Early-life patterns of growth are linked to levels of phenotypic trait covariance and postfledging mortality across avian species.

Life history studies have established that trade-offs between growth and survival are common both within and among species. Identifying the factor(s) that mediate this trade-off has proven difficult, however, especially at the among-species level. In this study, we examined a series of potentially interrelated traits in a community of temperate-zone passerine birds to help understand the putative causes and consequences of variation in early-life growth among species. First, we examined whether nest predation risk (a proven driver of interspecific variation in growth and development rates) was correlated with species-level patterns of incubation duration and nestling period length. We then assessed whether proxies for growth rate covaried with mean trait covariance strength (i.e., phenotypic correlations ( rp), which can be a marker of early-life stress) among body mass, tarsus length, and wing length at fledging. Finally, we examined whether trait covariance strength at fledging was related to postfledging survival. We found that higher nest predation risk was correlated with faster skeletal growth and that our proxies for growth corresponded with increased trait covariance strength ( rp), which subsequently, correlated with higher mortality in the next life stage (postfledging period). These results provide an indication that extrinsic pressures (nest predation) impact rates of growth, and that there are costs of rapid growth across species, expressed as higher mean rp and elevated postfledging mortality. The link between higher levels of trait covariance at fledging and increased mortality is unclear, but increased trait covariance strength may reflect reduced phenotypic flexibility (i.e., phenotypic canalization), which may limit an organism's capacity for coping with environmental or ecological variability.

Egg Incubation Temperature Affects Development of Innate Immune Function in Nestling American Robins (Turdus migratorius).

The innate immune system provides important first-line defenses against invading pathogens and is considered especially important for developing organisms. However, we know little about how early-life conditions influence these defenses in wild animals. For oviparous species such as birds, embryonic development occurs in the egg, which can be subject to variation in thermal conditions. There is evidence from cavity-nesting species and species with precocial young that reduced incubation temperatures can result in reduced measures of innate immunity. Whether and how this thermal variation impacts innate immunity for open-cup-nesting species with altricial offspring has not been examined. In this study, we experimentally manipulated egg incubation temperature for American robins (Turdus migratorius) and compared the bacteria-killing ability (BKA) of the nestlings' blood plasma. We collected baseline and poststressor samples on day 7 and day 10 after hatch to gain additional insights into the ontogeny of this immune measure, as well as into whether any changes were linked to levels of the glucocorticoid hormone corticosterone (CORT). We found that nestlings incubated at the low treatment (36.1°C) had significantly reduced BKA compared with nestlings incubated at the high treatment (37.8°C) when controlling for the posthatch nest environment. We also documented a significant reduction in poststressor levels of BKA, as well as an increase in BKA from day 7 to day 10. We found a weak inverse association between CORT and BKA but no other indications that BKA was mediated via treatment-induced variation in CORT. Our results suggest that incubation temperature can affect development of innate immunity in open-cup-nesting passerines.

Variation in Immune Defense Shapes Disease Outcomes in Laboratory and Wild Daphnia.

Host susceptibility may be critical for the spread of infectious disease, and understanding its basis is a goal of ecological immunology. Here, we employed a series of mechanistic tests to evaluate four factors commonly assumed to influence host susceptibility: parasite exposure, barriers to infection, immune responses, and body size. We tested these factors in an aquatic host-parasite system (Daphnia dentifera and the fungal parasite, Metschnikowia bicuspidata) using both laboratory-reared and field-collected hosts. We found support for each factor as a driver of infection. Elevated parasite exposure, which occurs through consumption of infectious fungal spores, increased a host's probability of infection. The host's gut epithelium functioned as a barrier to infection, but in the opposite manner from which we predicted: thinner anterior gut epithelia were more resistant to infectious spores than thick epithelia. This relationship may be mediated by structural attributes associated with epithelial cell height. Fungal spores that breached the host's gut barrier elicited an intensity-dependent hemocyte response that decreased the probability of infection for some Daphnia. Although larger body sizes were associated with increased levels of spore ingestion, larger hosts also had lower frequencies of parasite attack, less penetrable gut barriers, and stronger hemocyte responses. After investigating which mechanisms underlie host susceptibility, we asked: do these four factors contribute equally or asymmetrically to the outcome of infection? An information-theoretic approach revealed that host immune defenses (barriers and immune responses) played the strongest roles in mediating infection outcomes. These two immunological traits may be valuable metrics for linking host susceptibility to the spread of infectious disease.

Haemosporidian Parasite Infections In the Dickcissel ( Spiza americana): Prevalence and Costs.

Haemosporidian parasites are widespread in birds and are commonly used to evaluate ecological, evolutionary, and behavioral consequences of pathogen infection. While haemosporidian parasites may be important agents of selection, information on the incidence and outcomes of infection in many wild birds remains incomplete and warrants further inquiry. We examined the prevalence and costs of haemosporidian parasites in a wild population of dickcissels ( Spiza americana; n = 170) breeding in central Illinois. Over 2 breeding seasons, prevalence of haemosporidian parasites was 41% and was similar between sexes and years. Within each sex, there was no association between proxies of fitness (body condition and number of fledglings produced) and infection status. While we found no evidence that haemosporidian parasites have a direct, negative impact on dickcissels, we discuss how extra-pair paternity and changes across the life cycle of dickcissels and haemosporidian parasites may have masked or prevented detection of negative effects in our study. Ultimately our findings contribute to a growing literature providing improved clarity on how and when parasite infections can negatively impact wild host organisms and can aid in guiding future work examining such associations.

Early Life Stress Strengthens Trait Covariance: A Plastic Response That Results in Reduced Flexibility.

Stress exposure during development can impact both the expression of individual traits and associations between traits, but whether stress results in stronger or weaker associations between traits is unclear. In this study, we examined within- and among-trait associations for morphological and physiological traits in zebra finches (Taeniopygia guttata) exposed to corticosterone (CORT) during the nestling and fledging stages as well as in control birds. Birds exposed to CORT exhibited stronger within-trait correlations over time and stronger associations among traits. We found preliminary evidence that birds that died before the median age of death had stronger within- and among-trait correlations independent of treatment, and among CORT-treated birds, smaller birds were more likely to survive beyond the median age than larger birds. These findings suggest that stress hormone exposure in early life can result in reduced developmental flexibility, with potential fitness ramifications, and that these costs may be greater for larger offspring. Furthermore, our results provide experimental evidence for pleiotropic effects of hormones during development through altered patterns of phenotypic correlation.

Incubation temperature impacts nestling growth and survival in an open-cup nesting passerine.

For oviparous species such as birds, conditions experienced while in the egg can have long-lasting effects on the individual. The impact of subtle changes in incubation temperature on nestling development, however, remains poorly understood, especially for open-cup nesting species with altricial young. To investigate how incubation temperature affects nestling development and survival in such species, we artificially incubated American robin (Turdus migratorius) eggs at 36.1°C ("Low" treatment) and 37.8°C ("High" treatment). Chicks were fostered to same-age nests upon hatching, and we measured mass, tarsus, and wing length of experimental nestlings and one randomly selected, naturally incubated ("Natural"), foster nest-mate on days 7 and 10 posthatch. We found significant effects of incubation temperature on incubation duration, growth, and survival, in which experimentally incubated nestlings had shorter incubation periods (10.22, 11.50, and 11.95 days for High, Low, and Natural eggs, respectively), and nestlings from the Low treatment were smaller and had reduced survival compared to High and Natural nestlings. These results highlight the importance of incubation conditions during embryonic development for incubation duration, somatic development, and survival. Moreover, these findings indicate that differences in incubation temperature within the natural range of variation can have important carryover effects on growth and survival in species with altricial young.

Developmental corticosterone treatment does not program immune responses in zebra finches (Taeniopygia guttata).

Developmental conditions may impact the expression of immune traits throughout an individual's life. Early-life challenges may lead to immunological constraints that are mediated by endocrine-immune interactions. In particular, individual differences in the ability to mount immune responses may be programmed by exposure to stressors or glucocorticoid hormones during development. To test this hypothesis, we experimentally elevated levels of the glucocorticoid hormone corticosterone during the nestling and fledgling periods in captive zebra finches (Taeniopygia guttata). We subsequently challenged birds with the antigen lipopolysaccharide (LPS) on days 60 and 100 post-hatch to determine if developmental exposure to elevated corticosterone impacted the later response to LPS. As measures of immune function, we quantified bacteria killing ability, haptoglobin concentrations, and LPS-specific antibody responses at multiple time points. We also measured circulating corticosterone concentrations during the experimental period and on day 60 before and after endotoxin challenge. During the experimental period, corticosterone treatment elevated corticosterone levels. Corticosterone treatment did not induce programming effects on immune function or corticosterone production. Independent of treatment, individuals with higher corticosterone concentrations during the nestling period had lower bacteria killing ability on day 36 and higher baseline corticosterone concentrations on day 60 post-hatch. These results suggest a limited role for corticosterone exposure during early life to mediate immunological constraints later in life. Manipulation of cortisol may be necessary to conclusively determine if developmental glucocorticoid exposure can program immune function in birds. To determine if developmental stress can program the immune response, exposure to environmentally relevant stressors should also be manipulated.

Rates of parasitism, but not allocation of egg resources, vary among and within hosts of a generalist avian brood parasite.

Brown-headed cowbirds (Molothrus ater) deposit their eggs into the nests of other birds, which then raise the cowbird chick. Female cowbirds thus have limited options for impacting their offspring's development via maternal effects compared to most other passerines. Cowbirds can impact their offspring's phenotype by choosing among potential host nests, and by adjusting egg resources based on host characteristics. To examine whether cowbirds exhibit either or both of these strategies, we investigated rates of cowbird parasitism and egg investment (egg size, yolk-to-albumen ratio, and yolk testosterone and androstenedione) among and within host species in a shrubland bird community. We found that the probability of being parasitized by cowbirds, controlling for host status as a cowbird egg accepter or rejecter and ordinal date, varied significantly among host species, indicating an apparent preference for some hosts. Parasitism rates did not differ with host size, however, and despite variation in cowbird egg size among host species, this variation was not related to host size or cowbird preference. Among host species with eggs that are larger than those of the cowbird, cowbirds were significantly more likely to parasitize nests with relatively smaller eggs, whereas parasitism rates did not vary with relative egg size in host species with smaller eggs. There was no evidence for variation in cowbird egg components among or within host species. Our data indicate that cowbirds discriminate among host nests, but do not appear to adjust the composition of their eggs based on inter- or intraspecific host variation.

Prevalence of avian haemosporidian parasites is positively related to the abundance of host species at multiple sites within a region.

Parasite prevalence is thought to be positively related to host population density owing to enhanced contagion. However, the relationship between prevalence and local abundance of multiple host species is underexplored. We surveyed birds and their haemosporidian parasites (genera Plasmodium and Haemoproteus) at multiple sites across eastern North America to test whether the prevalence of these parasites in a host species at a particular site is related to that host's local abundance. Prevalence was positively related to host abundance within most sites, although the effect was stronger and more consistent for Plasmodium than for Haemoproteus. In contrast, prevalence was not related to variation in the abundance of most individual host species among sites across the region. These results suggest that parasite prevalence partly reflects the relative abundances of host species in local assemblages. However, three nonnative host species had low prevalence despite being relatively abundant at one site, as predicted by the enemy release hypothesis.

Early-life immune activation increases song complexity and alters phenotypic associations between sexual ornaments.

Early-life adversity can have long-lasting effects on physiological, behavioural, cognitive, and somatic processes. Consequently, these effects may alter an organism's life-history strategy and reproductive tactics.In response to early-life immune activation, we quantified levels of the acute phase protein haptoglobin (Hp) during development in male zebra finches (Taeniopygia guttata). Then, we examined the long-term impacts of early-life immune activation on an important static sexual signal, song complexity, as well as effects of early-life immune activation on the relationship between song complexity and a dynamic sexual signal, beak colouration. Finally, we performed mate-choice trials to determine if male early-life experience impacted female preference.Challenge with keyhole limpet hemocyanin (KLH) resulted in increased song complexity compared to lipopolysaccharide (LPS) treatment or the control. Hp levels were inversely correlated with song complexity. Moreover, KLH-treatment resulted in negative associations between the two sexual signals (beak colouration and song complexity). Females demonstrated some preference for KLH-treated males over controls and for control males over LPS-treated males in mate choice trials.Developmental immune activation has variable effects on the expression of secondary sexual traits in adulthood, including enhancing the expression of some traits. Because developmental levels of Hp and adult song complexity were correlated, future studies should explore a potential role for exposure to inflammation during development on song learning.Early-life adversity may differentially impact static versus dynamic signals. The use of phenotypic correlations can be a powerful tool for examining the impact of early-life experience on the associations among different traits, including sexual signals.

Imperfect past and present progressive: beak color reflects early-life and adult exposure to antigen.

Secondary sexual traits may convey information about individual condition. We assessed the capacity for immune challenge with lipopolysaccharide (LPS) or keyhole limpet hemocyanin (KLH) during the prenatal and early postnatal stages to impact beak color development and expression in captive zebra finches. In addition, we tested whether adult immune challenge impacted beak color, and if early-life experience was influential. Immune challenge with KLH early in life slowed development of red beak coloration, and males challenged with KLH as nestlings had reduced red coloration as adults. Following adult KLH challenge, males exhibited a decline in beak redness. Birds challenged with KLH during development produced more anti-KLH antibodies after adult challenge. There was a significant interaction between young treatment and anti-KLH antibody production; for males not challenged with KLH early in life, individuals that mounted a weaker antibody response lost more red coloration after challenge than males mounting a stronger antibody response. Based on models of avian vision, these differences in beak coloration should be detectable to the finches. In contrast to previous studies, we found no effect of early-life or adult challenge with LPS on any aspects of beak coloration. These results provide evidence that beak color reflects developmental and current conditions, and that the signal is linked to critical physiological processes.

Local host specialization, host-switching, and dispersal shape the regional distributions of avian haemosporidian parasites.

The drivers of regional parasite distributions are poorly understood, especially in comparison with those of free-living species. For vector-transmitted parasites, in particular, distributions might be influenced by host-switching and by parasite dispersal with primary hosts and vectors. We surveyed haemosporidian blood parasites (Plasmodium and Haemoproteus) of small land birds in eastern North America to characterize a regional parasite community. Distributions of parasite populations generally reflected distributions of their hosts across the region. However, when the interdependence between hosts and parasites was controlled statistically, local host assemblages were related to regional climatic gradients, but parasite assemblages were not. Moreover, because parasite assemblage similarity does not decrease with distance when controlling for host assemblages and climate, parasites evidently disperse readily within the distributions of their hosts. The degree of specialization on hosts varied in some parasite lineages over short periods and small geographic distances independently of the diversity of available hosts and potentially competing parasite lineages. Nonrandom spatial turnover was apparent in parasite lineages infecting one host species that was well-sampled within a single year across its range, plausibly reflecting localized adaptations of hosts and parasites. Overall, populations of avian hosts generally determine the geographic distributions of haemosporidian parasites. However, parasites are not dispersal-limited within their host distributions, and they may switch hosts readily.

Host immune responses to experimental infection of Plasmodium relictum (lineage SGS1) in domestic canaries (Serinus canaria).

Understanding the complexity of host immune responses to parasite infection requires controlled experiments that can inform observational field studies. Birds and their malaria parasites provide a useful model for understanding host-parasite relationships, but this model lacks a well-described experimental context for how hosts respond immunologically to infection. Here, ten canaries (Serinus canaria) were infected with the avian malaria parasite Plasmodium relictum (lineage SGS1) in a controlled laboratory setting with ten uninfected (control) birds. A suite of immunological blood parameters, including the concentration of four white blood cell types, the concentration of the acute phase protein haptoglobin, and the bacteria-killing ability of blood plasma, were repeatedly measured over a 25-day period covering the acute phase of a primary infection by P. relictum. Three infected and one control bird died during the course of the experiment. A multivariate statistical analysis of the immune indices revealed significant differences between infected and uninfected individuals between 5 and 14 days postinfection (dpi). Group differences corresponded to reduced concentrations of lymphocytes (5 dpi), heterophils (8 dpi), and monocytes (11 and 14 dpi), and an increase in haptoglobin (14 dpi), in infected birds relative to uninfected controls, and no change in bacteria-killing. Upon re-running the analysis with only the surviving birds, immunological differences between infected and control birds shifted to between 11 and 18 dpi. However, there were no clear correlates relating immune parameters to the likelihood of surviving the infection. The results presented here demonstrate the dynamic and complex nature of avian immune function during the acute phase of malaria infection and provide a context for studies investigating immune function in wild birds.

A blurring of life-history lines: Immune function, molt and reproduction in a highly stable environment.

Rufous-collared sparrows (Zonotrichia capensis peruviensis) from valleys in the Atacama Desert of Chile, live in an extremely stable environment, and exhibit overlap in molt and reproduction, with valley-specific differences in the proportion of birds engaged in both. To better understand the mechanistic pathways underlying the timing of life-history transitions, we examined the relationships among baseline and stress-induced levels of corticosterone (CORT), testosterone, and bacteria-killing ability of the blood plasma (BKA), as well as haemosporidian parasite infections and the genetic structure of two groups of sparrows from separate valleys over the course of a year. Birds neither molting nor breeding had the lowest BKA, but there were no differences among the other three categories of molt-reproductive stage. BKA varied over the year, with birds in May/June exhibiting significantly lower levels of BKA than the rest of the year. We also documented differences in the direction of the relationship between CORT and BKA at different times during the year. The direction of these relationships coincides with some trends in molt and reproductive stage, but differs enough to indicate that these birds exhibit individual-level plasticity, or population-level variability, in coordinating hypothalamo-pituitary-adrenal axis activity with life-history stage. We found weak preliminary evidence for genetic differentiation between the two populations, but not enough to indicate genetic isolation. No birds were infected with haemosporidia, which may be indicative of reduced parasite pressure in deserts. The data suggest that these birds may not trade off among different life-history components, but rather are able to invest in multiple life-history components based on their condition.

Epaulet size and current condition in red-winged blackbirds: examining a semistatic signal, testosterone, immune function, and parasites.

Some sexually selected signals are thought to convey information about the current condition and genetic/epigenetic quality of the individual signaling, including the ability to resist parasites. However, it is unclear whether semistatic sexual signals that develop periodically and remain stable over protracted periods, such as avian breeding plumage, can relate to measures of current condition and health. We examined a semistatic signal (wing epaulet size) in male red-winged blackbirds (Agelaius phoeniceus) during the breeding season and looked for relationships between this trait and circulating testosterone (T), hematocrit, bacteria-killing ability (BKA) of the blood, and the infection status, richness, and abundance of four functional categories of parasite. We found that epaulet size was positively related to circulating levels of T and ectoparasite infections. We found no relationships between T and parasite infections. In adult males there was a negative relationship between T and BKA, whereas in yearling males there was no relationship. We found no evidence for a general reduction in immunocompetence in males with larger epaulets but rather an increase in susceptibility to specific types of parasites. Our results suggest that semistatic signals can be linked to measures of current condition, and we postulate that these relationships are modulated via activity levels related to breeding-season activities.

Pre and post-natal antigen exposure can program the stress axis of adult zebra finches: evidence for environment matching.

Both maternal exposure to stressors and exposure of offspring to stressors during early life can have lifelong effects on the physiology and behavior of offspring. Stress exposure can permanently shape an individual's phenotype by influencing the development of the hypothalamic-pituitary-adrenal (HPA) axis, which is responsible for the production and regulation of glucocorticoids such as corticosterone (CORT). In this study we used captive zebra finches (Taeniopygia guttata) to examine the effects of matching and mismatching maternal and early post-natal exposure to one of two types of antigens or a control on HPA axis reactivity in adult offspring. Prior to breeding, adult females were injected with lipopolysaccharide (LPS), keyhole limpet hemocyanin (KLH) or a control. Offspring of females in each of the three treatments were themselves exposed to LPS, KLH or a control injection at 5 and 28days post-hatch. When offspring were at least 18months of age, standardized capture and restraint stress tests were conducted to determine the impact of the treatments on adult stress responsiveness. We found significant interaction effects between maternal and offspring treatments on stress-induced CORT levels, and evidence in support of the environment matching hypothesis for KLH-treated birds, not LPS-treated birds. KLH-treated offspring of KLH-treated mothers exhibited reduced stress-induced CORT levels, whereas LPS-treated or control offspring of KLH-treated mothers exhibited elevated stress-induced CORT levels. Although the treatment effects on baseline CORT were non-significant, the overall pattern was similar to the effects observed on stress-induced CORT levels. Our results highlight the complex nature of HPA axis programming, and to our knowledge, provide the first evidence that a match or mismatch between pre and post-natal antigen exposure can have life-long consequences for HPA axis function.

Maternal antibody transfer can lead to suppression of humoral immunity in developing zebra finches (Taeniopygia guttata).

Maternally transferred antibodies have been documented in a wide range of taxa and are thought to adaptively provide protection against parasites and pathogens while the offspring immune system is developing. In most birds, transfer occurs when females deposit immunoglobulin Y into the egg yolk, and it is proportional to the amount in the female's plasma. Maternal antibodies can provide short-term passive protection as well as specific and nonspecific immunological priming, but high levels of maternal antibody can result in suppression of the offspring's humoral immune response. We injected adult female zebra finches (Taeniopygia guttata) with one of two antigens (lipopolysaccharide [LPS] or keyhole limpet hemocyanin [KLH]) or a control and then injected offspring with LPS, KLH, or a control on days 5 and 28 posthatch to examine the impact of maternally transferred antibodies on the ontogeny of the offspring's humoral immune system. We found that offspring of females exposed to KLH had elevated levels of KLH-reactive antibody over the first 17-28 days posthatch but reduced KLH-specific antibody production between days 28 and 36. We also found that offspring exposed to either LPS or KLH exhibited reduced total antibody levels, compared to offspring that received a control injection. These results indicate that high levels of maternal antibodies or antigen exposure during development can have negative repercussions on short-term antibody production and may have long-term fitness repercussions for the offspring.

Breaking down seasonality: androgen modulation and stress response in a highly stable environment.

Previous studies show that most birds inhabiting temperate regions have well defined life history stages, and they modulate the production of testosterone (T) and corticosterone (CORT) in response to changes in seasonality. In this study we aimed to examine baseline and stress-induced levels of CORT and circulating T in relation with life history stages in the rufous-collared sparrow, Zonotrichia capensis. We carried out this study for a year in a population inhabiting riparian habitats in the Atacama Desert in Chile, one of the most climatically stable and driest places in the world. This environment shows minimal yearly change in average temperature and precipitation is virtually zero. We found individuals breeding, molting and overlapping breeding and molt year round, although most individuals were molting during March and in breeding condition during October. T levels were not related to individual breeding condition, and at population level they were not significantly different across sampling months. Baseline levels of CORT did not vary across the year. Stress-induced levels of CORT were suppressed during March when most of the birds were molting. This phenomenon was also observed in birds not molting during this period suggesting a mechanism other than molt in determining the stress-response suppression. Our results strongly suggest that in this study site, long-term extremely stable conditions could have relaxed the selective pressures over the timing of life history stages which was evidenced by the breeding and molt schedules, its overlap and endocrine profiles.

Immune function in an avian brood parasite and its nonparasitic relative.

Organisms that breed multiple times must trade off resources between current and future reproduction. In many species, sexual selection can lead to reduced levels of immune function in males because they invest heavily in current reproduction at the expense of self-maintenance. Much less is known about whether the same trend is seen in species such as the brood-parasitic brown-headed cowbird Molothrus ater (hereafter "cowbird"), in which females invest heavily in current reproduction. We examined two measures of immune function (bactericidal capacity of the plasma and the phytohemagglutinin swelling response) and baseline levels of corticosterone in both sexes of the cowbird and its nonparasitic relative the red-winged blackbird Agelaius phoeniceus (hereafter "redwing") during the breeding and subsequent nonbreeding seasons. We found that female cowbirds exhibited significantly lower levels of both measures of immune function than did male cowbirds and female redwings during the breeding season but had comparable levels during the nonbreeding season. Female redwings, in contrast, exhibited higher or comparable levels of immune function when compared with male redwings during the breeding season. In conjunction with published accounts documenting significantly higher rates of mortality for female cowbirds compared with male cowbirds and the fact that female cowbirds produce very high numbers of eggs (25-65) in a single breeding season, our results suggest that female cowbirds invest heavily in current reproduction at the expense of self-maintenance.