Travis E. Wilcoxen

Dietary protein restriction impairs growth, immunity, and disease resistance in southern leopard frog tadpoles.

The immune system is a necessary, but potentially costly, defense against infectious diseases. When nutrition is limited, immune activity may consume a significant amount of an organism’s energy budget. Levels of dietary protein affect immune system function; high levels can enhance disease resistance. We exposed southern leopard frog [Lithobates sphenocephalus (=Rana sphenocephala)] tadpoles to high and low protein diets crossed with the presence or absence of the pathogenic amphibian chytrid fungus (Batrachochytrium dendrobatidis; Bd) and quantified: (1) tadpole resistance to Bd; (2) tadpole skin-swelling in response to phytohaemagglutinin (PHA) injection (a measure of the T cell-mediated response of the immune system); (3) bacterial killing ability (BKA) of tadpole blood (a measure of the complement-mediated cytotoxicity of the innate immune system); and (4) tadpole growth and development. Tadpoles raised on a low-protein diet were smaller and less developed than tadpoles on a high-protein diet. When controlled for developmental stage, tadpoles raised on a low-protein diet had reduced PHA and BKA responses relative to tadpoles on a high-protein diet, but these immune responses were independent of Bd exposure. High dietary protein significantly increased resistance to Bd. Our results support the general hypothesis that host condition can strongly affect disease resistance; in particular, fluctuations in dietary protein availability may change how diseases affect populations in the field.

Selection on innate immunity and body condition in Florida scrub-jays throughout an epidemic.

Opportunities to investigate selection in free-living species during a naturally occurring epidemic are rare; however, we assessed innate immunocompetence in Florida scrub-jays before the population suffered the greatest over-winter mortality in 20 years of study. Propitiously, three months prior to the epidemic, we had sampled a number of male breeders to evaluate a suite of physiological measures that are commonly used to estimate the overall health-state of an individual. There was a significant, positive selection gradient for both Escherichia coli bacterial killing capability and body condition, suggesting that directional selection had occurred upon each of these traits during the disease epidemic.

The influence of nest attendance and provisioning on nestling stress physiology in the Florida scrub-jay.

Stressful conditions during development may have lasting consequences for an animals lifetime fitness. We investigated the degree to which parental provisioning and nest attendance influenced baseline levels of the stress hormone corticosterone in nestling Florida scrub-jays. Provisioning rates of male and female breeders and nest attendance of female breeders were recorded during focal watches conducted between days 3 and 5 post-hatch. A small blood sample was taken from each nestling on day 11 post-hatch and used to quantify levels of baseline corticosterone. The proportion of time spent by female breeders at a considerable distance from the nest was positively related to nestling corticosterone levels. Nestling corticosterone was also negatively related to parental provisioning rate, although this effect seemed to be secondary to the effect of the females time away from the nest. These results indicate that parental behavior contributes to nestling stress physiology, which may in turn direct the formation of the adult phenotype and influence an individuals chances of survival.

Age-related differences in baseline and stress-induced corticosterone in Florida scrub-jays

In physiological studies of free-living species, it is essential to consider the context of the life history stage at which an individual was observed in order to link measures of physiology with ecological parameters. One such measure that is important to consider is the age of an individual. We tested whether baseline or stress-induced corticosterone levels vary with age in free-living Florida scrub-jays (Aphelocoma coerulescens) during the pre-breeding period. Corticosterone (CORT), the primary avian stress hormone, is released in response to stressful stimuli, and stimulates gluconeogenesis; however, it also serves as a chemical messenger that can influence other physiological processes, reproduction, and behavior. We monitored both baseline CORT levels longitudinally throughout a five-year period and stress-induced CORT responses over a shorter two-year period. We predicted that older jays would have lower baseline CORT levels and a dampened stress response compared to younger birds, as has been shown in other avian species. We found no significant differences in baseline CORT levels with age. We found a decrease in total corticosterone responses to a stressor with age, however, the oldest birds in the population showed greater total corticosterone responses to a stressor. These results may be a product of age-related changes in physiological processes related to the stress response or a result of selection acting on the population, resulting in only the most responsive individuals surviving to old age.

Corticosterone administration does not affect timing of breeding in Florida scrub-jays (Aphelocoma coerulescens).

Providing supplemental food to Florida scrub-jays (Aphelocoma coerulescens) causes a reliable advance in clutch initiation of 1 to 2 weeks. In some years, supplemental food appeared to not only advance laying date but also decrease baseline concentrations of corticosterone (CORT) relative to controls. The coincidence of low CORT levels and early breeding led us to hypothesize that CORT serves to communicate information about environmental conditions to the hypothalamic-pituitary-gonadal (HPG) axis, which ultimately influences the timing of breeding. To test this hypothesis, we administered small oral doses of CORT three times each day to female breeders that were provisioned with supplemental food. We compared clutch initiation dates of the CORT-dosed females to females with supplementation but no exogenous CORT and to females with neither CORT nor supplemental food. CORT administration had a strong temporary effect on circulating CORT concentrations but clutch initiation did not differ between the two groups of supplemented birds, both of which laid eggs approximately 10 days earlier than nonsupplemented birds. Furthermore, during the year of our study we found no reduction in baseline CORT concentrations in our undosed supplemental groups, as had been observed in past studies.

Older can be better: physiological costs of paternal investment in the Florida scrub-jay

In species that undergo actuarial senescence, the value of current reproduction is predicted to increase relative to the value of future reproduction with age, as the probability of survival to another reproductive event is reduced. Therefore, life history theory predicts that aging animals should increase their investment in reproduction. However, an increase in reproductive investment may carry significant costs to the breeding individuals. We recorded provisioning rates of Florida scrub-jay male breeders, followed by their immediate capture to assess body condition and collect blood for an in vitro test of immunocompetence and an assay of baseline corticosterone for a measure of stress. Older males provisioned offspring and brooding mates at the highest rates. There was no evidence of any physiological deficits in males with high provisioning rates, independent of age. It appears that birds that survive to old age are high quality birds that maintain good physiological condition, which complements the value of experience and permits maximal investment in offspring.

Corticosterone, brood size, and hatch order in free-living Florida scrub-jay (Aphelocoma coerulescens) nestlings.

It is well known that variation in developmental conditions can have profound effects upon lifetime fitness. In altricial avian species, nestlings undergo a substantial portion of development in the nest after hatching, often in the presence of nest-mates. This can result in the formation of brood hierarchies based on age, size, and competitive ability. Measurement of baseline corticosterone (CORT) levels in developing birds may provide a means to assess whether individuals within a brood experience stress due to sibling competition or nutritional state. However, few studies have attempted to correlate corticosterone concentrations with brood hierarchies in free-living, developing birds. We investigated the degree to which corticosterone levels in nestling Florida scrub-jays (Aphelocoma coerulescens) were predicted by brood size, hatching order, and body mass. Nestling corticosterone levels were inversely correlated with body mass but did not differ among brood sizes. Within broods of two, second-hatched nestlings had lower CORT than first-hatched nestlings, although there was no effect of hatch order in broods of three or four nestlings. The results of this study suggest that conditions within the nest, particularly those associated with within-brood hierarchies, are manifested through differential body condition and nestling corticosterone secretion. The consequences of this variation in nestling corticosterone are unknown, but development of the adult phenotype, as well as life-long survival, may be impacted.

Are roads and traffic sources of physiological stress for the Florida scrub-jay?

Abstract Context. Anthropogenic disturbances induce physiological and behavioural responses in numerous species. The negative effects of human disturbance are of special concern to threatened and endangered species. Aims. The present study aims to compare physiological stress measures and reproductive success of Florida scrub-jays (Aphelocoma coerulescens) living near roads with jays that live away from roads. Specifically, it aims to test whether roads are stressful. Methods. We assessed physiological measures that can serve as indicators of ‘stress’ to determine whether a highway that bisects our study site had physiological effects on adult male Florida scrub-jays. We captured male breeders from three territory types, including scrub habitat that (1) bordered a highway with a grassy shoulder that created an ‘edge’ habitat (roadside), (2) bordered human-maintained habitat (a plowed firebreak of sand with adjacent pasture) that served as a control for the edge-effect of the road (pasture) and (3) contained only natural scrub habitat (interior). We measured baseline concentrations of the stress hormone corticosterone (CORT), body mass, and a suite of longitudinal body measures to generate a body condition index in males from each territory type over three breeding seasons. Key results. Roadside jays had greater body mass than did interior and pasture jays, although there were no differences in overall baseline CORT concentrations or body condition among territory classes. There was no difference in clutch initiation date or size and nestling and independent-young survival. Conclusions. Our results suggest that the road through our study site has physiological effects on Florida scrub-jays; however, there is mixed evidence as to whether it acts as a chronic stressor. Implications. Our findings provide evidence that roads and road-associated disturbance has neutral or potentially beneficial physiological effects of roads on Florida scrub-jays. Knowledge of these effects of roads and disturbance on jays will hopefully provide additional opportunities to improve conservation of this species.

Factors Influencing Seed Species Selection by Wild Birds at Feeders

Abstract Seed preferences of wild birds may be a result of factors such as bill and seed morphology, handling time, nutritional content of the seed, and foraging behavior of the bird. To examine factors influencing seed choice of 10 species of birds that regularly visit bird feeders in the United States and Canada, we collected data on bird visits, bill size, seed size, and nutritional content of seeds for 10 seed types commonly used in bird seed blends. The presence of an outer hull, protein content, and the ratio of bill volume to seed volume were the most important variables determining seed choice. Fiber and fat content were less important. Results from this study help us understand why birds that use feeders use the seed types they do, and ultimately, may improve the composition of seed species and individual seeds found in bird food blends.

Invasive fire ants depredate nest of Florida Scrub-Jay.

Abstract We observed depredation of a Florida Scrub-Jay (Aphelocoma coerulescens) nest of 4-day-old nestlings in April 2008 by the red imported fire ant (Solenopsis invicta). This exotic species is known to depredate nests of other avian species, but we believe this is the first account of this species of fire ant preying upon young of the Florida Scrub-Jay. This observation is particularly important, as it entails an introduced species directly reducing reproductive success of a federally threatened species.