Zonotrichia Leucophrys Gambelii Research Articles

Migratory sleeplessness in the white-crowned sparrow (Zonotrichia leucophrys gambelii).

Twice a year, normally diurnal songbirds engage in long-distance nocturnal migrations between their wintering and breeding grounds. If and how songbirds sleep during these periods of increased activity has remained a mystery. We used a combination of electrophysiological recording and neurobehavioral testing to characterize seasonal changes in sleep and cognition in captive white-crowned sparrows (Zonotrichia leucophrys gambelii) across nonmigratory and migratory seasons. Compared to sparrows in a nonmigratory state, migratory sparrows spent approximately two-thirds less time sleeping. Despite reducing sleep during migration, accuracy and responding on a repeated-acquisition task remained at a high level in sparrows in a migratory state. This resistance to sleep loss during the prolonged migratory season is in direct contrast to the decline in accuracy and responding observed following as little as one night of experimenter-induced sleep restriction in the same birds during the nonmigratory season. Our results suggest that despite being adversely affected by sleep loss during the nonmigratory season, songbirds exhibit an unprecedented capacity to reduce sleep during migration for long periods of time without associated deficits in cognitive function. Understanding the mechanisms that mediate migratory sleeplessness may provide insights into the etiology of changes in sleep and behavior in seasonal mood disorders, as well as into the functions of sleep itself.

Evidence for a Novel Gonadotropin-Releasing Hormone in Hypothalamic and Forebrain Areas in Songbirds

One or more of three distinct forms of gonadotropin-releasing hormones (GnRHs), has been found in all vertebrates studied. Birds are known to contain two types of GnRH, chicken GnRH-I and -II (cGnRH-I and -II), but to date nobody has investigated the possibility of the presence of lamprey GnRH-III in birds. Thus, our objective was to investigate the presence and distribution of immunoreactive lamprey (ir-lamprey) GnRH-III in hypothalamic and forebrain areas in house sparrows (Passer domesticus) and Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelii). In addition to its distribution in the CNS, we investigated whether or not exogenous lamprey GnRH-III peptide could elicit luteinizing hormone (LH) release in vivo. This study presents two novel findings: 1) The widespread presence in the central nervous system of an immunoreactive third GnRH in birds that is clearly hypophysiotropic and has gonadotropin-releasing capabilities. Intravenous injection of 100 ng lamprey-GnRH-III peptide elicited a rapid rise in circulating luteinizing hormone as compared to controls. This third GnRH, ir-lamprey GnRH-III, likely has multiple functions, as suggested by its widespread distribution. 2) Ir-lamprey GnRH-III is present in abundance in telencephalic areas, including the hippocampal formation and the song control system. In no vertebrate has a GnRH been localized in these ‘higher’ control regions before. The fact that ir-lamprey-GnRH-III is present in auditory processing areas (higher vocal center, or HVc, area X, caudomedial neostriatum) and song-producing areas (HVc, robust nucleus of the neostriatum, or RA) suggests a direct way in which auditory processing and song production can affect circulating gonadotropins.

Modulating the Corticosterone Stress Response: A Mechanism for Balancing Individual Risk and Reproductive Success in Arctic-Breeding Sparrows?

Abstract Modulation of the adrenocortical stress response with respect to the degree of parental activity was investigated in three Arctic-breeding species of songbirds faced with limited opportunities to breed. The hypothesis that the strength of the response can be lowered when reproductive effort or investment is greatest was tested by measuring the adrenocortical response to handling stress during two breeding stages that represented different levels of reproductive effort (i.e. before young were present [preparental] and while feeding nestlings [parental]). Comparisons of the corticosterone stress response (baseline at capture and subsequent samples 5, 10, 30, and 60 min after capture) were made within and between both sexes of American Tree Sparrows (Spizella arborea), White-crowned Sparrows (Zonotrichia leucophrys gambelii), and Savannah Sparrows (Passerculus sandwichensis) breeding above the Arctic Circle at Toolik Lake, Alaska. In general, body mass and baseline corticosterone did not differ between sexes within each of the three species, nor did they change during the two stages of breeding (tree and White-crowned sparrows only). In all three species, males had stronger adrenocortical responses than females during the preparental period, a time when males may expend less in reproductive effort than females. However, during the parental stage (tree and White-crowned sparrows only), the stress response of males declined and became similar to that of females because both sexes fed young. Females of both species (having considerable reproductive effort throughout both stages via egg laying or incubation and feeding young) showed no change in stress response with breeding stage. These results support the hypothesis that individuals with limited opportunities to breed can modulate the adrenocortical stress response when reproductive effort or investment is greatest as a way to maximize breeding success.

Short-term fasting affects locomotor activity, corticosterone, and corticosterone binding globulin in a migratory songbird

Unpredictable events such as severe storms lead to an increase in circulating corticosterone (CORT) in breeding birds. This increase is often accompanied by elevations in foraging and irruptive behavior. We were interested in determining if acute food restriction (such as might occur during inclement weather) is a sufficient cue to elicit an increase in locomotor activity, increase CORT secretion, and/or decrease circulating levels of corticosterone binding globulin (CBG) in white-crowned sparrows ( Zonotrichia leucophrys gambelii). Male Z.l. gambelii were housed individually in environmental chambers on long days (LD 20:4) to simulate breeding season daylength. Birds were fed ad libitum, and on select days, food was removed 2 h after lights on (fasted treatment), or was removed and replaced (control). We analyzed CORT and CBG levels after 1, 2, 6, 22 (lights on), and 23 h under fasted and control conditions. We also measured activity during the 23-h experiment. Activity levels were increased under fasted conditions during the daytime relative to control conditions, but activity levels did not differ between treatments during the night. Fasting as little as 1, 2, and 6 h significantly increased total CORT levels above baseline (control), although after 22 h, total CORT levels under fasted conditions matched those under control conditions. Plasma CBG decreased after the 22-h fast, and remained low after the 23-h fast. This change was sufficient to significantly elevate free CORT levels in fasted birds relative to ad libitum food conditions, despite the lack of difference in total CORT levels.

MODULATING THE CORTICOSTERONE STRESS RESPONSE: A MECHANISM FOR BALANCING INDIVIDUAL RISK AND REPRODUCTIVE SUCCESS IN ARCTIC-BREEDING SPARROWS?

Abstract Modulation of the adrenocortical stress response with respect to the degree of parental activity was investigated in three Arctic-breeding species of songbirds faced with limited opportunities to breed. The hypothesis that the strength of the response can be lowered when reproductive effort or investment is greatest was tested by measuring the adrenocortical response to handling stress during two breeding stages that represented different levels of reproductive effort (i.e. before young were present [preparental] and while feeding nestlings [parental]). Comparisons of the corticosterone stress response (baseline at capture and subsequent samples 5, 10, 30, and 60 min after capture) were made within and between both sexes of American Tree Sparrows (Spizella arborea), White-crowned Sparrows (Zonotrichia leucophrys gambelii), and Savannah Sparrows (Passerculus sandwichensis) breeding above the Arctic Circle at Toolik Lake, Alaska. In general, body mass and baseline corticosterone did not differ betw...

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.

Latitudinal variation in plasma testosterone levels in birds of the genus Zonotrichia

Birds breeding in northern latitudes generally have elevated plasma testosterone levels throughout the breeding season with a peak at the onset of the breeding season. In contrast, tropical birds tend to have extremely low plasma testosterone levels year round with only a slight increase during breeding. While these patterns have been consistent in the species investigated, closely related species have not been investigated across a range of latitudes. Birds of the genus Zonotrichia present an ideal opportunity to investigate latitudinal variation in plasma testosterone levels as breeding populations occur from northern Alaska to southern Argentina. We studied three taxa of Zonotrichia: (1) Gambel’s white-crowned sparrows, Zonotrichia leucophrys gambelii, breeding at high latitude in northern Alaska, (2) Puget Sound white-crowned sparrows, Z. l. pugetensis, breeding at mid-latitude in Washington state, and (3) an equatorial population of the rufous-collared sparrow, Z. capensis, in Ecuador. To compare both baseline breeding and maximal testosterone levels, males from the three taxa were either bled immediately upon capture during the breeding season or first challenged with gonadotropin-releasing hormone (GnRH) and then bled. As a control, another group of birds experienced a saline challenge. Two-way ANCOVA, with treatment and taxa as factors and testis volume as a covariate, showed a significant effect of treatment on plasma testosterone levels. There was also a significant interaction between taxa and treatment. Contrary to expectations, the equatorial species exhibited greater plasma testosterone levels in response to the GnRH challenge than the high latitude species. There were no differences between the mid- and high-latitude species. The equatorial species had the smallest average testis size and within each taxa there were no relationships between plasma testosterone and testis size. These data suggest that latitudinal clines in plasma testosterone levels in Zonotrichia do not follow previously described patterns and that tropical birds can have levels of testosterone similar to northern latitude species.

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.

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 (<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.

ANNUAL SURVIVAL RATES OF WINTERING SPARROWS: ASSESSING DEMOGRAPHIC CONSEQUENCES OF MIGRATION

Abstract The 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 tend...

Avian orientation at steep angles of inclination: experiments with migratory white-crowned sparrows at the magnetic North Pole.

The Earth's magnetic field and celestial cues provide animals with compass information during migration. Inherited magnetic compass courses are selected based on the angle of inclination, making it difficult to orient in the near vertical fields found at high geomagnetic latitudes. Orientation cage experiments were performed at different sites in high Arctic Canada with adult and young white-crowned sparrows (Zonotrichia leucophrys gambelii) in order to investigate birds' ability to use the Earth's magnetic field and celestial cues for orientation in naturally very steep magnetic fields at and close to the magnetic North Pole. Experiments were performed during the natural period of migration at night in the local geomagnetic field under natural clear skies and under simulated total overcast conditions. The experimental birds failed to select a meaningful magnetic compass course under overcast conditions at the magnetic North Pole, but could do so in geomagnetic fields deviating less than 3 degrees from the vertical. Migratory orientation was successful at all sites when celestial cues were available.

Intracerebroventricular corticotropin-releasing factor decreases food intake in white-crowned sparrows

Neuropeptides such as corticotropin-releasing factor (CRF) may play a role in regulating the pronounced seasonal changes in food intake shown by white-crowned sparrows ( Zonotrichia leucophrys gambelii). White-crowned sparrows held on short day length received injections into the third ventricle (icv) of saline or 5.0, 15.0, and 30 μg/kg. Meal size over the subsequent 180 min was significantly depressed in a dose-dependent fashion. Other non-specific behaviors such as preening, hopping, and immobile behaviors appeared to not be affected by a dose that suppressed food intake. This experiment suggests that white-crowned sparrows, when weight-stable, respond to CRF in a manner comparable with several mammalian species.

Effects of complex radiative and convective environments on the thermal biology of the white-crowned sparrow (Zonotrichia leucophrys gambelii).

The energy budgets of small endotherms are profoundly affected by characteristics of the physical environment such as wind speed, air temperature and solar radiation. Among these, solar radiation represents a potentially very large heat load to small animals and may have an important influence on their thermoregulatory metabolism and heat balance. In this investigation, we examined the interactive effects of wind speed and irradiance on body temperature, thermoregulatory metabolism and heat balance in the white-crowned sparrow (Zonotrichia leucophrys gambelii). We measured changes in metabolic heat production by exposing birds to different wind speeds (0.25, 0.5, 1.0 and 2.0 m s(-1)) and irradiance combinations (<3 W m(-2) and 936+/-11 W m(-2); mean +/- s.d.) at an air temperature of 10 degrees C. Body temperature was not affected by wind speed, but was significantly higher in animals not exposed to simulated solar radiation compared with those exposed at most wind speeds. In the absence of solar radiation, metabolic heat production was strongly affected by wind speed and increased by 30 % from 122 to 159 W m(-2) as wind speed increased from 0.25 to 2.0 m s(-1). Metabolic heat production was even more strongly influenced by wind speed in the presence of simulated solar radiation and increased by 51% from 94 to 142 W m(-2) as wind speed increased from 0.25 to 2. 0 m s(-1). Solar heat gain was negatively correlated with wind speed and declined from 28 to 12 W m(-2) as wind speed increased from 0.25 to 2.0 m s(-1) and, at its maximum, equaled 11% of the radiation intercepted by the animal. The overall thermal impact of the various wind speed and irradiance combinations on the animal's heat balance was examined for each treatment. Under cold conditions, with no solar radiation present, an increase in wind speed from 0.25 to 2.0 m s(-1) was equivalent to a decrease in chamber air temperature of 12.7 degrees C. With simulated solar radiation present, a similar increase in wind speed was equivalent to a decrease in chamber air temperature of 16 degrees C. Overall, shifting environmental conditions from a wind speed of 0.25 m s(-1) and irradiance of 936 W m(-2) to a wind speed of 2.0 m s(-1) with no short-wave radiation present was equivalent to decreasing chamber air temperature by approximately 20 degrees C. The sensitivity to changes in the convective environment, combined with the complex effects of changes in irradiance levels revealed by re-analyzing data published previously, significantly complicates the task of estimating the heat balance of animals in nature.

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.

Effects of N-methyl-D-aspartate on luteinizing hormone release and Fos-like immunoreactivity in the male White-crowned sparrow (Zonotrichia leucophrys gambelii).

Seasonal breeding is terminated in the White-crowned sparrow by the onset of absolute photorefractoriness, a condition in which the reproductive system is switched off indefinitely until it is dissipated by short day lengths. Absolute photorefractoriness is controlled by the central nervous system; however, the mechanisms underlying GnRH quiescence in photorefractory birds have yet to be elucidated. Using the excitatory amino acid glutamate agonist N-methyl-D-aspartate (NMDA), plasma LH levels in White-crowned sparrows were significantly elevated regardless of the reproductive or photoperiodic condition. NMDA also significantly induced Fos-like immunoreactivity (FLI) within the infundibular nucleus and median eminence, regions previously shown to express FLI after a photoperiodically driven LH rise. NMDA did not induce FLI within GnRH I neurons; instead, it significantly activated cells within the organum vasculosum of the lamina terminalis in close proximity to GnRH I perikarya. These findings provide the first evidence that photorefractoriness is not due to depletion of GnRH stores, as LH and presumably GnRH were secreted in response to excitatory amino acid stimulation. NMDA activation of FLI in the region of the organum vasculosum of the lamina terminalis and the basal tuberal hypothalamus suggests that seasonal reproductive neuroendocrine control may be mediated via cells in the region of the GnRH I perikarya and terminals.

Effects of N-Methyl-D-Aspartate on Luteinizing Hormone Release and Fos-Like Immunoreactivity in the Male White-Crowned Sparrow (Zonotrichia leucophrys gambelii)

Effects of N-Methyl-D-Aspartate on Luteinizing Hormone Release and Fos-Like Immunoreactivity in the Male White-Crowned Sparrow (Zonotrichia leucophrys gambelii)

Energy metabolism, testosterone and corticosterone in white-crowned sparrows

The influence of the steroid hormones testosterone and corticosterone on energy metabolism and activity of birds is largely enigmatic. We measured resting metabolic rate during night and day in 12 long-term castrated and 12 intact male white-crowned sparrows (Zonotrichia leucophrys gambelii) under short-day (8:16 SD), long-day (20:4 LD), LD+testosterone implant and LD−testosterone implant conditions. Each male was sequentially measured under all four conditions. Photostimulation increased testosterone, resting metabolic rate, food intake, hopping activity and body mass in castrates and intact males. Surprisingly, testosterone levels and metabolic rates did not differ between intact and castrated males. Testosterone implantation increased activity and food intake, but decreased body mass and resting metabolic rate in both groups. Removing testosterone implants reversed the effects on resting metabolic rate, activity and food intake. Corticosterone levels, measured immediately at the end of metabolism measurements, showed birds were not stressed. Corticosterone had no apparent relationship with resting metabolic rate and there was no interaction between corticosterone and testosterone. Overall, positive changes in testosterone levels resulted in a decrease of resting metabolic rate. We speculate that testosterone increases activity, and birds compensate for increased activity metabolism by reducing resting metabolic rate.

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.

Winter and Breeding-Season Energetics of Nonmigratory White-Crowned Sparrows

The lives of most temperate-latitude birds are organized on an annual cycle that includes reproduction, molt, dispersal and/or migration, and winter maintenance. How food and energy requirements vary with stage of the annual cycle, and which stage (if any) constitutes an energy bottleneck, are poorly known, but two alternative hypotheses have generally been recognized (Masman et al. 1989). The reallocation holds that abundant food and moderate temperatures during the breeding season result in decreased thermoregulatory and foraging costs, which allows energy to be reallocated to breeding. This hypothesis predicts little seasonal variation in field metabolic rate (FMR). The increased-demand holds that breeding results in a substantial increase in the energy demand of adults. By breeding during the season with maximum food availability, parent birds are able to meet their increased energy demands. This hypothesis predicts that FMR should reach a seasonal maximum during the breeding season. Early attempts to deduce the annual cycle in FMR by extrapolating laboratory metabolic measurements to the field produced conflicting patterns. Kendeigh (1973) derived a monthly energy budget for central Illinois House Sparrows (Passer domesticus) in which FMR attained an annual maximum of 117 kJ day-' in January and February and then gradually decreased to an annual nadir of 83 kJ day-' in August. The seasonal change in FMR in Kendeigh's analysis was driven largely by seasonal variation in basal metabolic rate and thermostatic costs. In contrast, Farner (1980) derived an annual energy budget for a migratory race of Whitecrowned Sparrow (Zonotrichia leucophrys gambelii) in which FMR was 100 kJ day-' during the winter and breeding seasons. Subsequent studies that estimated FMR using more direct techniques revealed a diversity of patterns (see Weathers and Sullivan 1993), with some species following the House Sparrow model and others showing little seasonal change in FMR. Which pattern is followed appears to be related to diet in that FMR increases little during the breeding season in species with energetically economical foraging modes, whereas FMR of species that feed on mobile animal prey is higher during the breeding season than during winter (Weathers and