Eco-endo-immunology across avian life history stages.

Broadly distributed songbirds, particularly those that migrate, encounter a wide range of pathogens. Both pathogen exposure and energy available for immune responses are expected to be affected by environmental variation in climate, habitat quality, and social interactions as well as hormonal mechanisms. Comparisons of Aves in the field have begun to build the framework for understanding how such environmental variation interacts with disease environments as reflected in endocrine and immune responses. In this review, the roles of hormones and immune function across the various life history stages that make up the avian annual cycle are considered with an emphasis on free-living songbirds and the various hormones known to mediate the innate and acquired immune systems including melatonin, prolactin, growth hormone, and several neuroendocrine hormones. Finally, hormone-immune interactions are considered within the framework of disease ecology.

ANIMAL MIGRATION AS A MOVING TARGET FOR CONSERVATION: INTRA-SPECIES VARIATION AND RESPONSES TO ENVIRONMENTAL CHANGE, AS ILLUSTRATED IN A SOMETIMES MIGRATORY SONGBIRD.

Identifying important "migratory species" and the characteristics of their migrations might sound like a simple starting point for efforts to conserve and protect animal migrations. However, migrations are dynamic phenomena that vary over space and time, and migratory behaviors can vary substantially among closely related species, subspecies, races, or populations, and even among individual animals within a single population. The migratory behaviors of populations or individuals can also change rapidly-or be lost entirely-in response to habitat alteration or climate change. These complexities present both challenges and opportunities for initiatives to conserve animal migrations. In this Article, we discuss the concepts of intra-species variation in migration and the sensitivity of migrations to environmental change, and we consider the implications of these topics for legal, policy, management, and research agendas.

The effect of winter sex ratio on immune function and condition in a differential migrant.

The dark-eyed junco (Junco h. hyemalis) is a differential migrant in which females migrate farther south in winter than males. Hypotheses to explain this difference in migratory behavior have given limited consideration to physiological differences between the sexes, particularly with respect to immune function. We hypothesized that female migratory behavior could either be directly dependent on climate if migratory distance traveled is a product of interactions between climate and inherent sex differences in physiology, or indirectly dependent on climate owing to interactions between climate and aggressive encounters with males which are known to be behaviorally dominant to females and restrict their access to food. We tested the latter of these two hypotheses by measuring condition in females held in two sex ratio regimes in a common winter environment. Holding density constant in 28 replicate populations, we compared flocks composed of 100% female to mixed-sex flocks composed of 20% female. We assessed condition by measuring immune function via complement activity and response to a foreign antigen, keyhole limpet hemocyanin, as well as by indices of mass, fat, and pectoral muscle. Unexpectedly, females in mixed-sex flocks did not differ in condition from females housed only with females, despite increased aggressive interactions in mixed flocked cages. Combined, our results suggest that overwintering with males has no significant effect on female winter condition. Differential migration in the junco may be mediated by sex differences in ability to withstand harsh northern climate, as has been demonstrated in several mammalian systems, rather than dominance interactions.

Experimentally-elevated testosterone, female parental care, and reproductive success in a songbird, the Dark-eyed Junco (Junco hyemalis).

In male dark-eyed juncos (Junco hyemalis), experimentally elevated testosterone (T) decreases male parental care and offspring survival, but results in higher overall fitness through greater mating success. To help address the ensuing question of what prevents selection from favoring higher levels of T in male juncos, we manipulated T in female juncos. A previous study demonstrated no effect of experimentally elevated T on female incubation behavior, suggesting that female parental behavior might be insensitive to T. In this study we asked whether experimentally elevated T mediates other female parental behaviors and whether variation in T-mediated parental behavior might influence reproductive success. We videotaped free-living control- and T-females during nesting to quantify brooding behavior when young were 3 days old and provisioning behavior when young were 6 days old. Nest defense was measured by quantifying responses to a mounted predator placed near the nest. Reproductive success was assessed via fecundity, nestling quality, and nest survival. T-females spent less time than control females brooding but did not differ in provisioning rate. T-females performed fewer dives at the predator mount and, unlike controls, failed to increase defense as nesting progressed. T-females also had lower daily nest survival and lower nest success (odds of producing at least one fledgling). We conclude that some aspects of female parental behavior are sensitive to experimentally elevated T while others are not and consider the implications for the evolution of T-mediated characters in both sexes.

Consequences of elevating plasma testosterone in females of a socially monogamous songbird: evidence of constraints on male evolution?

To explore whether selection for testosterone-mediated traits in males might be constrained by costs of higher testosterone to females, we examined the effects of experimental elevation of plasma testosterone on physiological, reproductive, and behavioral parameters in a female songbird, the dark-eyed junco (Junco hyemalis). We used subcutaneous implants to elevate testosterone (T) in captive and free-living female juncos. In captive birds, we measured the effects of high T on body mass, feather molt, and brood patch formation. In the field, we monitored its effects on the timing of egg laying, clutch size, egg size, egg steroid levels, incubation, and nest-defense behavior. Females implanted with testosterone (T-females) had significantly higher circulating levels of testosterone than did control females (C-females). Captive T-females had lower body mass, were less likely to develop brood patches, and delayed feather molt relative to C-females. Among free-living females, the interval between nest completion and appearance of the first egg was longer for T-females than for C-females and egg yolk concentrations of testosterone were higher, but there were no significant differences in estradiol levels, clutch size, or egg size. Incubation and nest defense behavior were also similar between T- and C-females. Our results suggest that selection on males for higher testosterone might initially lead to a correlated response in females producing changes in body mass and feather molt, both of which could be detrimental. Other possible female responses would be delayed onset of reproduction, which might reduce reproductive success, and higher yolk testosterone, which might have either positive or negative effects on offspring development. We found no reason to expect reduced parental behavior by females as a negative fitness consequence of selection for higher testosterone in males.