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

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 30 min 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. Received: 14 November 1997 / Accepted: 9 March 1998     

Parasite assemblages distinguish populations of a migratory passerine on its breeding grounds

We attempted to establish migratory connectivity patterns for American redstarts Setophaga ruticilla between their breeding and wintering grounds by characterizing the composition of their haematozoan parasite assemblages in different parts of their range. We detected significant but limited geographic structuring of haematozoan parasite lineages across the breeding range of the redstart. We found that redstarts from the south-eastern (SE) region of the breeding range had a significantly different haematozoan parasite assemblage compared with populations sampled throughout the rest of the breeding range. Evidence using stable isotopes from feathers previously demonstrated that redstarts from the SE of the breeding range also have a unique and separate wintering range. Thus, although two methods of estimating migratory connectivity have now both shown the SE US breeding sub-population of redstarts to be distinct from other populations on both the breeding and wintering grounds, conclusive migratory connectivity for this species as a whole could not be established.     

Food supply controls the body condition of a migrant bird wintering in the tropics

The hypothesis that migratory bird populations are limited during the non-breeding season is increasingly supported by empirical studies that also suggest consequences that carry-over into subsequent seasons. Although variation in food supply is the best supported explanation for non-breeding season limitation, the ecological mechanisms and physiological consequences are not well understood. We both supplemented and reduced Ovenbird (Seiurus aurocapilla) food availability on replicate plots in Jamaica in each of 3 years to determine the direct role of food in limiting the physical condition of Ovenbirds. Annual variation in rainfall and food supply created a natural experiment in parallel with manipulations. Sex and age-classes of Ovenbirds did not respond differently in terms of body condition to either food manipulation or natural variation in environmental conditions, suggesting that this population is not structured by strong dominance relationships. Ovenbird body mass, fat, and pectoralis muscle shape were positively and predictably related to manipulated food availability. Feather regrowth rate also responded positively to food supplementation and negatively to food reduction in the drier of 2 years. Prior to manipulation, annual variation in body mass corresponded to annual variation in food supply and rainfall, providing additional, correlational evidence of food limitation. Since multiple intercorrelated body condition indices of Ovenbirds responded directly to food supply, and since food supply influenced body condition independently of other habitat features, we argue that food is a primary driver of non-breeding season population limitation. Moreover, since these effects were observed during the late non-breeding period, when individuals are preparing to migrate, we infer that food availability likely initiates carry-over effects.     

The role of behavioral dominance in structuring patterns of habitat occupancy in a migrant bird during the nonbreeding season

Several species of territorial migratory birds exhibit sexualhabitat segregation on their wintering grounds, with some habitatscontaining mostly males and others mostly females. The objectiveof this study was to determine if in the American redstart(Setophaga ruticilla) in Jamaica habitat segregation is dueto social mechanisms or due to sex-specific habitat specialization.I used habitat-specific patterns of arrival by young males andfemales, observations of territorial displacements, removalexperiments, and simulations of territorial intrusions to differentiatebetween these two mechanisms. Redstarts were studied in twohabitat types, a male-biased mangrove forest and a female-biasedscrub habitat. In autumn, male and female hatch-year redstartsinitially settled in equal numbers in each habitat, and segregationof the sexes occurred gradually and mostly later in the arrivalperiod. This shift corresponded with an increase in densityof older birds and an increase in territorial displacements.Removal experiments showed that vacancies in male-biased habitatwere filled more rapidly and with greater frequency than thosein female-biased habitat and that vacated male territoriesin mangrove were replaced more often by females than by males.Simulations of territorial intrusions and analyses of body sizeindicated that levels of aggression and body size of both malesand females were greater in mangrove habitat, suggesting thatthese factors may be important in determining the outcomesof dominance interactions. I conclude that patterns of sexualhabitat segregation in redstarts are structured by the dominancebehavior of older and more dominant individuals, and these aremostly males.     

Patterns and correlates of extrapair paternity in American redstarts (Setophaga ruticilla)

We examined correlates and hypotheses pertaining to extrapairfertilizations in socially monogamous American redstarts (Setophagaruticilla). DNA fingerprinting revealed extrapair fertilizationin 59% of broods (19 of 32), involving 40% of nestlings (43of 108). Fewer broods than expected had mixed paternity, asdetermined from a binomial distribution of extrapair young inthe population. This result is consistent with the "good genes"hypothesis, but not with the "genetic diversity" hypothesis.There was a negative association between the age of putativefathers and the proportion of extrapair young in their broods.Irrespective of age, males with prior residency were cuckoldedless often than males new to the study area. Extrapair fatherswere' immediate neighbors in 7 of 10 cuckolded broods whereall neighbors were sampled. Males were more likely to sire offspringin the territories of younger neighbors than in those of olderneighbors. Plumage characteristics of adult males, breedingsynchrony of females, and breeding densities were not significantlyassociated with cuckoldry. Realized reproductive gain from cuckoldrywas small because of high nest predation in our area. Extrapairfertilizations allowed one-quarter of males whose own nestshad failed to achieve some reproductive success. Only 2 of 17males whose own nests were successful also had extrapair young.There was no egg dumping by females. We conclude that male ageand prior residency were predictors of cuckoldry in Americanredstarts. In the context of the heavy predation experiencedby our birds, extrapair fertilizations allowed many males tosalvage some reproductive success and did not increase the varianceof success across males     

Mycetophilidae (Diptera), an insect group vulnerable to forestry practices? A comparison of clearcut,managed and semi-natural spruce forests in southern Norway

The mycetophilid fauna and environmental variables were studied at 15 sites within a spruce forest in southern Norway. There were five replications of each of the following categories: semi-natural forests, clearcuts, and managed forests (clearcut 70–120 years ago). Clearcutting seems to induce a long lasting effect on the Mycetophilidae fauna. The semi-natural forests were more speciesrich and contained more potentially rare species than the two other categories. Even though managed forests and clearcuts differed in faunal composition, their species richness was not significantly different. Continuity is probably a main factor for maintaining the diversity of Mycetophilidae species. Lumping the 15 sites together, the number of species was strongly correlated with the (temporal) continuity of tree cover and substrates, which may reflect an increased diversity of fungal habitats both in dead wood and on the ground. The degree of continuity was recognized by means of indicator species of fungi, lichen and vascular plants. The amount of dead wood was correlated with the species diversity, but was probably dependent on the continuity factor, since clearcuts with much dead wood had relatively fewer species.Important elements in a strategy for conservation of the diversity of mycetophilid species seem to be: (i) to identify and protect the remnant patches of forests with long continuity, (ii) as far as possible, to practice timber harvesting in earlier clearcut forests instead of semi-natural forests.