The internet connection was down for nearly 24 hours at the hotel, so I was unable to update you all on the talks I attended yesterday afternoon, which caused me to express much crankiness. Hopefully, I will be able to get that done sometime within the next 24 hours (i.e.; before I return to NYC). Today is the third day of the conference and I am getting tired and overwhelmed by the intense flood of presentations and posters, so now I am attending only presentations that focus explicitly on evolutionary biology or ornithology. Below the fold are the bird presentations that I attended;
- Beck. Human-caused habitat fragmentation and disturbance are becoming increasingly common problems for many natural populations. Habitat fragmentation or reduction may have drastic effects on a population by greatly reducing the number of individuals present or their reproductive success. However, less drastic habitat modification may have more subtle effects that are only detected by examining a population in detail. More subtle effects of habitat modification could include differences in the stress response or changes in the development of young. To better understand this, Beck and colleagues examined the effects of habitat modification on nestling growth and condition in the Prothonotary warbler, Protonotaria citrea. In this case, habitat modification was defined as areas where timber had been harvested in the last 10 years such that the forest was thinned by 40-60%. Beck predicted that nestlings raised in these thinned areas would be in worse condition, would be less mature at fledging, and would grow at a slower rate than those raised in unharvested areas. Beck also predicted that nestlings in harvested areas would have higher levels of the stress hormone, corticosterone, than those raised in unharvested areas. But interestingly, Beck and colleagues found no significant differences in growth rates, condition, or maturity of nestlings between the harvested and unharvested areas. Furthermore, they also found that plasma stress hormone concentrations did not differ significantly between nestlings raised in the harvested and unharvested habitats. Beck and colleagues showed that prothonotary warblers are not negatively affected by thinning the forest surrounding their breeding sites. However, it is possible that forest thinning that encompasses aquatic areas where they nest might negatively affect the birds because they strongly prefer to breed in shaded areas.
- Deviche. As you have probably surmised from previous presentations, changes in baseline and stress-induced plasma corticosterone (CORT) concentrations have been documented in many avian species. However, few studies have investigated systematic variations of these concentrations throughout the annual cycle for intact free-ranging birds, and correlated how changes in plasma CORT are related to those of other hormones, particularly sex steroids, such as the androgens and estrogens. To document these changes in CORT, adult male Rufous-winged Sparrows, Aimophila carpalis, were sampled in the field over a 1.5 year-long period, and seaasonal baseline plasma CORT levels were examined. CORT concentrations were high during the summer monsoon season (Jul-Sept) when birds were breeding and low during the fall post-breeding molt and throughout the winter non-breeding season. In one year, but not in the next, plasma CORT was as elevated in March, when males were beginning to develop testes and not yet in breeding condition, as they were during the summer breeding period. This yearly difference was correlated with a difference in local rainfall, indicating that proximate factors associated with precipitation may stimulate the hypothalamo-pituitary-adrenal axis that triggers CORT synthesis. Capture and handling of these birds for 30 minutes stimulated CORT secretion. Deviche found that baseline and acute stress-induced plasma CORT levels were individually correlated but the amplitude of this stress response due to capture and handling of the birds did not vary during the annual cycle, suggesting that there is no seasonal change in sensitivity to acute stress. Additionally, baseline plasma testosterone (T) was seasonally high during the summer breeding season but low in March, indicating no significant year-round relationship between the secretion of this androgen and of CORT. Further, acute stress, while increasing plasma CORT, it also consistently decreased plasma T by almost 50%.
- Boyd. As you already know, animals secrete glucocorticoids -- either cortisol or corticosterone -- in response to stressful events. These elevated concentrations of glucocorticoids suppress reproduction and regulate the immune system to promote survival in the face of acute stress but this response can be deleterious in the long term. In this presentation, Boyd and colleagues investigated the link between the acute stress response and development of long term chronic stress. They tested whether intermittent increases in the glucocorticoid, corticosterone (CORT), at different frequencies can cause similar effects to chronic stress in White-crowned sparrows, Zonotrichia leucophrys. They found a significant affect on gonadal development (testis mass) in birds treated with CORT three times per day for five weeks compared to their control group. This experimental group also experienced a decline in health, and changes in white blood cell count that were consistent with chronic immune suppression. These results reveal that intermittent increases of corticosterone simulating acute stress responses, if they occur frequently, can cause deleterious effects associated with chronic stress.
- O'Neal. Hormonal manipulations allow scientists to create novel phenotypes so we can learn how existing phenotypes evolve and why they persist. In the male dark-eyed junco, Junco hyemalis, experimental elevation of the androgen, testosterone (T), is known to decrease both male parental care and offspring survival, but causes higher overall fitness due to increased breeding success. To understand this effect and also to understand the selective forces that influence variations in T concentrations in birds, O'Neal and colleagues focused their attention on the females. In this study, O'Neal asked whether T might mediate parental care behaviors other than incubation. So T levels in females were experimentally elevated to their spring maximum concentrations using subcutaneous T implants. O'Neal and colleagues videotaped free-living females at their nests. They measured female brooding behavior when the young were three days old and feeding behavior when the young were six days old. They also measured female nest defense behavior by quantifying responses to a mounted predator (a chipmunk) placed near the nest. They found that T-implanted females showed a significant reduction in brooding time when compared to control birds. However, the experimental females did not differ from controls in the number of nestlings fed per hour or in overall provisioning rate. Interestingly, males mated to T-implanted females changed their behaviors: they fed young significantly more than males mated to control females. Additionally, T-implanted females performed significantly more dives at the mounted nest predator than did control females but the experimental females did not differ in other nest defense behaviors. O'Neal and colleagues thus conclude that some aspects of female parental behavior are sensitive to T while others are not.
- Coverdill. Analysis of migratory behavior may increase scientists' understanding of nocturnal activity in captive birds and may reflect distinct migratory strategies displayed by wild birds. For example, expression of migratory restlessness (MR) in captive birds has been considered a characteristic of migratory species. Paradoxically, some sedentary bird species also express MR. To understand this issue, Coverdill and colleagues first defined migratory behavior as including the quiescent phase (QP), a transitional state between daytime activity and nocturnal flight, and specific behaviors that define nocturnal MR such as beak-up (BU) and beak-up flight (BUF) as identified in Gambel's white-crowned sparrow, Zonotrichia leucophrys gambelii. Coverdill and colleagues then compared three related subspecies of the white-crowned sparrow, representing a spectrum of migration from long-distance flight to short-distance flight to non-migratory resident species, all exposed to spring photoperiods. The long-distance migrant, Z.l.gambelii, expressed QP prior to dark phase and showed MR consistently with high frequency throughout the night. The short-distance migrant, Z. l. pugetensis, expressed QP immediately following onset of night. MR was observed in most individuals but with variable frequency. In Z. l. nuttalli, the sedentary sunspecies, QP was absentand they showed a low intensity nocturnal activity. Additionally, migratory fattening was observed only in the migratory species. These results support observations that resident congeners of migratory species show nocturnal activity but this activity may not represent expression of migration but a predilection for nocturnal movement within confines of a territory. In light of these data, application of a more specific definition of migratory behavior may prove to be the most revealing in evaluating the spectrum of migratory strategies among the Pacific races of white-crowned sparrows and possibly other bird species as well.
- Jawor. Song production and learning is controlled in songbirds by a series of brain regions, or "nuclei", that are collectively known as the song control system (SCS). Many songbirds are strongly sexually dimorphic in song production and SCS morphology; females typically possess smaller repertoires and SCS nuclei than males. Additionally, SCS nuclei volume can vary seasonally with larger volume in the breeding season; this has been linked to seasonally varying testosterone (T) levels. Jawor and colleagues investigated the sexual and seasonal differences in the SCS of northern cardinals, Cardinalis cardinalis, a species where males and females both sing at approximately the same rates. Jawor and colleagues also compared SCS volumes with levels of T in both sexes. They found a significant seasonal effect between the sexes on the volume of the song nucleus HVC: female HVC was smaller than male HVC and HVC in both sexes was smaller during the non-breeding season. Their results suggest that even in species with near monomorphic song production, the female HVC is smaller. This may be the result of an overall sexual differences in plasma T concentrations in adults, or to exposrure to differeing hormone levels during development. Curiously, even though the male HVC volume differed with season, the amount of this change was not as pronounced as has been observed in other avian species. This may allow male cardinals to have a prolonged period of song production, or allow them to produce song outside of the breeding season.
- Salvante. In some songbird species, the various properties of a male's song may reflect his quality and hence his suitability as a prospective mate. As a result, females choose their mates based on variation among males in their song. Female European starlings, Sturnus vulgaris, prefer males with long songs over those that primarily sing short songs. Additionally, regions of the auditory telencephalon are sensitive to this song-length variation. However, exposure to other social information, such as the prevalence of long- versus short-song males, can modulate the sensitivity of the auditory telencephalon to song length. Due to the presence of noradrenergic input to the auditory forebrain and, in some species, the loss of song-based preferences when this input is lesioned, Salvante and colleagues hypothesized that norepinephrine secretion mediates the effects of "song culture" (i.e.; long-term exposure either long or short songs) on the sensitivity of the telencephalic to song length. Consistent with this hypothesis, Salvante and colleagues found that a 1-week exposure to short songs reduced forebrain immunoreactivity for the enzyme that converts dopamine to norepinephrine (dopamine beta hydroxylase, DBH-ir) compared to females exposed to long song or no song. Thus, song-culture-dependent modulation of noradrenergic activity in the auditory telencephalon may contribute to the mechanism for integration of social information in the female's mate-choice decisions and to the social modulation of female choosiness.