Vernal Migration Research Articles

Photoperiodic modulation of melatonin receptor and immune genes in migratory redheaded bunting

The transcriptional regulation of innate immune function across annual life history states (LHS) remains obscure in avian migrants. We, therefore, investigated this in a migratory passerine songbird, redheaded bunting (Emberiza bruniceps), which exhibits long-distance vernal migration from India to Central Asia. We exposed the birds (N = 10) to differential photoperiodic conditions to induce a non-migratory (NM), pre-migratory (PM), migratory (MIG), and refractory (REF) state, and performed gene expression assays of melatonin receptors (MEL1A and MEL1B), and innate immunity-linked genes (IL1B, IL6, TLR4, and NFKB) in spleen and blood. We found a significant reduction in splenic mass and volume, and a parallel increase in fat accumulation, and testicular growth in birds under migratory state. The gene expression assay revealed an upregulation of MEL1A and MEL1B mRNA levels in both the tissues in MIG. Additionally, we found a nocturnal increase of splenic IL1B expression, and IL1B, IL6, and TLR4 expression in the blood. The mRNA expression of melatonin receptors and proinflammatory cytokine showed a positive correlation. These results suggest that melatonin relays the photoperiodic signal to peripheral immune organs, which shows LHS-dependent changes in mRNA expression of immune genes.

Autumn migratory departure is influenced by reproductive timing and weather in an Arctic passerine

Numerous studies of spring migration have discovered that environmental conditions experienced on the wintering grounds and/or during vernal migration itself can have “carry-over effects” altering timing or success of the subsequent breeding season. Few studies have evaluated whether breeding and pre-basic molt have carry-over effects on autumn migration. The aim of this study was to test the expectations that (1) at broad temporal scales carry-over effects from breeding and molt constrain migratory departure but that (2) at finer temporal scales local weather will further refine this decision. We monitored nests of Gambel’s white-crowned sparrows breeding in the low Arctic in Alaska over three years and used radio-telemetry to track autumn migratory departure. We found that reproductive timing and weather parameters, but not molt timing, contributed to variation in autumn departure from the breeding site. Birds that terminated parental care late in the summer departed from the breeding grounds late relative to other birds. Birds were more likely to leave on nights without precipitation, when barometric pressure was increasing, and when ground level south winds were prominent. We also observed that, on average, birds departed 2.5 h after sunset and shifted the hour of departure as sunset advanced over the migration season. Our findings, in conjunction with observations of migration from earlier studies, raise the possibility that global climate change may be delaying autumn migratory departure in Gambel’s white-crowned sparrows breeding in Alaska.

Development of vernal migration in redheaded buntings: concurrent behavioral, physiological and neural changes under stimulatory photoperiods

We investigated changes in behavior, physiology and selected brain regions during the development of vernal migration and reproduction phenotypes in migratory redheaded buntings. We monitored 24 h activity-rest pattern and measured food intake, fat deposition, and body mass of buntings exposed for 12 weeks to short (SP, 8L : 16D) and long (LP, 13L : 11D) photoperiods at 22 ± 2 °C temperature. Under LP, not SP, buntings exhibited a photostimulated spring migration phenotype (hyperphagia, fat deposition and body mass gain). However, there were sex differences in the development of vernal migration, as shown by faster and earlier induction of Zugunruhe (nocturnal migratory restlessness) in males than in females. In the next experiment, increasing photoperiods over 12 weeks following the vernal equinox induced behavioural and physiological changes associated with vernal migration phenotypes in both male and female buntings, but in a sex-dependent manner. In a subsequent experiment over 8 weeks corresponding to the spring migration period we found an increased expression of CART, not NPY, in INc, and decreased expression of GnRH-I in POA in the brain by week 6 of the observation under increasing photoperiods. There was also an increased expression of doublecortin (a marker of neuronal incorporation) in the olfactory bulb and song control nuclei (Area X and HVC, higher vocal centre) in male birds. These results demonstrate changes in the brain peptides and neuronal recruitment along with changes in the behaviour and physiology, and give insights into the concurrent photoperiodic induction of the seasonal response at multiple levels in migratory songbirds.

Rainbow Trout Migration and Use of a Nature‐Like Fishway at a Great Lakes Tributary

AbstractRainbow Trout Oncorhynchus mykiss were monitored over two consecutive vernal migration periods at a nature‐like fishway on the Beaver River, Ontario, to assess attraction efficiency, passage efficiency, multiple fish passage metrics, and interannual return rates from Georgian Bay, Lake Huron. Fishway evaluations have shifted to fill knowledge gaps related to the passage of nonsalmonids; however, surprisingly little work has been conducted with Rainbow Trout, with no known study assessing attraction or passage at a nature‐like fishway. Attraction efficiency was 53% and passage efficiency was 100% in 2017; only two of the radio‐tagged fish returned to the fishway in 2018. Upstream passage through the fishway required an average time of 152 ± 122 min. Fish spent 19–43 d upstream before returning to Lake Huron, where downstream passage required as little as 15 min. Overall, there were no significant relationships between any of the fish passage metrics and fish size or condition. These results can be used as a foundation for anadromous O. mykiss subspecies passage research and suggest that fisheries managers may need to adjust annual fishway counts. Future research should focus on developing methods to directly integrate temporal passage metrics into estimates of fishway efficiency.

Regulation of complex behavioural transitions: migration to breeding

The evolution of migration combines the selective advantages of breeding with ephemeral resources at high latitude or altitude with the benefits of overwintering at distant sites with adequate resources to promote survival. Migration between sites involves changes in behaviour, especially the transition from migration to nesting, or in autumn, the transition to wintering stage. We focus here on the vernal transition. Because migration and onset of nesting are mutually exclusive events, transition from a social, hyperphagic and highly mobile state to territoriality and pair bonding involves complex behavioural changes. In Arctic regions, weather in spring is unpredictable adding another dimension of complexity to the migration/breeding transition. Nevertheless, the brief Arctic summer requires that songbirds arrive on the breeding grounds while conditions may still be unfavourable, thus delaying nesting until conditions ameliorate. This requires flexibility and coordination of migration behaviour to be able to leave if weather deteriorates, or to become territorial, attract a mate and begin nesting within hours when conditions permit. In Gambel's white-crowned sparrows, Zonotrichia leucophrys gambelii, neuroendocrine regulation of reproductive development progresses throughout vernal migration so that males and females are mature when they arrive on the nesting grounds, but final ovarian maturation leading to ovulation and reproductive behaviours is suppressed. What provides the ‘brake’ to the onset of nesting? We focus on the adrenocortical response to acute stress that is greatly increased at arrival, especially in males, probably to enable flexibility in behavioural responses to severe weather. More recent evidence suggests that a hypothalamic peptide, gonadotropin-inhibiting hormone (GnIH) may provide the brake for the onset of nesting without suppressing development of a functional reproductive system. Experimental investigations show that gene silencing of GnIH in the brain increases singing and reproductive behaviours accompanying the onset of nesting. Thus we suggest that corticosteroids and GnIH play important regulatory roles specifically at the transition from vernal migration to the onset of nesting.

Overlapping life-history stages in migrating songbirds: variation in circulating testosterone and testosterone production capacity

Understanding the extent of overlap between life-history stages is fundamental to understanding full-life cycle biology, especially for migratory species. Testosterone levels vary throughout the annual cycle in seasonally reproducing vertebrates. In migratory songbirds, testosterone increases associated with breeding preparation may overlap with the vernal migratory period; however, this overlap remains largely unexplored. We test the hypothesis that both circulating testosterone and the capacity to elevate testosterone increases throughout vernal migration in long-distance songbird migrants. Here we relate testosterone in songbirds sampled en route with the stable hydrogen isotope ratios in their feathers as a metric of breeding ground proximity. We determined the capacity to elevate testosterone using field gonadotropin-releasing hormone bioassays and related this to feather hydrogen ratios as well. Males that were closer to their breeding grounds had higher circulating testosterone, whereas there was no relationship between testosterone and breeding ground proximity in females. Similarly, while capacity to elevate testosterone was not related to breeding ground proximity in female migrants, this capacity was greater in males closer to their breeding grounds than those further away. These results reveal that male migrants prepare for breeding during their vernal migration, whereas the schedule for breeding preparation among females is less clear and may be more complex.

Regulation of vernal migration in Gambel's white-crowned sparrows: Role of thyroxine and triiodothyronine

Appropriate timing of migratory behavior is critical for migrant species. For many temperate zone birds in the spring, lengthening photoperiod is the initial cue leading to morphological, physiological and behavior changes that are necessary for vernal migration and breeding. Strong evidence has emerged in recent years linking thyroid hormone signaling to the photoinduction of breeding in birds while more limited information suggest a potential role in the regulation of vernal migration in photoperiodic songbirds. Here we investigate the development and expression of the vernal migratory life history stage in captive Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) in a hypothyroidic state, induced by chemical inhibition of thyroid hormone production. To explore possible variations in the effects of the two thyroid hormones, triiodothyronine and thyroxine, we subsequently performed a thyroid inhibition coupled with replacement therapy. We found that chemical inhibition of thyroid hormones resulted in complete abolishment of mass gain, fattening, and muscle hypertrophy associated with migratory preparation as well as resulting in failure to display nocturnal restlessness behavior. Replacement of thyroxine rescued all of these elements to near control levels while triiodothyronine replacement displayed partial or delayed rescue. Our findings support thyroid hormones as being necessary for the expression of changes in morphology and physiology associated with migration as well as migratory behavior itself.

Patterns of testosterone in three Nearctic–Neotropical migratory songbirds during spring passage

Preparation for breeding may overlap extensively with vernal migration in long-distance migratory songbirds. Testosterone plays a central role in mediating this transition into breeding condition by facilitating changes to physiology and behavior. While changes in testosterone levels are well studied in captive migrants, these changes are less well known in free-living birds. We examined testosterone levels in free-living Nearctic–Neotropical migrants of three species during their vernal migration. Testosterone levels increased during the migratory period in males of all three species but significantly so in only two. Testosterone levels in females remained the same throughout their migration. Our results support the extensive overlap between vernal migration and breeding preparation in male songbirds. The pattern of testosterone changes during vernal migration is far from clear in females.

Regulatory mechanisms for the development of the migratory phenotype: Roles for photoperiod and the gonad

This article is part of a Special Issue “Energy Balance”.Male white-crowned sparrows, Zonotrichia leucophrys gambelii, were studied to investigate roles of natural day length and the testes in regulating development and expression of the vernal migration phenotype. Previous work suggested that a pulse of androgen during winter months followed by the vernal increase in photoperiod promotes fueling (fat deposition) to support long distance flight; however, other traits required for successful migration remain untested. To investigate these points, birds were captured on their wintering grounds and castrated prior to winter solstice following Mattocks (1976). A subset of the castrates received 8mm Silastic implants of testosterone (T-castrates) and others blank implants (Blank-castrates) for 16days in February. Shams were surgical controls. Migratory traits measured were as follows: 24h locomotor activity, prenuptial molt, body mass, fat score, flight muscle profile, cloacal protuberance (CPL) and plasma androgens measured over 28weeks divided into 3 experimental periods (pre-implant, implant, and post-implant). Under short day lengths, castration increased diurnal locomotor activity over Shams. Testosterone implants temporarily enhanced CPL, plasma androgens and flight muscle enlargement, but failed to induce migratory restlessness. Whereas all groups exhibited seasonal increases in mass, fat score and muscle profile, only Shams showed timely onset and completion of prenuptial molt and migratory restlessness. Thus, for castrated males exposed to naturally increasing day lengths, the organizational effects of a transient testosterone surge were not sufficient to actuate a timely spring molt and migratory behavior. A fully functional testis that can organize central processes is required for the entire expression of the spring migratory phenotype.

Apparent dissociation of photoperiodic time measurement between vernal migration and breeding under dim green light conditions in Gambel’s white-crowned sparrow Zonotrichia leucophrys gambelii

Abstract In seasonally breeding birds, the annual cycle of photoperiod is a principal environmental cue for temporal arrangement of different life-history stages, such as migration and breeding. In the past, most research has focused on the mechanisms of photoperiodic control of breeding with less attention paid to migration. In Gambel’s white-crowned sparrow Zonotrichia leucophrys gambelii (GWCS), photoreceptors for induction of breeding are known to reside in the basal hypothalamus. However, it is unknown whether the sites of photoperiodic reception for vernal migration are the same as those for breeding. Therefore, we hypothesized that they may be controlled separately. In this study, we exposed photosensitive GWCSs to low-penetration green light (wavelength at 510 nm) under a regime of 1 lux during the day and <0.1 lux at night, and switched the photoperiodic conditions from short day (10 h daytime) to long day (18 h daytime). The results showed that the experimental birds developed traits associated with vernal migration including mass increase, fat deposition and migratory restlessness behavior when transferred from short day to long day green light cycles, while control birds maintained continuously on short day green light conditions did not express any migration related characteristics. Neither experimental nor control groups showed gonadal recrudescence under either green light cycles. In support of our hypothesis, we were able to apparently dissociate the photoperiodic responses regulating vernal migration and breeding, which suggests separate mechanisms of photoperiodic time measurement. Such distinct photoperiodic mechanisms may drive the fine-tuned temporal arrangement of the two life history stages.

Seasonal Variations of Steroid Hormones and Triiodothyronine Concentrations in Migratory Northern Shovelers

Endogenous serum testosterone, estradiol, and triiodothyronine concentrations were measured in the northern shoveler (Anas clypeata, n = 20) during different stages of migratory condition at their wintering grounds to look at how these hormones may be linked to the development of migratory condition. These hormones were assayed by enzyme-linked immunosorbent assay (ELISA). The changes in hormones concentrations were parallel to each other and correspond to the increasing day lengths of spring. Furthermore, the northern shovelers showed a distinct seasonal cycle in serum concentrations of triiodothyronine, estradiol, and testosterone. It is suggested that these seasonal cycles in the northern shovelers are endogenously programmed and their entrainment to the environmental photoperiod ensures the occurrence of different physiological functions at temporally fixed time of the year. The results of this study offer a captive model of the seasonal changes in physiology and behavior that occur in the months preceding and including vernal migration of the northern shovelers.

Migration phenology and winter habitat quality are related to circulating androgen in a long-distance migratory bird

In migratory birds, the timing of departure from wintering grounds is often dependant on the quality of habitat on an individual’s territory and may influence individual fitness, resulting in an interaction of life history stages across large geographical distances. American redstart Setophaga ruticilla males who overwinter in high quality habitats arrive early to breed and subsequently produce more offspring than late arrivers. Since many migratory species overlap vernal migration with the physiological transition to breeding, we examined if breeding preparation plays a role in this seasonal interaction. We tested the hypothesis that early arriving male redstarts from high quality winter habitats are in superior breeding condition by simultaneously measuring winter habitat quality (stable-carbon isotopes) and breeding preparation (circulating androgen, cloacal protuberance (CP) diameter) upon arrival at breeding grounds. Compared with late arrivers, early arriving males were from higher quality winter habitats and had higher androgen, but smaller CPs. Males arriving with higher androgen were in more advanced physiological migratory condition, as measured by haematocrit. Early arrivers were more likely to successfully breed, but there was no significant relationship between androgen upon arrival and breeding success. One possible explanation for these relationships is that androgen measured during arrival is most relevant in a migratory context, such that birds with high androgen may benefit from effects on migratory condition, positively influencing fitness through earlier arrival.

Plasma Corticosterone Levels of Semipalmated Sandpiper Calidris pusilla Overwintering in a Tropical Coastal Lagoon of Northeastern Venezuela

Semipalmated sandpiper Calidris pusilla arrives on the coasts of Venezuela between the middle of August and early September, staying there until May when birds prepare for vernal migration. To investigate possible relationships between corticosterone levels and body mass changes during preparation for migration, we sampled birds during wintering (January) and vernal premigration (May) over 2 years. Body mass and fat increased significantly from wintering to vernal premigration in both years. Capture stress levels of corticosterone varied between 30 and 150 ng mL(-1), the highest being during wintering. These results strongly suggest that increase in body mass of C. pusilla during preparation for vernal migration in a tropical environment was promoted by a decreased corticosterone secretion response to stress.

Migratory behaviour of captive white-crowned sparrows, Zonotrichia leucophrys gambelii, differs during autumn and spring migration

AbstractGambel's white-crown sparrow (Zonotorichia leucophrys gambelii) is a long-distance, over-land migrant. In captivity birds display many characteristics of the autumn and spring migratory life history stages that include hyperphagia, fattening and high intensity nocturnal activity termed migratory restlessness or Zugunruhe. We recorded the behaviour of captive birds while simultaneously collecting 24 h locomotor activity. These data were used to define the behaviour displayed by captive birds during autumn and spring in order to compare the two migratory stages and to draw inferences for free-living birds. The predominant behaviour during day and nighttime was rest. Feeding occurred only during daylight hours but at a greater frequency in autumn than spring. Birds generally used their feet as the primary source of locomotion during the day termed 'jump'. During the night, two distinct behaviours, 'beak-up flight' and 'beak-up' involving high intensity wing motions were observed and considered components of migratory restlessness. The frequency of the 'beak-up flight' was greatest during spring and associated with the enhanced tempo of vernal migration. In both stages, migratory restlessness was preceded by a quiescent phase, the occurrence of which differed and related to time available for foraging and length of the night. Given these findings, we hypothesize that diel behaviours displayed by autumn and spring migrants in captivity highlight distinctions between the two life history stages.

Flexibility in annual cycles of birds: implications for endocrine control mechanisms

Life cycles of birds and other vertebrates are composed of series of life history stages each with unique combinations of morphological, physiological and behavioral characteristics. For example, in the white-crowned sparrow, Zonotrichia leucophrys, the nonbreeding stage (winter), vernal migration, breeding, moult and autumn migration stages occur in a fixed and repeated sequence where each cycle is 1 year. The sequence of stages cannot be reversed. Transition from one life history stage to the next and the duration of each stage are dependent upon a combination of genetic factors and environmental cues. The latter include the annual change in photoperiod and the former may involve endogenous circannual rhythms. All vertebrates also express the emergency life history stage in response to perturbations of the environment that allow individuals to cope with the unpredictable. Each stage has a unique repertoire of sub-stages (physiological and behavioral, and to a lesser extent morphological), which can be expressed in any sequence or combination to give the state of the individual at any point in its life cycle. This state is presumably maximally adapted to the environmental conditions at that time. Although the sequence of life history stages appears to be innate, the rate of transition from stage to stage, and the expression of sub-stages can be modified by the local environmental factors and, particularly, by social cues. These environmental cues acting on the phenotype result in neuroendocrine and endocrine secretions that regulate development of the life history stage, its onset once mature capability has been attained, and then terminate it at the appropriate times. The environmental cues (from the physical and social environment) impart a strong experiential component. Because, there is a set number of life history stages and their sub-stages, there is a finite number of states that can be expressed in response to the environmental variation experienced by the individual. The more life history stages a phenotype expresses, the less flexibility is there in the overall timing of these stages owing to the time taken to develop one stage and terminate the last (about 1 month). However, many phenotypes have increased flexibility in their life cycles by overlapping some life history stages (i.e., with overlapping mature capability of two or perhaps even more stages). Another potential strategy is to dissociate some components of a life history stage so they are expressed at other times of year thus spreading out potential costs associated with that life history stage. Examples of both overlap and dissociation of life history stages are given including implications for hormonal control mechanisms.

Passage Rates and Timing of Bird Migration in Montana

Abstract Rates and timing of bird passage in the proposed Norris Hill Wind Resource Area (NHWRA) and vicinity in southwestern Montana were investigated using two marine surveillance radars between August 1995 and August 1996. The scanning radar array displayed movements in a horizontal plane within 360° while the vertical radar displayed altitudes of birds in and out of the NHWRA to the east and west. Radars were also used to record raptor movements within NHWRA in summer. Spatio-temporal profile of migration was determined by adjusting observed numbers of events by detection probability by radar, derived from point- and line-transect bird sampling techniques. Autumn migration was more protracted than vernal migration. Altitude of birds flying in and within 2 km east and west of NHWRA averaged 209 m in autumn and 388 m in spring. Higher altitudes in spring were a function of birds ascending after leaving Ennis Lake, whereas birds were descending to visit the lake in autumn. More birds passed over valleys ...

Influence of Photoperiod Duration on Fecal Androgen Content in Migratory Reed Bunting Emberiza schoeniclus and Non-migra tory Meadow Bunting Emberiza cioides Males

Fecal androgen levels of migratory reed bunting Emberiza schoeniclus and non-migratory meadow bunting Emberiza cioides males were measured by radioimmunoassay after extraction with ether.When the photoperiod conditions were changed stepwise from 10 h light/14 h darkness (LD 10:14) to LD 15:9, in the migratory reed bunting the fecal androgen contents increased significantly at LD 12:12, and these relative high levels were maintained at LD 14:10.The photoperiod conditions resulting in high fecal androgen contents were similar to those resulting in nocturnal migratory restlessness.On the other hand, no significant change was observed in the non-migratory meadow bunting.The fecal androgen content would reflect the levels circulating in the plasma, as has been reported for the Japanese Quail.It has been suggested that androgen may be an endocrine regulator of vernal migration.Thus, androgen may be involved in the control of the vernal migration in the reed bunting.

Environmental and hormonal control of vernal migration in redheaded bunting (Emberiza bruniceps)

This paper reviews the factors and mechanisms which result in the development of the metabolic state characteristic of migration with special reference to a palaeotropic migrant the redheaded bunting, Emberiza bruniceps. Changes in climatic conditions and food supply act as proximate triggers of migratory behaviour in partial migrants. Typical migrants like buntings use daylength as a cue but the exact mechanism of how photoperiodic information is translated in terms of migratory events is still not known. Almost entirely the photoperiodic effects have been explained on the basis of the involvement of hypothalamo/hypophyseal system. We feel mechanism(s) other than those acting through neuroendocrine system may be equally important. Furthermore the role of temperature has not been adequately explored so far. Our observations indicate the possibility that redheaded buntings might integrate the information received from photoperiod with environmental temperature (and other factors?) resulting in the development of migratory state. The physiological control of avian migration is much less understood. Majority of papers have centered around the 'gonadal hypothesis' of Rowan supporting or contradicting it without providing conclusive evidence. Pituitary prolactin has also been shown to be implicated although the mechanism of action is only speculative. Conclusive evidence for the involvement of thyroid hormones (thyroxine, T4; triiodothyronine, T3) in the physiological timing of migration has been produced attributing independent roles to T4 and T3. It is suggested that seasonal variation in peripheral conversion of T4 to T3 could serve as an effective strategy to render available the required thyroid hormones T4 and/or T3 during different phases of the year thus accounting for the metabolic switch over from T4-dependent moult to T3-dependent migratory fat deposition and zugunruhe and also ensuring preclusion of simultaneous occurrence of these mutually incompatible events. Considering that the number of environmental and physiological factors influence this mechanism and considering that thyroid hormone molecule has been put to a wide range of usage during the course of evolution the mechanism(s) of peripheral conversion of T4-T3 may assume great flexibility and have selective value-especially in migration which is known to have evolved several times in diverse avian families. The attractiveness of this hypothesis lies in the fact that it has potential to explain the both physiological development of the metabolic state of migration and at the same time the physiological timing of migration not only with respect to the cycle of environment but also with respect to other conflicting seasonal events (moult and reproduction).

A north-temperate migratory bird: a model for the fate of lipids during exercise of long duration

Changes in deposition and the utilization of lipids during one night of migratory activity (nocturnal physical activity) were investigated in dark-eyed Juncos (Junco hyemalis) held in large outdoor aviaries. During vernal migration (May), captive Juncos were sampled at the beginning and conclusion of one night of nocturnal restlessness. Comparisons of variables were drawn with control samples collected from birds in March. Measurements included body weight, fat stored in subcutaneous depots (adiposity), adipose and muscle lipoprotein lipase activity, and fat cell lipolysis. During the migratory period, body weight and adiposity were increased over levels measured in the March birds (p less than 0.01). On the other hand, neither body weight nor adiposity were significantly altered as a result of nocturnal physical activity and no significant changes were observed in adipose lipoprotein lipase activity. Fat cell lipolysis was lower at the beginning than at the end of nocturnal physical activity while, the opposite was observed for muscle lipoprotein lipase activity (p less than 0.05). These results suggest that the amount of work of one night of nocturnal physical activity modifies both muscle lipoprotein lipase activity and fat cell lipolysis in an interrelated fashion. This phenomenon could act to direct a steady supply of fatty acids to the site of energy utilization, i.e., flight muscles.

Endocrine and Environmental Control of Vernal Migration in Male White-Crowned Sparrows, Zonotrichia leucophrys gambelil

Migratory birds in captivity continue to show migratory restlessness throughout the normal breeding period. We investigated whether reduced endocrine activity of the testes of captive male white-crowned sparrows causes this extension of migratory state. Experimentally elevated circulating levels of testosterone at the time it attains maximum levels in free-living male conspecifics after arrival on their territories did not suppress the migratory activity of captive males. Also, exposure to estradioltreated females did not affect migratory activity. Testosterone treatment at the end of the reproductive period postponed the seasonal termination of migratory activity and the onset of postnuptial molt. It is suggested (1) that unknown environmental or neuroendocrine cues suppress migratory behavior during breeding; and (2) that testosterone indirectly delays the final termination of vernal migratory behavior of captive males by its delaying effect on the onset of photorefractoriness, which ultimately terminates the tendency to migrate to the breeding area.