Prebasic Molt Research Articles

Corticosterone and insulin interact to regulate plasma glucose but not lipid concentrations in molting starlings

Captive starlings ( Sturnus vulgaris) undergoing a prebasic molt were given exogenous insulin (INS) and corticosterone (CORT), to determine how these counterregulatory hormones would affect glucose and triglyceride concentrations during stress. Experiments were conducted at both morning (11:00) and night (23:00) to monitor daily variation in these responses. Concentrations of CORT, glucose, and triglycerides were measured in blood plasma within 3 min of disturbance (basal) and at 40, 70, and 150 min thereafter (stress-induced) to monitor the effect of injecting saline, INS, CORT, or INS + CORT. Saline injection (which included the stress of handling and restraint) increased CORT concentrations, decreased triglycerides, but had no effect on circulating glucose. Daily variation was not evident in CORT or glucose, but concentrations of triglycerides were higher at night than during the day. INS markedly suppressed glucose concentrations, but had no effect on plasma CORT or triglycerides. Glucose levels did not change in response to stress, but exogenous CORT elicited hyperglycemia during the day. Injected CORT also hastened the recovery of glucose concentrations from INS-induced hypoglycemia at night, and had no effect on circulating triglycerides. Basal concentrations of CORT, glucose, and triglycerides exhibited photoperiodic (mimicking seasonal) changes when combined with data from an earlier study in starlings held on long- and short-day photoperiods. During the prebasic molt, all three measurements were lower compared to other photoperiods. Together, these data suggest that glucose and triglycerides concentrations are regulated differently during molt, but INS and CORT maintain their traditional effects.

Molt-Migration in Western Tanagers (Piranga Ludoviciana): Age Effects, Aerodynamics, and Conservation Implications

AbstractWe describe timing and location of the prebasic molt in Western Tanagers (Piranga ludoviciana), focusing on age class differences in premolt movements. Most adults migrate south to the American Southwest where they stop to molt before moving on to their wintering grounds. Molting adults are found in southern mountain regions (Sierra Madres and southern Rockies) and in the Mexican monsoon region, where late summer rains cause a substantial increase in productivity. In contrast, juvenile Western Tanagers move to nearby montane habitats to molt before migrating, a previously undocumented strategy in western passerines that show molt-related movements. By molting in nearby montane habitats, juveniles avoid the aerodynamic cost of migrating in their “fluffy”, aerodynamically inefficient juvenal plumage. Western Tanagers are the fifth species known to use the Mexican monsoon region during the prebasic molt, further affirming the importance of that area to the conservation of Neotropical migrants that breed in western North America.

MOLT, PLUMAGE ABRASION, AND COLOR CHANGE IN LAWRENCE'S GOLDFINCH

Abstract We examined molting, plumage abrasion, and seasonal color changes of Lawrence's Goldfinch, Carduelis lawrencei, to determine to what extent the reported brightening of male colors at the approach of the breeding season results from plumage renewal. Lawrence's Goldfinch has only one molt per year, a complete postbreeding prebasic molt. Color changes during spring result entirely from plumage abrasion and fading, not from a prealternate molt as previously was thought. The yellow breast feathers of the males, but not the females, are unusually resistant to wear, so that the yellow on the breast appears to expand and brighten, as less durable surrounding gray feathers abrade. This may be due to a one-third greater thickness of the rachillae of the yellow barbs in males, so that instead of the rachillae progressively losing yellow-pigmented material, the brown pigmented barbules break off, leaving intact the rest of the barb with only yellow pigment. In contrast, the yellow breast feathers of females ...

MOLT PATTERNS AND MOLTING GROUNDS OF LUCY'S AND VIRGINIA'S WARBLERS: SIMILAR YET DIFFERENT

Using museum specimens, we documented the molt cycles and molting grounds of adult Lucy's (Vermivora luciae) and Virginia's (V. virginiae) warblers. During prebasic molts, both species replace all body plumage. Prebasic primary molt takes a mean of 71 days for Lucy's Warblers, but a mean of just 42 days for Virginia's Warblers. Prebasic molt occurs exclusively on the breeding grounds. We found no evidence of a prealternate molt in Lucy's Warblers, and limited evidence of a prealternate molt in Virginia's Warblers. In both species, the seasonal change in crown color is a function of the freshly replaced gray-tipped feathers acquired during the prebasic molt being abraded during the winter, such that the basal red coloration of those same feathers is exposed during spring. These species differ from other western-breeding passerines in that they do not appear to directly exploit the late season food resources in the southwestern United States and northwestern Mexico. This difference is suggested by the lack of a shift from northern parts of their breeding range to molt in those potentially more productive regions of the southwestern monsoon region. The possibility of winter territoriality may play a role in the rapid prebasic molt exhibited by Virginia's Warblers. On the other hand, the lengthy prebasic molt of Lucy's Warblers may be explained by the use of poor nutritional resources during late summer, and/or the lack of territoriality during late summer and winter. We conclude that our data, in conjunction with data from previous studies, suggest multiple contrasting molt migration strategies among breeding passerines in western North America.

MOLT-MIGRATION IN WESTERN TANAGERS (PIRANGA LUDOVICIANA): AGE EFFECTS, AERODYNAMICS, AND CONSERVATION IMPLICATIONS

We describe timing and location of the prebasic molt in Western Tanagers (Piranga ludoviciana), focusing on age class differences in premolt movements. Most adults migrate south to the American Southwest where they stop to molt before moving on to their wintering grounds. Molting adults are found in southern mountain regions (Sierra Madres and southern Rockies) and in the Mexican monsoon region, where late summer rains cause a substantial increase in productivity. In contrast, juvenile Western Tanagers move to nearby montane habitats to molt before migrating, a previously undocumented strategy in western passerines that show molt-related movements. By molting in nearby montane habitats, juveniles avoid the aerodynamic cost of migrating in their “fluffy”, aerodynamically inefficient juvenal plumage. Western Tanagers are the fifth species known to use the Mexican monsoon region during the prebasic molt, further affirming the importance of that area to the conservation of Neotropical migrants that breed in western North America.

Molt of the Ivory Gull

-Molting is a fundamental biological process common to all birds, and understanding molt strategies is an integral part of studying any species' life history. Molt of the monotypic Ivory Gull (Pagophila eburnea) has received variable and often contradictory treatment in the literature. Based upon examination of 152 museum specimens, the unique molt strategy of this species is clarified. Juveniles molt directly into definitive (= adult) basic plumage via a complete molt during April to August of their second calendar year. Adults have only a single molt per cycle, a strategy unique among gulls: their prebasic molt starts in March and April, progresses quickly, and is mostly completed before the pre-laying to hatching period (mid June to early August); adults typically suspend primary molt at this time and conclude molt of outermost primaries in late August or September. Thus molt and breeding overlap broadly in the Ivory Gull, presumably an adaptation to the constraints of its high-latitude winter range and the brief Arctic summer. Received iJuly 2001, accepted 12 August 2001.

Daily and photoperiod variations of basal and stress-induced corticosterone concentrations in house sparrows (Passer domesticus).

Corticosterone concentrations were measured in captive house sparrows (Passer domesticus) and found to vary both daily and with different photoperiods. Basal corticosterone was highest during the dark hours of the daily cycle and lowest during the light hours. This trend remained constant when the birds were held on short-day and long-day light cycles, and while the birds were undergoing a prebasic molt. At all times, corticosterone concentrations significantly increased in response to the stress of handling and restraint. Stress-induced corticosterone concentrations, however, only reflected a daily rhythm when the birds were held on short-days. Furthermore, even though mean basal corticosterone concentrations were equivalent over the short-day, long-day, and molt, total corticosterone output in response to stress was lower in molting birds, especially at night. Therefore, these data indicate that captive house sparrows modulate corticosterone in daily cycles that change in response to photoperiod.

Regulation of the hypothalamic-pituitary-adrenal axis in free-living pigeons.

We studied feral free-living pigeons (Columba livia) to determine whether either unstressed or stress-induced corticosterone release was altered during a prebasic molt. The pigeons were at various stages of molt throughout the study, but corticosterone responses in molting and nonmolting birds did not differ. This was further reflected in equivalent adrenal responses to exogenous adrenocorticotropic hormone (ACTH), suggesting equivalent steroidogenic capacity of adrenal tissues during both physiological states. There was a slight change, however, in pituitary regulation during molt. Whereas exogenous arginine vasotocin (AVT) elevated corticosterone levels in nonmolting birds, during molt an equivalent dose of AVT was ineffective, suggesting that the pituitary is less sensitive to an AVT signal during molt. AVT also appears to be more effective than corticotropin-releasing factor at eliciting ACTH release in pigeons. Overall, these data indicate that pigeons regulate their corticosterone release differently during molt than other avian species studied to date.

Two-Year Delay in Plumage Maturation of Male and Female ‘Elepaio

Abstract I studied molt and plumage in the ‘Elepaio (Chasiempis sandwichensis), a monarch flycatcher endemic to the Hawaiian Islands, at Hakalau Forest National Wildlife Refuge on the island of Hawai‘i from 1994–1997. I captured birds in mist nets, examined them for molt, measured their plumage color with Munsell color chips, and followed them for several years to document plumage changes. Male and female Hawai‘i ‘Elepaio exhibited three distinct post-juvenal plumages, first basic, second basic, and definitive basic, and thus have a two-year delay in plumage maturation. ‘Elepaio have a single annual molt, so there are no alternate plumages. The first prebasic molt is partial, but all subsequent molts are complete. The first basic plumage is juvenile-like and sexually monochromatic. The second basic and definitive basic plumages are sexually dimorphic in throat color. In each sex the second basic plumage resembles definitive plumage. Age-related plumage changes include increases in the whiteness, number, a...

AGE AND SEX DETERMINATION OF THE MAUI PARROTBILL

Abstract We determined the best plumage and morphometric variables for ageing and sexing the Maui Parrotbill (Pseudonestor xanthophrys), an endangered Hawaiian honeycreeper found only on east Maui, Hawaii, by examining and measuring 30 museum specimens and 71 live birds captured in mist nets. Juvenal plumage was identified by the presence of pale-tipped wing bars on the middle and greater coverts, grayish olive dorsal plumage, and dingy white underparts and superciliaries. Birds undergoing first prebasic molt retained the juvenal remiges, rectrices, and wing coverts. Birds in first basic plumage possessed juvenal wing bars and a dull juvenal-like plumage. Subsequent molts were complete, and adults lacked wing bars. Adult males had bright yellow plumage on the cheeks, throat, and superciliaries, as did 27% of adult females. All other adult females had less yellow in the underparts. The dorsal plumage of adult females was more variable than adult males and was either yellow-olive like the males or grayish o...

Definitive Prebasic Molt of Gray Catbirds at two Sites in New England

AbstractWe studied the pattern and timing of prebasic molt in adult Gray Catbirds (Dumetella carolinensis) at two New England sites: Block Island, Rhode Island (BIRI), and the Vermont Institute of Natural Science (VINS) in Woodstock, Vermont. Catbirds at VINS initiated molt earlier and molted at a significantly slower rate than catbirds at BIRI. Mean individual molt durations spanned approximately 54 days at VINS and 44 days at BIRI. The two groups ended molt at about the same time. Catbirds at VINS were more variable in the timing of their molt. At both sites, second-year catbirds began and ended molt significantly earlier than after second-year catbirds. Males and females did not differ significantly in their rate or timing of molt at either site. Behavioral observations at BIRI indicated that catbirds spent less time foraging during the heaviest period of molt, but that increased foraging during late molt stages coincided with increases in fat stores, indicating overlap of molt and hyperphagy. We found no evidence that Gray Catbirds at either site departed for migration prior to completing remigial molt. The later molt schedule of BIRI birds likely resulted from the extended breeding season of some individuals. We believe that molt schedules at the two sites were influenced less by latitude per se than by site-specific differences in vegetation, food abundance, and temperature, resulting from differing elevations and conditions of coastal and inland environments.

Molt of the Gray Vireo

AbstractUsing museum specimens, I document the molt cycles and molting grounds of the Gray Vireo (Vireo vicinior). During prebasic molts, adult female Gray Vireos replace their primaries in 57 days, whereas adult males take 70 days; all body plumage is replaced during this molt. Prebasic molts occur almost exclusively on the breeding grounds; just 3 of 41 specimens replacing primaries were collected away from breeding grounds. No molting specimens were collected from wintering areas. Prealternate molt occurs on the wintering grounds, and appears limited to the replacement of innermost secondaries and a limited molt of body plumage. By performing prebasic molt on breeding grounds, the Gray Vireo differs from several other western breeding passerine species that use desert regions of the southwestern United States and northwestern Mexico to exploit late summer food resources. The areas of these southwestern desert regions used by other species form a small portion of the breeding grounds, and encompass the entire wintering grounds, of Gray Vireos. I hypothesize that this contrast in molting regions is not due to differences in the general timing of prebasic molts among these species; rather, the contrast may be due to constraints imposed on Gray Vireos by a dietary shift to fruit during winter and the need to defend winter territories.

Time-Activity Budgets of Harlequin Ducks Molting in the Gannet Islands, Labrador

AbstractWe studied the time-activity budgets of Harlequin Ducks (Histrionicus histrionicus) molting at the Gannet Islands, Labrador in the summer of 1998. For the entire population, a large proportion of time was spent hauled out of the water (61.4%), and resting (53.5%). Only a small proportion of time was spent foraging (11.6%). Male Harlequin Ducks undergoing the pre-basic molt were hauled out of the water significantly more (92.2%) than males in basic plumage (8.1%). Males undergoing the pre-basic body feather molt foraged significantly less (1.7%) than males in basic plumage (17.7%). Harlequin Ducks do not appear to increase their food intake to meet the nutritional requirements of molt. Instead they may try to reduce thermoregulatory and maintenance costs by engaging in activities that do not consume much energy, and by staying out of cold water while their plumage is not intact. Furthermore, they may deliberately lose body mass while molting to regain the ability to fly at an earlier stage of wing molt.

Daily and Seasonal Variation in Response to Stress in Captive Starlings (Sturnus vulgaris): Corticosterone

Captive starlings were used to examine daily and seasonal changes in basal and stress-induced corticosterone levels. Birds were bled at 4 times during the daily cycle and during three different simulated seasons: under a short-day photoperiod (mimicking winter), under a long-day photoperiod (mimicking summer), and while undergoing a prebasic molt. Basal corticosterone samples were assayed from blood collected within 3 min of disturbance and corticosterone increases in response to handling and restraint were monitored in blood taken at 15, 30, and 45 min postdisturbance. Handling and restraint elicited robust increases in corticosterone at all times of the day and during all three seasons. Both basal and stress-induced levels varied with the time of day (with the exception of basal samples during molt). Levels were higher at night, during the bird's inactive period, and decreased during the day. These data indicate that starlings have daily rhythms in both basal corticosterone levels and in their response to stress, with more corticosterone released during the night in response to identical stimuli. Starlings also show pronounced seasonal variation in both basal and stress-induced corticosterone levels. Although birds held on short and long days had equivalent corticosterone levels, both basal and stress-induced levels were lower during molt. This parallels data from free-living birds and provides a laboratory model for studying seasonal corticosterone regulation.

Daily and Seasonal Variation in Response to Stress in Captive Starlings (Sturnus Vulgaris): Glucose

We investigated the seasonal and daily variation in plasma glucose levels in response to stress in captive wild starlings. Starlings were captured from the wild during the winter, held on short days (11L:13D, mimicking winter), and then shifted to long days (19L:5D, mimicking summer). Birds were maintained on long days until they began a prebasic molt, the energetically costly replacement of feathers. Throughout the daily cycle we took a basal blood sample within 3 min of disturbance and took subsequent blood samples at 15 and 30 min. Birds were kept in cloth bags (restraint) between bleeds. Experiments were repeated during all three seasons (short day, long day, and molt). Starlings showed no sexual difference in circulating glucose levels at any time of the day or in any season. Both basal and stress-induced glucose levels, however, showed a significant effect of season, with birds held on long days exhibiting the highest levels, molting birds showing intermediate levels, and birds held on short days exhibiting the lowest levels. Basal glucose levels also showed a circadiel rhythm in all three seasons. Regardless of season, however, the daily peak in basal levels occurred at midday with nadir in the middle of the scotophase. This trend was paralleled in the overall weights of the birds. Although stress-induced glucose levels showed no circadiel rhythm, the stress-induced elevation of glucose above baseline showed a significant daily rhythm, indicating that stress elevated plasma glucose levels only during the scotophase in all three seasons.

TIMING, PATTERN, AND EXTENT OF FIRST PREBASIC MOLT OF WHITE-WINGED CROSSBILLS IN ALASKA

Abstract The timing, pattern, and extent of the first prebasic wing molt of White-winged Crossbills (Loxia leucoptera) were determined during a 6-yr banding study in interior Alaska. Hatching-year birds were not caught in substantial numbers until May, and all crossbills molted from juvenal to first basic plumage between September and December. A subsample consisting of 65 males and 55 females in first basic plumage was used to determine the pattern and extent of wing covert molt. On average, birds molted three greater coverts (GC) and retained two juvenal medial coverts (MC) per wing. The number of molted GCs was negatively correlated with that of juvenal MCs. Twenty-three percent of the individuals molted no GC, and no crossbill replaced all these feathers. The pattern and extent of covert molt was similar in males and females and was symmetrical in 69% of the individuals, but differed by one or two feathers in the remaining birds. Males in first basic plumage and with partially red or pink contour feat...

Year-Class Differences in the Reproductive System, Plasma Prolactin and Corticosterone Concentrations, and Onset of Prebasic Molt in Male Dark-Eyed Juncos (Junco hyemalis) during the Breeding Period

Year-class differences in reproductive function were investigated in a free-living population of adult male Dark-eyed Juncos, Junco hyemalis, breeding in interior Alaska. Second-year males (SY, entering their first breeding season) were compared with after-second-year males (ASY, entering at least their second breeding season). We measured body mass, size of the cloacal protuberance (CP), testis mass, onset of prebasic molt, and concentrations of plasma luteinizing hormone (LH), testosterone (T), corticosterone (CORT), and prolactin (PRL) throughout the reproductive season (April to mid-July). There were no differences in SY and ASY body weights but SY males had smaller CPs and testis masses than ASY males during gonadal recrudescence and at the end of the breeding season. Plasma LH was elevated from April until mid-June and then decreased in the same way in both year classes. In contrast, plasma T was high from April until mid-May and was lower in SY than in ASY juncos shortly after they arrived on their breeding grounds at the end of April, but not at other times. In July, SY males started to molt earlier, suggesting that they became photorefractory earlier than ASY males. Plasma PRL increased progressively in both year classes between April and early June and decreased in early July. At this time, plasma PRL decreased earlier in SY than in ASY males. Plasma CORT changed seasonally, but did not differ between SY and ASY juncos. Thus, year-class differences in CP sizes and testis mass apparently did not result from SY males secreting less LH or more PRL or CORT than ASY males. It is suggested that differences in reproductive condition in SY and ASY juncos are mediated by interactions with conspecific birds and do not result from an intrinsic effect of age.

Postbreeding Movements of American Avocets and Implications for Wetland Connectivity in the Western Great Basin

Abstract Wetlands in the western Great Basin of the United States are patchily distributed and undergo extensive seasonal and annual variation in water levels. The American Avocet (Recurvirostra americana) is one of many shorebird species that use these wetlands as breeding and migratory stopover sites and must adjust to variable conditions. We used radio telemetry to determine postbreeding, premigratory movement patterns of avocets throughout the region. In 1996 and 1997, 185 breeding adults were captured and fitted with radio transmitters at five breeding areas in Oregon, California, and Nevada. Regular aerial and ground surveys were conducted at the five main study areas from June through September, or until all avocets had left a site. Other wetlands in the western Great Basin also were surveyed by aircraft for the presence of radio-marked birds. Fifty-six percent of radio-marked avocets were still detected in the region at least eight weeks after capture. Each of these individuals was detected at an average of 2.1 lakes (range 0 to 6), with 74% found at more than one lake system. Forty radio-marked individuals moved at least 200 km between wetlands prior to migration, most of which dispersed northward. Male and female patterns did not differ significantly. Overall, movements may be associated with a prebasic molt, exploitation of a superabundant food source in northern lakes, and reconnaissance for future breeding efforts or staging sites. These results also demonstrate wide-ranging patterns of dispersal in this species and suggest a need for the consideration of large-scale habitat connectivity issues in establishing conservation strategies for shorebirds in the western Great Basin.

MOLT OF THE GRAY VIREO

Using museum specimens, I document the molt cycles and molting grounds of the Gray Vireo (Vireo vicinior). During prebasic molts, adult female Gray Vireos replace their primaries in 57 days, whereas adult males take 70 days; all body plumage is replaced during this molt. Prebasic molts occur almost exclusively on the breeding grounds; just 3 of 41 specimens replacing primaries were collected away from breeding grounds. No molting specimens were collected from wintering areas. Prealternate molt occurs on the wintering grounds, and appears limited to the replacement of innermost secondaries and a limited molt of body plumage. By performing prebasic molt on breeding grounds, the Gray Vireo differs from several other western breeding passerine species that use desert regions of the southwestern United States and northwestern Mexico to exploit late summer food resources. The areas of these southwestern desert regions used by other species form a small portion of the breeding grounds, and encompass the entire wintering grounds, of Gray Vireos. I hypothesize that this contrast in molting regions is not due to differences in the general timing of prebasic molts among these species; rather, the contrast may be due to constraints imposed on Gray Vireos by a dietary shift to fruit during winter and the need to defend winter territories.

DEFINITIVE PREBASIC MOLT OF GRAY CATBIRDS AT TWO SITES IN NEW ENGLAND

Abstract We studied the pattern and timing of prebasic molt in adult Gray Catbirds (Dumetella carolinensis) at two New England sites: Block Island, Rhode Island (BIRI), and the Vermont Institute of Natural Science (VINS) in Woodstock, Vermont. Catbirds at VINS initiated molt earlier and molted at a significantly slower rate than catbirds at BIRI. Mean individual molt durations spanned approximately 54 days at VINS and 44 days at BIRI. The two groups ended molt at about the same time. Catbirds at VINS were more variable in the timing of their molt. At both sites, second-year catbirds began and ended molt significantly earlier than after second-year catbirds. Males and females did not differ significantly in their rate or timing of molt at either site. Behavioral observations at BIRI indicated that catbirds spent less time foraging during the heaviest period of molt, but that increased foraging during late molt stages coincided with increases in fat stores, indicating overlap of molt and hyperphagy. We foun...