levels in both sexes did not vary with sex ratio. Collectively, these results suggest that during the breeding season, social instability is greater in male-biased populations, and instability may lead to decreased general health and vigour.