Gambel's White-crowned Sparrows Research Articles

Testosterone treatment diminishes sickness behavior in male songbirds

Males of many vertebrate species are typically more prone to disease and infection than female conspecifics, and this sexual difference is partially influenced by the immunosuppressive properties of testosterone (T) in males. T-induced immunosuppression has traditionally been viewed as a pleiotropic handicap, rather than an adaptation. Recently, it has been hypothesized that suppression of sickness behavior, or the symptoms of infection, may have adaptive value if sickness interferes with the expression of T-mediated behaviors important for male reproductive success. We conduct a classic hormone replacement experiment to examine if T suppresses sickness behavior in a seasonally-breeding songbird, Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). Triggered experimentally by bacterial lipopolysaccharide (LPS), sickness behavior includes decreased activity, anorexia, and weight loss. Gonadectomized (GDX) males that were treated with silastic implants filled with T exhibited suppression of behavioral and physiological responses to LPS compared to GDX and sham-GDX controls given empty implants. Sickness responses of control groups were statistically indistinguishable. T-implanted birds had significantly higher plasma T than control groups and levels were within the range associated with aggressive interactions during male-to-male contests. These findings imply that suppression of sickness behavior could occur when T is elevated to socially-modulated levels. Alternatively, it is possible that this suppressive effect is mediated through a stress-induced mechanism, as corticosterone levels were elevated in T-implanted subjects compared to controls. We propose that males wounded and infected during contests may gain a brief selective advantage by suppressing sickness responses that would otherwise impair competitive performance. The cost of immunosuppression would be manifested in males through an increased susceptibility to disease, which is presumably offset by capitalizing upon limited reproductive opportunities.

Time course of changes in Gambel's white-crowned sparrow song behavior following transitions in breeding condition

Seasonal changes in behavior and in its underlying neural substrate are common across animal taxa. These changes are often triggered by steroid sex hormones. Song in seasonally breeding songbirds provides an excellent example of this phenomenon. In these species, dramatic seasonal changes mediated by testosterone and its metabolites occur in adult song behavior and in the neural circuitry controlling song. While song rate can quickly change in response to seasonal breeding cues, it is unknown how quickly other aspects of song change, particularly the stereotypy of song phonology and syntax. In this study we determined whether and how quickly song rate, phonology, and syntax change in response to breeding and non-breeding physiological cues. We asked these questions using Gambel's white-crowned sparrows ( Zonotrichia leucophrys gambelii), a closed-ended learner with well-characterized changes in the neural circuitry controlling song behavior. We exposed ten photosensitive sparrows to long-day photoperiod and implanted them with subcutaneous testosterone pellets (day 0) to simulate breeding conditions. We continuously recorded song and found that song rate increased quickly, reaching maximum around day 6. The stereotypy of song phonology changed more slowly, reaching maximum by day 10 or later. Song syntax changed minimally after day 6, the earliest time point examined. After 21 days, we transitioned five birds from breeding to non-breeding condition. Song rate declined precipitously. These results suggest that while song rate changes quickly, song phonology changes more slowly, generally following or in parallel with previously investigated changes in the neural substrate.

ENVIRONMENTAL CONDITIONS AFFECT THE BEHAVIOR OF CAPTIVE, MIGRATORY WHITE-CROWNED SPARROWS

Abstract Many aspects of migration have been studied extensively, but little is known of how environmental conditions influence the behaviors displayed by migrants. Field studies suggest that such environmental factors as atmospheric conditions, nocturnal illumination, and food availability can affect migratory activity. We used 24 hr locomotor activity records and specific behaviors displayed by captive migrants to determine how nocturnal illumination and food restriction altered the migratory behavior of Gambel's White-crowned Sparrows (Zonotrichia leucophrys gambelii). Our results indicated that nocturnal locomotor activity was enhanced with increased nocturnal illumination at the source from 0 to 9 lux, even though the intensity of illumination that reached the birds remained <1 lux. Food deprivation had little effect on migratory restlessness per se, but resulted in increased locomotor activity during daylight hours and behaviors associated with attempts to escape from the cage. Thus, we suggest that...

Circadian and Masking Control of Migratory Restlessness in Gambel's White-Crowned Sparrow (Zonotrichia leucophrys gambelii)

Avian migration is a seasonal activity that requires intricate timing on both an annual and daily basis. With increasing evidence for endogenous regulation of daily activities in migrant species, we tested whether a circadian oscillator may be involved with the expressions of daily locomotor activities and specific behaviors of the long-distance migrant, Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). Our previous studies have identified both daytime and nighttime behavioral patterns under a photoperiod of 18L:6D. In 2 separate trials, birds in the vernal migratory life-history stage were exposed to constant dim light, (DD)dim, and constant bright light, LL, while locomotor activity and behavioral observations were collected. Under (DD)dim, the daytime behaviors that included active and quiescent components observed under 18L:6D were lost as migratory restlessness, the intense nighttime activity, persisted nonstop for 36.4 h. Furthermore, the specific behaviors of migratory restlessness that are normally confined to the dark phase of 18L:6D, beak-up and beak-up flight, were expressed also during the subjective day of (DD)dim. Birds exposed to LL retained similar patterns of activity to the 18L:6D condition for 3 days, after which they became arrhythmic. Behavioral observations of intense locomotor activity observed during the subjective night of LL revealed no beak-up and beak-up flight. Thus, the complete expression of migratory restlessness that includes beak-up and beak-up flight may not be regulated by a circadian oscillator but instigated by very low light intensity. Locomotor activity and associated daytime behaviors appear to be influenced by a circadian oscillator, given their persistent patterns under LL. Therefore, we suggest that the separate components of migratory behavior are regulated differentially by a circadian oscillator and ambient lighting conditions.

Plasma Metabolite Profiles: Effects of Dietary Phospholipids in a Migratory Passerine (Zonotrichia leucophrys gambelii)

Plasma metabolites, including triglycerides, beta -hydroxybutyrate, and glycerol, can be used to estimate mass change in birds. Although dietary fatty acids can be ingested and absorbed as phospholipids, they have been largely overlooked as a potential indicator of mass change. The plasma ratio of triglyceride to phospholipid could also provide insight into diet quality because a high ratio in food items indicates high relative energy content. Variability in dietary phospholipid content and triglyceride : phospholipid may also affect the relationships between metabolites and mass change. We fed Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) diets differing in phospholipid content and triglyceride : phospholipid and measured metabolites during mass loss and mass gain. Plasma phospholipids were higher and triglyceride : phospholipid was lower in birds fed a diet higher in phospholipid content and lower in triglyceride : phospholipid. Contrary to our expectations, plasma phospholipids were negatively related to mass change. Plasma triglyceride levels were positively related to mass change and unaffected by diet. The relationships between mass change and both plasma beta -hydroxybutyrate and glycerol were affected by diet. Plasma triglyceride appears to be the most reliable metabolite predicting body mass changes, but inclusion of plasma phospholipids and triglyceride : phospholipid into metabolite profiles may provide additional information on diet quality.

Hypothalamic Pro‐GnRH‐GAP, GnRH‐I and GnRH‐II During the Onset of Photorefractoriness in the White‐Crowned Sparrow (Zonotrichia leucophrys gambelii)

Gambel's white-crowned sparrow is a long distance migrant that undergoes spontaneous gonadal regression as a result of long day exposure. This termination of breeding is caused by the development of photorefractoriness and the birds become insensitive to long days, including continuous light. The present study investigated its possible mechanisms by examining the activity of the gonadotrophin-releasing hormone (GnRH) system under different photoperiodic regimes. We investigated the localisation and distribution of GnRH-I, its precursor pro-GnRH-GAP and GnRH-II in Gambel's white-crowned sparrow brain using immunocytochemistry with specific antibodies during photostimulation and the development of photorefractoriness. The study revealed that photoperiodic treatment, including the onset of photorefractoriness, had no significant effect on the size or number of GnRH-I, pro-GnRH-GAP or GnRH II immunoreactive cells, or the density of the GnRH-I, pro-GnRH-GAP immunoreactive fibres at the median eminence. GnRH-II was not found in the median eminence, suggesting that it does not regulate pituitary gonadotrophin secretion. GnRH-I measurement in hypothalamic extracts by radioimmunoassay did not reveal any significant difference between birds that were photostimulated or in the early stages of photorefractoriness. Furthermore, the action of the excitatory amino acid glutamate agonist N-methyl-D-aspartate on GnRH neurones in photorefractory birds was demonstrated by the significant blockade of luteinising hormone release with a specific GnRH antagonist. Taken together, these results suggest that, in Gambel's white-crowned sparrow, a decrease in GnRH-I secretion is the initial step for the onset of photorefractoriness and not a decrease in GnRH-I biosynthesis.

Plasma corticosterone increases during migratory restlessness in the captive white-crowned sparrow Zonotrichia leucophrys gambelli

Plasma corticosterone increases during the period of spring migration in a variety of bird species. Long-distance migrants show elevations in corticosterone specifically in association with the stage of flight, suggesting that corticosterone may support flight-related processes, for example, locomotor activity and/or energy mobilization. The pattern of corticosterone secretion as it relates to migratory flight has hitherto not been clearly described in migrants that frequently interrupt flight to refuel, for example, the Gambel's white-crowned sparrow ( Zonotrichia leucophrys gambelii). The Gambel's white-crowned sparrow fuels by day and expresses peak migratory activity during the first few hours of night. To determine if plasma corticosterone increases in association with the stage of migratory flight also in this short-bout migrant, we induced captive white-crowned sparrows to enter into the migratory condition by placing photosensitive birds on long days (16L:8D) and then evaluated birds for plasma corticosterone and locomotor activity during four time points of the day. Patterns found in long-day birds were compared to those observed in short-day controls (8L:16D). Differences in energy metabolism as determined from plasma metabolites were also evaluated. We found that locomotor activity and corticosterone were significantly elevated at the onset of the dark period, but only in long-day birds. Plasma β-hydroxybutyrate (a ketone body) was also elevated. Thus, findings suggest that plasma corticosterone and ketogenesis increase in association with migratory restlessness in a short-bout migrant. In fact, corticosterone may play a regulatory role, because it shows a trend to increase already before night-time activity.

Gonadotropin-inhibitory hormone in Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii): cDNA identification, transcript localization and functional effects in laboratory and field experiments.

The neuropeptide control of gonadotropin secretion is primarily through the stimulatory action of the hypothalamic decapeptide, GnRH. We recently identified a novel hypothalamic dodecapeptide with a C-terminal LeuPro-Leu-Arg-Phe-NH2 sequence in the domestic bird, Japanese quail (Coturnix japonica). This novel peptide inhibited gonadotropin release in vitro from the quail anterior pituitary; thus it was named gonadotropin-inhibitory hormone (GnIH). GnIH may be an important factor regulating reproductive activity not only in domesticated birds but also in wild, seasonally breeding birds. Thus, we tested synthetic quail GnIH in seasonally breeding wild bird species. In an in vivo experiment, chicken gonadotropin-releasing hormone-I (cGnRH-I) alone or a cGnRH-I/quail GnIH cocktail was injected i.v. into non-breeding song sparrows (Melospiza melodia). Quail GnIH rapidly (within 2 min) attenuated the GnRH-induced rise in plasma LH. Furthermore, we tested the effects of quail GnIH in castrated, photostimulated Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), using quail GnIH or saline for injection. Again, quail GnIH rapidly reduced plasma LH (within 3 min) compared with controls. To characterize fully the action of GnIH in wild birds, the identification of their endogenous GnIH is essential. Therefore, in the present study a cDNA encoding GnIH in the brain of Gambel's white-crowned sparrow was cloned by a combination of 3' and 5' rapid amplification of cDNA ends and compared with the quail GnIH cDNA previously identified. The deduced sparrow GnIH precursor consisted of 173 amino acid residues, encoding one sparrow GnIH and two sparrow GnIH-related peptides (sparrow GnIH-RP-1 and GnIH-RP-2) that included Leu-Pro-Xaa-Arg-Phe-NH2 (Xaa=Leu or Gln) at their C-termini. All these peptide sequences were flanked by a glycine C-terminal amidation signal and a single basic amino acid on each end as an endoproteolytic site. Although the homology of sparrow and quail GnIH precursors was approximately 66%, the C-terminal structures of GnIH, GnIH-RP-1 and GnIH-RP-2 were all identical in two species. In situ hybridization revealed the cellular localization of sparrow GnIH mRNA in the paraventricular nucleus (PVN) of the hypothalamus. Immunohistochemical analysis also showed that sparrow GnIH-like immunoreactive cell bodies and terminals were localized in the PVN and median eminence respectively. Thus, only the sparrow PVN expresses GnIH, which appears to be a hypothalamic inhibitory factor for LH release, as evident from our field injections of GnIH into free-living breeding white-crowned sparrows. Sparrow GnIH rapidly (within 2 min) reduced plasma LH when injected into free-living Gambel's white-crowned sparrows on their breeding grounds in northern Alaska. Taken together, our results indicate that, despite amino acid sequence differences, quail GnIH and sparrow GnIH have similar inhibitory effects on the reproductive axis in wild sparrow species. Thus, GnIH appears to be a modulator of gonadotropin release.

The effect of flight, fasting and p,p′-DDT on thyroid hormones and corticosterone in Gambel's white-crowned sparrow, Zonotrichia leucophrys gambelli

This study examined the effects of p,p′-dichlorodiphenyltrichloroethane ( p,p′-DDT), fasting and flight on thyroid hormones and corticosterone in Gambel's White-crowned Sparrows ( Zonotrichia leucophrys gambelli). Female sparrows were dosed daily with either 5 mg p,p′-DDT per kg body mass or corn oil vehicle over 3 days. On the fifth day the sparrows were divided into 3 groups: (1) unstressed – non-stressed control sparrows; (2) fasted – sparrows fasted for intervals ranging from 20 min to 9 h; or (3) flown – sparrows flown in a wind tunnel for intervals between 20 min and 2.5 h while fasting. Half the sparrows from each group received DDT (DDT-dosed sparrows) and the other half corn oil vehicle only (vehicle sparrows). Trunk blood plasma was analyzed for thyroxine, triiodothyronine and corticosterone using radioimmunoassay. In the flown group, corticosterone was elevated (DDT-dosed 35.52 ng/ml, P≤0.05), and thyroxine was depressed (DDT-dosed 4.09 ng/ml, P≤0.05; vehicle 4.33 ng/ml, P≤0.05). Elevated corticosterone likely decreased thyroid hormone production through a negative feedback mechanism originating at the hypothalamus. Mean triiodothyronine concentrations did not differ among any of the test groups. Relative to time fasted and flown, thyroxine decreased in flown birds dosed with DDT ( P<0.001) and triiodothyronine decreased in fasted birds dosed with DDT ( P=0.004). The increased rate of hormone diminution may be a result of the ability of DDT to induce microsomal enzyme production.

Three-dimensional analysis of avian song control nuclei

Analysis of seasonal and developmental changes in the morphology of avian song control nuclei has traditionally been performed using two-dimensional (2-D) cross-sectional traces from brain sections. This method, although reliable, does not encompass the possibility that subdivisions of a nucleus might change in size to different degrees. Three-dimensional (3-D) analysis of song nuclei under different conditions could provide insight on this issue. This approach could also be of value in guiding and evaluating the use of lesions to study the functions of subdivisions of song nuclei. We used customized computer software to produce 3-D images of song nuclei from 2-D brain sections of spotted towhees ( Pipilo maculatus) in different hormonal status, and from Gambel's white-crowned sparrows ( Zonotrichia leucophrys gambelii) with unilateral lesions of the higher vocal center (HVc). 3-D images show that some sub-regions of song nuclei indeed change in size to a greater extent than others. 3-D analysis of HVc lesions provides a clearer view of the size and shape of the lesion site within the target nucleus and relative to the surrounding tissue. Used in conjunction with 2-D analysis, the 3-D method will aid investigations of the song system and contribute to the understanding of its regulation by hormones.

Steroid Hormone Interrelationships with Territorial Aggression in an Arctic-Breeding Songbird, Gambel's White-Crowned Sparrow, Zonotrichia leucophrys gambelii

The breeding season is very brief for arctic-breeding passerines, and any interruptions of parental care by aggressive interactions over territory may reduce reproductive success. We tested both the “testosterone insensitivity” and “corticosterone insensitivity” hypotheses in the arctic-breeding Gambel's white-crowned sparrow, Zonotrichia leucophrys gambelii. Additionally, we tested whether simulated territorial intrusions (STIs), known to stimulate increases in luteinizing hormone (LH) and testosterone (T) in mid-latitude breeding Z. l. pugetensis, would also be effective in either the early or late phases of the brief breeding season of Z. l. gambelii. Plasma levels of T and LH were high early in the breeding season and declined as egg laying began. Exposure of free-living males to 10 min of STI significantly increased LH but not T secretion. Nonetheless, the pituitary–gonadal axis is sensitive as jugular injection of gonadotrophin-releasing hormone increased plasma T at 10 min relative to saline-challenged controls. T implants failed to increase territorial aggression following STI during incubation. These data are consistent with the T insensitivity hypothesis and contrast sharply with the response of the southerly breeding subspecies, Z. l. pugetensis, in which the territorial response to T administration is retained throughout its relatively long breeding season. However, corticosterone implants during the incubation period decreased territorial aggression during STI. This responsiveness to corticosterone is not consistent with the corticosterone insensitivity hypothesis of stress modulation. Z. l. gambelii retain sensitivity to corticosterone levels that may occur naturally in response to environmental perturbations resulting in suppression of territorial behavior.

Large-Scale Geographic Variation in Songs of Gambel's White-Crowned Sparrows

Abstract An earlier study failed to find dialects in the songs of male Gambel's White-crowned Sparrows (Zonotrichia leucophrys gambelii) in Alaska. We recorded songs of males at five localities over a much greater portion of the subspecies' range, spanning 2250 km east to west and 1650 km north to south. On this scale, distinct geographical variants are evident, based on the song's terminal phrase and the fine details of other phrases. In White-crowned Sparrows, the extent of dialects is apparently positively correlated with the distance migrated by the subspecies; a demographic variable such as survivorship or natal philopatry likely results in this link. Variación Geográfica a Gran Escala en el Canto de Zonotrichia leucophrys gambelii Resumen. Un estudio anterior no encontró dialectos en los cantos de machos de Zonotrichia leucophrys gambelii en Alaska. Nosotros grabamos cantos en cinco localidades a través de un área de 2250 km este-oeste por 1650 km norte-sur, que abarca una gran parte del área de distribución de la subespecie. A esta escala, es evidente una clara variación geográfica, basada en la parte terminal de los cantos, y en detalles de las otras frases. En esta especie, la extension geográfica de los dialectos aparentemente está correlacionada de manera positiva con la distancia de migración de las subespecies. Esta correlación tal vez sea producto de factores demográficos como la supervivencia o de filopatría.

CORTICOSTERONE RESPONSES IN WILD BIRDS: THE IMPORTANCE OF RAPID INITIAL SAMPLING

Abstract Corticosterone concentrations in birds usually rise in response to capture and handling, and it is often assumed that this change is predictable. We tested this assumption by leaving Gambel's White-crowned Sparrows (Zonotrichia leucophrys gambelii), House Sparrows (Passer domesticus), and Lapland Longspurs (Calcarius lapponicus) in nets or traps for 15 min following capture and comparing their corticosterone response over the next 60 min with birds removed immediately. White-crowned Sparrows and House Sparrows left in mist nets for 15 min and then bled had significantly elevated corticosterone concentrations compared to controls that were immediately removed from the net and bled. Corticosterone concentrations over the next 45 min of handling and restraint were similar between groups. In another experiment, White-crowned Sparrows and Lapland Longspurs were captured using seed-baited Potter traps. The corticosterone response of White-crowned Sparrows left in the trap for 15 min did not differ from...

Annual Survival Rates of Wintering Sparrows: Assessing Demographic Consequences of Migration

AbstractThe demographic consequences of migration have important implications for both evolutionary ecology and conservation biology. We investigated local survival rates for six populations of sparrows at a wintering site. Recent developments in mark–recapture statistics were applied to a 13 year dataset with large numbers of marked individuals (n = 1,632 to 4,394). The study taxa were closely related, and included one resident species (Song Sparrow [Melospiza melodia gouldii]), one short-distance migrant (“Puget Sound” White-crowned Sparrow [Zonotrichia leucophrys pugetensis]), two moderate-distance migrants (Lincoln's [Melospiza lincolnii] and Fox [Passerella iliaca] sparrow), and two long-distance migrants (“Gambel's” White-crowned [Zonotrichia leucophrys gambelii] and Golden-crowned [Zonotrichia atricapilla] sparrow). A literature review demonstrated a cline in fecundity among these sparrows: resident and short-distance migrants laid multiple clutches of few eggs, whereas long-distance migrants tended to produce one large clutch. Annual rates of local survival were low in the interval after first capture (&lt;0.35), possibly because of variation in true survival, site-fidelity, presence of transients and heterogeneity of capture. Estimates of local survival among birds that returned at least once were more robust and were comparable among Song (0.558 ± 0.054 SE), Puget Sound White-crowned (0.461 ± 0.026), Lincoln's (0.456 ± 0.066), Fox (0.352 ± 0.0), Golden-crowned (0.422 ± 0.023) and Gambel's White-crowned (0.432 ± 0.0) sparrows. Estimates of survivorship for Lincoln's and Fox sparrows are among the first values available for those species. Local survival was not higher among resident than migratory taxa, nor did it covary with migration distance among migratory species. These results did not support the time-allocation hypothesis of Greenberg (1980), but are consistent with aspects of bet-hedging theory. While these analyses have potential implications for conservation of migratory birds, further work is required to establish whether these patterns are applicable to Neotropical migrants.

Testosterone treatment increases the metabolic capacity of adult avian song control nuclei.

In songbirds, the size of brain nuclei that control song learning and production change seasonally. These changes are mainly controlled by seasonal changes in plasma testosterone (T) concentration. One hypothesis to explain why it may be adaptive for these areas to regress in the fall is that this would decrease the metabolic demand of maintaining a large song system when singing is reduced or absent. We used a marker for cellular metabolism to examine birds with regressed song nuclei and compared them to birds whose song nuclei were induced to grow by administration of exogenous T. Photorefractory male Gambel's white-crowned sparrows were captured during their autumnal migration and kept in outdoor aviaries on a natural photoperiod. We implanted birds with Silastic capsules containing T or with empty implants. Three weeks later the birds were sacrificed. We assayed the brains for cytochrome oxidase (CO) activity and measured the volume of four song nuclei: HVc, RA, 1MAN, and area X. All four nuclei increased in volume in response to T treatment. T treatment increased the metabolic capacity of area X, HVc, and RA relative to surrounding tissue but had no effect on the metabolic capacity of 1MAN. These results support the hypothesis that song nuclei are more metabolically active under the influence of T than they are when plasma T levels are low.

Rapid Behavioral Response to Corticosterone Varies with Photoperiod and Dose

The magnitude of the adrenocortical response to stress can be modulated by a variety of environmental and physiological factors, such as daylength and body condition. To determine the consequences of this modulation for the organism, one also needs to investigate behavioral sensitivity to glucocorticoids. We examined the behavioral response of Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) to elevated glucocorticoids. Using a behavioral assay in which a rapid and transient dose of corticosterone (CORT: the avian glucocorticoid) rapidly increases perch hopping, we first investigated the photoperiodic regulation of this behavioral response. Intermediate levels of CORT (∼24 ng/ml), which increased activity in sparrows exposed to a long-day (breeding) photoperiod, had no behavioral effect in sparrows exposed to a short-day (winter) photoperiod. Hence, the neural mechanisms which regulate the behavioral response to CORT appear to be less sensitive under a winter photoperiod. Using the same behavioral assay, we also measured a dose-response curve for CORT's effects on activity in sparrows exposed to the long-day photoperiod. Intermediate levels (24 and 40 ng/ml) increased activity to threefold background levels, whereas high physiological levels (65 and 97 ng/ml) had no effect. Given that the behavioral response does not increase linearly with CORT, we can no longer assume that modulation of the adrenocortical response to stress will result in linear changes in behavior.

Diel rhythms of basal and stress-induced corticosterone in a wild, seasonal vertebrate, Gambel's white-crowned sparrow

Diel rhythms of basal and stress-induced corticosterone in a wild, seasonal vertebrate, Gambel's white-crowned sparrow

Diel rhythms of basal and stress‐induced corticosterone in a wild, seasonal vertebrate, Gambel's white‐crowned sparrow

Glucocorticoids have a wide array of actions in vertebrates. Daily fluctuations in basal levels of glucocorticoids are thought to regulate homeostatic mechanisms. In contrast, elevated levels secreted in response to stress stimulate changes in physiology and behavior. These changes are thought to aid an animal in avoiding chronic stress or death. Twenty-four-hour rhythms in basal and stress-induced glucocorticoids have been detected in laboratory mammals, but studies in wild, seasonal vertebrates are rare. Identification of plasticity in hormone secretion in wild vertebrates is critical to understanding the effects of hormones on physiology and behavior, and therefore the success of an animal in its natural environment. In the present study, we characterized diel patterns of basal and stress-induced corticosterone (the avian glucocorticoid) under two photoperiods in Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). In contrast to previous findings in the white-crowned sparrow, we demonstrated a robust rhythm in basal corticosterone secretion, in which corticosterone reaches peak levels at the end of the inactive period, and has returned to trough levels just after the active period has begun. We also demonstrated a diel rhythm in secretion of corticosterone in response to a stressor, showing the greatest response at the beginning of the active period. Patterns of CORT secretion were similar under both photoperiods. These patterns show interesting similarities and differences to classical mammalian rhythms.

Seasonal changes in androgen receptor immunoreactivity in the song nucleus HVc of a wild bird

In seasonally breeding songbirds, song behavior and neural morphology change seasonally. Song control nuclei are larger during the breeding season, as determined by multiple cytological labels. Seasonal changes in song nuclei are regulated by testosterone (T), and several song nuclei contain intracellular androgen receptors (AR). Changes in AR levels may interact with changes in plasma T levels to regulate song nuclei morphology. We measured seasonal changes in AR-immunoreactive cells in the telencephalic song nucleus HVc using the affinity-purified PG21 antibody to rat AR. We caught wild adult male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) during spring breeding in Alaska and during autumn migration in Washington State. To enhance PG21 labeling, animals were treated with T for 90 minutes (as in Smith et al. [1996] J. Histochem. Cytochem. 44:1075-1080). AR+ cells were found in HVc and other song nuclei, hippocampus, nucleus taeniae (homologue to mammalian amygdala), and the hypothalamus. HVc volume was larger in spring (S) than autumn (A), in both the PG21- and Nissl-stained sections (S:A = 1.9 and 1.7, respectively). In spring, but not autumn, PG21 and Nissl measurements were slightly different (PG21:Nissl = 1.07), perhaps because PG21 labeled the most caudal extent of HVc more clearly. In HVc, AR+ cell density and number were greater in spring. The percentage of AR+ cells was also increased in spring. Qualitatively, the staining intensity of individual cells was higher in spring. In time course studies, the T injection enhanced PG21 staining within 15 minutes, suggesting that it increases labeling via AR translocation to and concentration in the cell nucleus.

Alterations in hypothalamic–pituitary–adrenal function associated with captivity in Gambel's white-crowned sparrows ( Zonotrichia leucophrys gambelii)

Gambel's white-crowned sparrows were captured and brought into captivity in order to study seasonal changes in the function of the hypothalamic–pituitary–adrenal (HPA) axis in captive birds. 30 min of restraint elicited a rise in corticosterone titers that varied depending upon the season and physiological state of the birds. Restraint elevated corticosterone titers significantly more during the fall (within 2 weeks of capture from the wild) than during either the winter or during a prealternate or prebasic molt. We also examined what changes in the HPA axis could account for altered corticosterone levels. Exogenous ACTH significantly elevated corticosterone levels beyond the response to restraint during the fall, indicating a dramatic enhancement of the adrenal's ability to secrete corticosterone. Exogenous ACTH was ineffective at other times, suggesting that the adrenal's ability to release corticosterone often limits circulating levels. We further inferred the pituitary's ACTH secretory ability by injecting exogenous corticotrophin-releasing factor, arginine vasotocin, and mesotocin and measuring corticosterone release. Pituitaries failed to respond to any exogenous releasing factor during the fall, suggesting that the pituitary may be the site in the HPA axis regulating corticosterone release at this time. When compared to wild-caught birds, these results suggest that captivity alters both adrenal and pituitary function during restraint in white-crowned sparrows, and that this change depends upon the season and/or physiological state of the animal. Captivity thus appears to have a profound affect on the function of the HPA axis, and these results reiterate the caution that must be used to extrapolate laboratory data to field conditions.