Rebecca L. Holberton

Corticosterone levels as indicators of habitat quality: effects of habitat segregation in a migratory bird during the non-breeding season

Abstract During the non-breeding season, many species of territorial migratory birds exhibit a non-random pattern of habitat distribution, with males and females occupying different habitats. In this study, we examined possible physiological consequences arising from such habitat segregation in one migrant passerine species, the American redstart (Setophaga ruticilla), on its non-breeding grounds in Jamaica, West Indies. For 2 years, we measured concentrations of corticosterone, at the time of capture (baseline) and 30u2009min after capture (profile of acute corticosterone secretion), in redstarts in two distinct habitats, one occupied predominately by males and one mostly by females. All redstarts in both habitat types exhibited similar concentrations of baseline corticosterone levels in fall (October), whereas in spring (March–April), redstarts in female-biased habitat exhibited significantly higher baseline levels regardless of age or sex. In fall, all individuals in both habitats exhibited significant increases in corticosterone concentration with capture and handling, but in spring only redstarts (both sexes) in male-biased habitat continued to exhibit acute corticosterone secretion. Redstarts in female-biased habitat had elevated baseline corticosterone levels and reduced acute corticosterone secretion. In spring, baseline corticosterone concentration was negatively correlated with body mass, suggesting muscle catabolism associated with high corticosterone concentrations or possibly that birds are leaner as a result of increased foraging effort. These results indicate that redstarts (primarily females) in female-biased habitats suffered a decline in physiological condition, which could in turn influence their departure schedules, migration patterns and even their condition and arrival schedules on the breeding grounds. Thus, segregation of populations into habitats of different quality during the non-breeding period may have ramifications throughout the annual cycle of such migratory species. Furthermore, these results show the usefulness of plasma corticosterone levels as indicators of physiological condition and thus habitat quality for birds during the non-breeding period.

Modulation of the adrenocortical stress response in neotropical migrants during autumn migration

easonal changes in corticosterone metabolism have been implicated in the migratory physiology of passerines. To investigate how this hormone might relate to migra- tory condition in free-living birds, we compared plasma levels of corticosterone in response to capture and handling stress in premigratory and migrating Gray Catbirds (Dumetella car- olinensis). Lean, premigratory catbirds exhibited stress responses typical of other nonlipogenic, nonhyperphagic bird species sampled on the breeding or wintering grounds (i.e. low levels of corticosterone at capture followed by significant increases over I h). In contrast, fat, migratory catbirds sampled later in the season showed significantly higher baseline levels of corticosterone at capture and no significant increase in hormone levels with handling time. We also sampled a group of migrating Yellow-rumped Warblers (Dendroica coronata) at a stopover site. Like catbirds, migrating warblers exhibited an absence of a stress response, with no significant change in corticosterone levels with capture and handling stress. Unlike birds sampled on the breeding grounds, there was no relationship between corticosterone levels at capture and fat scores in premigratory and migratory catbirds, or in the migratory warblers. These results support our Migration-Modulation Hypothesis regarding seasonal changes in corticosterone levels, viz.: (1) an absence of a relationship between fat deposition and baseline corticosterone levels illustrates the permissive nature of the hormone, which remains elevated throughout the migratory period to facilitate hyperphagia and lipogenesis independent of short-term changes in energetic condition; and (2) further elevation of cor- ticosterone in response to acute stress is suppressed during migration to protect skeletal muscle needed for flight. Received 25 April 1995, accepted 14 July 1995.

Status signalling in dark-eyed juncos, Junco hyemails: plumage manipulations and hormonal correlates of dominance

Status signalling in dark-eyed juncos was investigated by using plumage manipulations and observing interactions within pairs of birds. Plumage manipulations included darkening of the hood and mantle and the addition of white tail feathers to mimic the appearance of adult males. Plumage characteristics predicted dominance relationships within pairs of first-year males when body size and prior residence in cages was controlled. These characteristics were also strong enough to reverse previously established intra-class relationships within pairs of first-year males and first-year and adult females. The results fail to support the incongruence hypothesis (Rohwer: Behaviour, 1977, 61, 107–129; Rohwer &Rohwer: Anim. Behav., 1978, 26, 1012–1022). In contrast to recent studies using exogenous testosterone to investigate dominance behaviour, circulating plasma levels of testosterone were measured and a comparison made between those of dominants and those of subordinates. Plasma levels of corticosterone were also analysed as a measure of stress. No correlation between either hormone and dominance status was found. Hypotheses are proposed to explain the absence of a relationship between circulating levels of testosterone and winter aggression in flocking birds.

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.

Does stress response predict return rate in a migratory bird species? A study of American redstarts and their non-breeding habitat

In vertebrates, the adrenocortical stress response activates an emergency life-history stage, which is thought to promote survival by helping individuals escape life-threatening situations. Although the adrenocortical stress response promotes many behavioural and physiological changes, it remains unclear whether this stress response actually translates into higher survival in wild vertebrates. We measured the adrenocortical stress response of non-breeding American redstarts (Setophaga ruticilla), a migratory bird that wintered in habitats of either high (mangroves) or low suitability (scrubs), and subsequently monitored their return rate during the following non-breeding seasons. The intensity of the adrenocortical stress response was consistent within individuals across the non-breeding season and was positively correlated with return rates in redstarts that wintered in scrubs, but not in redstarts that wintered in mangroves. Thus, in a context-dependent manner, the ability of an individual to physiologically react to stress determines its ability of returning to its non-breeding territory the following winters. For an individual, the ability to mount an important adrenocortical stress response probably benefits to survival. However, this beneficial effect probably depends on an individuals environment and phenotypic characteristics because these two variables are likely to affect its probability of being confronted with life-threatening stressors during its annual life cycle.

Age-related dominance in male dark-eyed juncos: effects of plumage and prior residence

Abstract Age-related differences in dominance ability and the effect of prior residence on age-related dominance were investigated in captive male dark-eyed juncos, Junco hyemalis . To determine if adults and first-winter birds differed in their intrinsic dominance ability, pairs of adult and first-winter birds of equal body size and winning experience were established. Because plumage influences status, young birds were experimentally darkened to match their older cage-mates. Adult and first-winter males became dominant with equal frequency. Because of this result and the fact that adults are usually dominant in the field, the effect of prior residence on age-related dominance was also investigated. When given prior residence, birds of either age-class won significantly more interactions against intruders. Although first-winter residents (without darkened plumage) dominated intruding adults, they won significantly fewer interactions than did resident adults against intruding adults. This difference in relative dominance success may be due to age-related plumage differences. In the absence of advantages based on prior residence or winning experience, both adult and first-winter birds may rely on plumage differences to settle contests with others of their age class. Prior residence confers an advantage on both adult and first-winter birds. In nature, however, adults are more likely to experience this advantage as a consequence of earlier arrival at migration stopover sites and of site fidelity to the wintering grounds.

Effects of exogenous corticosterone on locomotor activity in the red-eared slider turtle, Trachemys scripta elegans.

We investigated the effects of exogenous corticosterone on the locomotor activity of captive red-eared slider turtles, Trachemys scripta elegans. An increase in plasma corticosterone often increases locomotor activity in mammals and birds, but there are no reported findings for turtles. In this study turtles implanted with corticosterone-filled Silastic((R)) implants showed a significant increase in caged locomotor activity when compared to control animals with empty implants. Corticosterone-treated turtles also showed a significant increase in plasma corticosterone concentration when pre-trial plasma samples were compared to post-trial plasma samples, while control turtles exhibited no such increase, validating the effectiveness of our implants to deliver corticosterone. Although corticosterone remained high at the end of the activity trials, the increase in activity was ephemeral in nature, peaking within 48 hr after the implant was in place. This suggests that the effects of corticosterone on behavior may be context-dependent (i.e., whether the turtles can find food) and concentration-dependent, and that there are underlying physiological mechanisms, perhaps mediated at the receptor level in the brain, involved in locomotor activity behavior in slider turtles. Environmental perturbations that cause a reduction in available food resources may cause the organism to increase its level of locomotor activity to increase food encounter rate but later reduce activity to conserve energy reserves. These data are important when considering behavioral and physiological mechanisms involved in a turtles response to changing conditions in habitat quality. J. Exp. Zool. 284:637-644, 1999.

Telomere length, non‐breeding habitat and return rate in male American redstarts

Summary 1. Telomeres are long repetitive noncoding sequences of DNA located at the ends of chromosomes. Recently, the study of telomere dynamics has been increasingly used to investigate ecological questions. However, little is currently known about the relationships that link environmental conditions, telomere dynamics and fitness in wild vertebrates. 2. Using a small migratory bird (American redstart, Setophaga ruticilla), we investigated how telomere dynamics can be affected by non-breeding habitat quality and to what extent telomere length can predict the return rate of males. 3. We show that telomeres shorten in most individuals over a 1-year period and, importantly, that telomeres of individuals wintering in a low-quality habitat shorten more than those of individuals wintering in a high-quality habitat. 4. In addition, we found that longer telomeres are associated with a higher return rate than shorter telomeres, although the relationship between return rate and telomere length did not depend on habitat quality. 5. Our study suggests that telomere dynamics are affected by environmental conditions and are related to indices of fitness in a migratory bird species.

Differential migration and an endocrine response to stress in wintering dark-eyed juncos (Junco hyemalis).

The dark–eyed junco (Junco hyemalis) exhibits differential migration in autumn that, in general, results in females overwintering south of males, and young within each sex overwintering north of older birds. Individuals overwintering at higher latitudes face less predictable and more challenging environmental conditions. Rapid increases in circulating levels of the energy-regulating glucocorticosteroid, corticosterone, occur in response to environmental stressors. To establish whether the strength of acute corticosterone secretion was correlated with the probability of encountering poor environmental conditions, we compared the corticosterone stress response (e.g. initial plasma concentrations at the time of capture and 30 min later) in dark–eyed juncos overwintering in Mississippi (MS), USA, near the southern limit of their wintering range, with juncos overwintering in New York (NY), USA, near the northern limit of their wintering range. During two winters, 22 males and one female were sampled in NY; 13 males, 12 females and one bird of undetermined sex were sampled in MS. Not unexpectedly, NY birds carried greater fat reserves that resulted in a significantly higher value of energetic condition (mass corrected for wing cord cubed). There was no difference between the two winters sampled at either site, nor was there an effect of sex on patterns of corticosterone secretion in MS birds. With sexes pooled, MS and NY birds had similar baseline corticosterone levels. However, as predicted, NY birds exhibited significantly higher corticosterone concentrations 30 min after capture. These results support the hypothesis that birds wintering in less predictable, more extreme environments show a higher amplitude corticosterone response, which may enable them to adjust their behaviour and physiology more rapidly in response to environmental stressors such as storms. Adrenocortical sensitivity may be a part of the physiological milieu associated with differential migration in juncos; whether it results from endogenous differences in the migratory programmes of individuals or from acclimatization to local environmental conditions remains to be determined.

Corticosterone levels in relation to migratory readiness in red-eyed vireos (Vireo olivaceus)

We examined the relationship between plasma levels of corticosterone and the migratory activity and directional preference of red-eyed vireos during fall migration at the northern coast of the Gulf of Mexico. Corticosterone is thought to play a role in physiological and behavioural processes before, during, and after long-distance migratory flights. An increase in corticosterone at the onset of migratory flights can be expected in birds that are energetically prepared to migrate in a seasonally appropriate southerly direction. Red-eyed vireos (Vireo olivaceus) were tested in orientation cages under clear twilight skies. Just prior to the orientation experiments, blood was sampled to assay baseline corticosterone levels. Average corticosterone level for all birds was 22.8xa0ng/ml. Red-eyed vireos with higher than average baseline levels of corticosterone were significantly more active in orientation cages compared to birds with lower levels of corticosterone. Moreover, birds with higher than average levels oriented in a southwesterly direction, which is consistent with a trans-Gulf flight, whereas individuals with levels below average showed a NNW mean direction. Although there was no significant difference in baseline levels of corticosterone between fat and lean birds, individual mass loss between capture and test was negatively correlated with corticosterone levels. Results from this study clearly demonstrate that corticosterone influences departure decisions and the choice of direction during migration.