Plumage manipulation alters associations between behaviour, physiology, the internal microbiome and fitness

Does Parental Care Modify the Association of Early-Life Size and Growth with Physiology?

AbstractSize and growth early in life are associated with physiological development, and these traits influence fitness. Life history theory predicts that the relationship between traits reflect constraints involving allocation and acquisition of resources. Using longitudinal data from 113 wild nestling barn swallows (Hirundo rustica erythrogaster), we first characterized developmental changes in glucose metabolism, a physiological trait involved in energy mobilization and response to stress. Next, we tested hypotheses from life history theory about allocation and acquisition of resources based on associations of nestling size and growth with glucose physiology and assessed whether these relationships are modified by parental care. Larger nestlings had higher baseline blood glucose and larger magnitude of change in glucose in response to a stressor than smaller nestlings. Furthermore, the relationship in which greater growth was associated with a stronger stress response, as indicated by a larger magnitude of increase in glucose levels, was most pronounced among birds in nests that received the lowest amount of parental care. These results suggest that physiological constraints may contribute to the early-life disadvantage of slow growth, especially in the context of lower parental care. While these findings are inconsistent with a trade-off involving differential allocation of resources between life history traits, they align with the differential acquisition hypothesis.

The Evolution of Using Shed Snake Skin in Bird Nests

AbstractMany species of birds use shed snake skin in nest construction, but this behavior remains poorly understood. Ecological context is likely key for understanding how this unusual, but widespread, behavior evolved. We use comparative and experimental approaches to suggest that the evolution of this behavior is mediated by nest morphology and predator communities. First, we reviewed the literature and found that 78 species from 22 families have been reported to use shed snake skin in nest construction. All but one of these species are passerines and, using comparative analyses, we show that this behavior is disproportionately observed in cavity-nesting species. Second, we examined a subsample of North American species, all of which are reported to use snake skin in nest construction, to see whether the proportion of nests with snake skin differs between cavity- and open cup-nesting species. This analysis suggested that the proportion of nests with snake skin is roughly 6.5 times higher in cavity- than in open cup-nesting species. Finally, we used a series of experiments and comparisons to test four hypotheses whereby snake skin could award fitness benefits (nest predation, nest microbiotas, nest ectoparasites, social signaling) and found support for the predation hypothesis. Snake skin reduced nest predation in cavity, but not open cup, nests. These unequal fitness benefits highlight different ecological conditions between nest morphologies and likely explains why, across species, cavity-nesting birds show this behavior more frequently than open cup-nesting birds.

Brain transcriptomics of a social challenge and maternal aggression in incubating female tree swallows

Aggressive behavior is ubiquitous across many contexts, including defense of territories, mates, and offspring. For decades, researchers have detailed the effect of aggressive behavior on physiology, but our understanding of these mechanisms in females lags behind that of males, despite the fact that female aggression is widespread, particularly in the context of maternal defense of eggs or offspring (i.e., maternal aggression). Here, we measured effects of a social challenge on brain gene expression in free-living incubating females. We hypothesized that the social challenge would generate at least one of three transcriptomic effects: (1) sensitizing the brain to otherwise low levels of sex steroids, (2) changing other neuroendocrine signaling pathways associated with social behavior (e.g., dopamine), or (3) broad shifts related to metabolism or immune function. We tested these hypotheses in incubating female tree swallows (Tachycineta bicolor), exposing 10 females to a 30-min simulated territorial intrusion, which elicited maternal aggression. After this challenge, we measured neural gene expression via RNA-seq and compared gene expression to 10 unchallenged controls. We saw no global treatment effect on gene expression. However, within the experimental group, more maternal aggression was correlated with upregulation of genes associated with immune activation and downregulation of genes associated with synaptic plasticity. Though more research is needed to understand the downstream effects of these transcriptional differences, our findings generate key questions about how the brain responds to social challenges across different contexts.

Manipulation of a social signal affects DNA methylation of a stress-related gene in a free-living bird

Social status directly affects the health of humans and other animals. Low status individuals receive more antagonistic encounters, have fewer supportive relationships and have worse health outcomes. However, the physiological and cellular processes that mediate the relationship between the social environment and health are incompletely known. Epigenetic regulation of the hypothalamic-pituitary-adrenal (HPA) axis, the neuroendocrine pathway that activates in response to stressors, may be one process that is sensitive to the social environment. Here, we experimentally manipulated plumage, a key social signal in female tree swallows (Tachycineta bicolor) and quantified methylation of four genes in the HPA axis before and after treatment. We found that dulling the white breast plumage affected methylation in one gene, CRHR1; however, the effect depended on the original brightness of the bird. Methylation in this gene was correlated with baseline corticosterone levels, suggesting that DNA methylation of CRHR1 helps regulate glucocorticoid production in this species. Methylation in two other genes, FKBP5 and GR, changed over the course of the experiment, independent of treatment. These results show that methylation of these genes is labile into adulthood and suggest that epigenetic regulation of the HPA axis could help birds respond to current environmental conditions.

Developmental stage-dependent effects of perceived predation risk on nestling tree swallows (Tachycineta bicolor)

The risk of predation directly affects the physiology, behavior, and fitness of wild birds. Strong social connections with conspecifics could help individuals recover from a stressful experience such as a predation event; however, competitive interactions also have the potential to exacerbate stress. Few studies have investigated the interaction between environmental stressors and the social landscape in wild bird populations. In 2 years of field studies, we experimentally simulated predation attempts on breeding female tree swallows (Tachicyneta bicolor). At the same time, we manipulated female breast plumage color, a key social signal. Simulated predation events on tree swallows early in the nestling period reduced young nestlings' mass by approximately 20% and shortened telomere lengths. Ultimately, only 31% of nestlings in the predation group fledged compared with 70% of control nestlings. However, the effects of experimental manipulations were timing dependent: the following year when we swapped the order of the experimental manipulations and simulated predation during incubation, there were no significant effects of predation on nestling condition or fledging success. Contrary to our expectations, manipulation of the social environment did not affect the response of tree swallows to simulated predation. However, manipulating female plumage during the nestling period did reduce nestling skeletal size and mass, although the effects depended on original plumage brightness. Our data demonstrate that transient stressors on female birds can have carry-over effects on their nestlings if they occur during critical periods in the breeding season.

Social signal manipulation and environmental challenges have independent effects on physiology, internal microbiome, and reproductive performance in tree swallows (Tachycineta bicolor)

The social environment that individuals experience appears to be a particularly salient mediator of stress resilience, as the nature and valence of social interactions are often related to subsequent health, physiology, microbiota, and overall stress resilience. Relatively few studies have simultaneously manipulated the social environment and ecological challenges under natural conditions. Here, we report the results of experiments in wild tree swallows (Tachycineta bicolor) in which we manipulated both ecological challenges (predator encounters and flight efficiency reduction) and social interactions (by experimental dulling of a social signal). In two experiments conducted in separate years, we reversed the order of these treatments so that females experienced either an altered social signal followed by a challenge or vice-versa. Before, during, and after treatments were applied, we tracked breeding success, morphology and physiology (mass, corticosterone, and glucose), nest box visits via an RFID sensor network, cloacal microbiome diversity, and fledging success. Overall, we found that predator exposure during the nestling period reduced the likelihood of fledging and that signal manipulation sometimes altered nest box visitation patterns, but little evidence that the two categories of treatment interacted with each other. We discuss the implications of our results for understanding what types of challenges and what conditions are most likely to result in interactions between the social environment and ecological challenges.

Cold temperatures induce priming of the glucose stress response in tree swallows

Capricious environments often present wild animals with challenges that coincide or occur in sequence. Conceptual models of the stress response predict that one threat may prime or dampen the response to another. Although evidence has supported this for glucocorticoid responses, much less is known about the effects of previous challenges on energy mobilization. Food limitation may have a particularly important effect, by altering the ability to mobilize energy when faced with a subsequent challenge. We tested the prediction that challenging weather conditions, which reduce food availability, alter the energetic response to a subsequent acute challenge (capture and restraint). Using a three-year dataset from female tree swallows measured during three substages of breeding, we used a model comparison approach to test if weather (temperature, wind speed, and precipitation) over 3- or 72-hour timescales predicted baseline and post-restraint glucose levels, and if so which environmental factors were the strongest predictors. Contrary to our predictions, weather conditions did not affect baseline glucose; however, birds that had experienced lower temperatures over the preceding 72 h tended to have higher stress-induced glucose when faced with an acute stressor. We also saw some support for an effect of rainfall on stress-induced glucose: around the time that eggs hatched, birds that had experienced more rainfall over the preceding 72 h mounted lower responses. Overall, we find support in a wild animal for the idea that the glucose stress response may be primed by exposure to prior challenges.

Timing of Breeding Reveals a Trade-Off between Immune Investment and Life History in Tree Swallows

The allocation of limited resources among life history traits creates trade-offs that constrain the range of possible phenotypes of organisms. In animals, the cost of maintaining an effective immune response may reduce the ability to invest in reproduction, resulting in altered susceptibility to disease. However, not all members of a population face identical constraints because differences in an individual's environmental context or physiological state can influence the degree to which traits are negatively associated. Here, we evaluated how variation in timing of breeding, a correlate of fitness, may result in different patterns of trait associations between immunity and reproduction. We measured constitutive immunity in breeding female tree swallows (Tachycineta bicolor) using a bacteria killing assay with blood plasma to assess the relationships between bacteria killing ability (BKA), reproductive effort, and reproductive success. We found that timing of breeding can influence the association between BKA and reproductive effort, but its effects are not homogeneous among all traits. Late-breeding tree swallows with stronger BKA laid smaller clutches, a pattern that was not apparent in early breeders. Regardless of the timing of breeding, birds with stronger BKA fed their nestlings less. Despite a negative association with reproductive effort, we found no association between immunity and reproductive success. We provide evidence that individual tree swallows do not experience some trade-offs equally, and that timing of breeding likely plays a role in how costs of immunity are weighed. To understand how investment in immunity can limit life history traits, we must consider how a variation among individuals influences the relative costs of immunity.

Individual variation in natural or manipulated corticosterone does not covary with circulating glucose in a wild bird

Animals respond to sudden challenges with a coordinated set of physiological and behavioral responses that enhance the ability to cope with stressors. While general characteristics of the vertebrate stress response are well described, it is not as clear how individual components covary between- or within-individuals. A rapid increase in glucocorticoids coordinates the stress response and one of the primary downstream results is an increase in glucose availability via reduced glucose utilization. Here, we asked whether between- and within-individual variation in corticosterone directly predicted variation in glucose. We collected 2,673 paired glucose and corticosterone measures from 776 tree swallows (Tachycineta bicolor) from four populations spanning the species range. In adults, glucose and corticosterone both increased during a standardized restraint protocol in all four populations. Moreover, in one population experimentally increasing a precursor that stimulates corticosterone release resulted in a further increase in both measures. In contrast, nestlings did not show a robust glucose response to handling or manipulation. Despite this group level variation, there was very little evidence in any population that between-individual variation in corticosterone predicted between-individual variation in glucose regulation. Glucose was moderately repeatable within-individuals, but within-individual variation in glucose and corticosterone were unrelated. Our results highlight the fact that a strong response in one aspect of the coordinated acute stress response (corticosterone) does not necessarily indicate that specific downstream components, such as glucose, will show similarly strong responses. These results have implications for understanding the evolution of integrated stress response systems.