Rising to the Challenge: Mounting an Acute Phase Immune Response Has No Long-Term Negative Effects on Captive Sparrow Migratory Body Composition or Migratory Restlessness.

Migratory animals may trade-off between investing energy in immune defense versus investing in energy reserves needed for seasonal migration. However, these trade-offs are often masked by other sources of variation and may not be detected through observational field studies of free-living animals. Moreover, observational studies can rarely distinguish the costs of pathogenic infection from those of mounting an immune response. To disentangle such effects, we conducted an immune challenge experiment. We captured song sparrows (Melospiza melodia) and white-throated sparrows (Zonotrichia albicollis) in autumn migratory condition, challenged the sparrows with non-infectious antigens that induce an acute-phase immune response, then monitored body composition and migratory restlessness behavior. For both species, body mass was higher the day after exposure to keyhole limpet hemocyanin (KLH) compared to controls. White-throated sparrows, but not song sparrows, increased lean mass 1 week after exposure to lipopolysaccharide (LPS), suggesting that effects of immune upregulation on body composition may be long-lasting and specific to certain combinations of hosts and antigens. White-throated sparrows exposed to KLH increased nocturnal migratory restlessness (Zugunruhe) for the week following exposure. These findings suggest that short-term activation of the acute immune response does not constrain migratory physiology in these songbirds.

Experimental Malaria Infection Affects Songbirds' Nocturnal Migratory Activity.

Migratory animals encounter multiple parasite communities, raising concerns that migration may aid transport of infectious disease. How migration affects disease spread depends fundamentally on how disease affects migration, specifically whether infection alters individuals' migratory physiology and behavior. We inoculated white-throated sparrows (Zonotrichia albicollis) with avian malaria parasites (Plasmodium sp.), monitored parasite loads for 5 wk as the birds reached spring migratory condition, and compared nocturnal migratory restlessness (Zugunruhe), body composition (fat, lean, and whole-body mass), and hematocrit among experimentally infected birds, sham-inoculated birds, and birds that were exposed to parasites but resisted infection. Migratory restlessness increased over time in the study, but the rate of change varied between sham (control) birds, infected birds, and birds that resisted infection. We were unable to detect any effects of malaria exposure on body condition. Our findings suggest that encountering parasites affects migratory activity, regardless of whether infection occurs or is resisted.

Old carbon reservoirs were not important in the deglacial methane budget.

Permafrost and methane hydrates are large, climate-sensitive old carbon reservoirs that have the potential to emit large quantities of methane, a potent greenhouse gas, as the Earth continues to warm. We present ice core isotopic measurements of methane (Δ14C, δ13C, and δD) from the last deglaciation, which is a partial analog for modern warming. Our results show that methane emissions from old carbon reservoirs in response to deglacial warming were small (<19 teragrams of methane per year, 95% confidence interval) and argue against similar methane emissions in response to future warming. Our results also indicate that methane emissions from biomass burning in the pre-Industrial Holocene were 22 to 56 teragrams of methane per year (95% confidence interval), which is comparable to today.

Food stress, but not experimental exposure to mercury, affects songbird preen oil composition.

Mercury is a global pollutant and potent neurotoxic metal. Its most toxic and bioavailable form, methylmercury, can have both lethal and sublethal effects on wildlife. In birds, methylmercury exposure can disrupt behavior, hormones, the neuroendocrine system, and feather integrity. Lipid-rich tissues and secretions may be particularly susceptible to disruption by lipophilic contaminants such as methylmercury. One such substance is feather preen oil, a waxy secretion of the uropygial gland that serves multiple functions including feather maintenance, anti-parasitic defense, and chemical signaling. If methylmercury exposure alters preen oil composition, it could have cascading effects on feather quality, susceptibility to ectoparasites, and mate choice and other social behaviors. We investigated whether exposure to methylmercury, either alone or in association with other stressors, affects preen oil chemical composition. We used a two-factor design to expose adult song sparrows (Melospiza melodia) to an environmentally relevant dietary dose of methylmercury and/or to another stressor (unpredictable food supply) for eight weeks. The wax ester composition of preen oil changed significantly over the 8-week experimental period. This change was more pronounced in the unpredictable food treatment, regardless of dietary methylmercury. Contrary to our prediction, we found no main effect of methylmercury exposure on preen oil composition, nor did methylmercury interact with unpredictable food supply in predicting the magnitude of chemical shifts in preen oil. While it remains critical to study sublethal effects of methylmercury on wildlife, our findings suggest that the wax ester composition of preen oil is robust to environmentally relevant doses of this contaminant.

Glucocorticoids and "Stress" Are Not Synonymous.

Reference to glucocorticoids as "stress hormones" has been growing in prevalence in the literature, including in comparative and environmental endocrinology. Although glucocorticoids are elevated in response to a variety of stressors in vertebrate animals, the primary functions of glucocorticoids are not responding to stressors and they are only one component of complex suite of physiological and behavioral responses to stressors. Thus, the use of the short-hand phrase "stress hormone" can be misleading. Further, simply measuring glucocorticoids is not equivalent to measuring a stress response, nor is manipulating glucocorticoids equivalent to exposing an animal to a stressor. In this commentary we highlight the problems with using functional names for hormones, and of treating cortisol or corticosterone as synonymous with stress. We provide recommendations to add clarity to the presentation of research on this topic, and to avoid conflation of glucocorticoids with stressors and the stress response in the design of experiments.

Los Glucocorticoides y el "Estrés" no Son Sinónimos (Glucocorticoids and "Stress" Are Not Synonymous) La referencia a los glucocorticoides como "hormonas del estrés" ha aumentado en prevalencia en la literatura, incluso en endocrinología comparativa y ecológica. Aunque los glucocorticoides están elevados en respuesta a una variedad de factores de estrés en animales vertebrados, las funciones primarias de los glucocorticoides no responden a los factores de estrés y son solo un componente de un conjunto complejo de respuestas fisiológicas y de comportamiento a los factores de estrés. Por lo tanto, el uso de la frase abreviada "hormona del estrés" puede ser engañoso. Además, simplemente medir glucocorticoides no es equivalente a medir una respuesta al estrés, ni manipular glucocorticoides equivalente a exponer a un animal a un factor estresante. En este comentario destacamos los problemas con el uso de nombres funcionales para las hormonas y con el tratamiento del cortisol o la corticosterona como sinónimo de estrés. Brindamos recomendaciones para agregar claridad a la presentación de investigaciones sobre este tema y para evitar la combinación de glucocorticoides con factores estresantes y la respuesta al estrés en el diseño de los experimentos. Translated to Spanish by J. Heras (herasj01@gmail.com).

Glucocorticóides e "Stress" Não São Sinônimos (Glucocorticoids and "Stress" Are Not Synonymous) A referência aos glicocorticóides como "hormônios do estresse" vem se tornando prevalente em literatura, inclusive na endocrinologia comparada e ecológica. Embora os glicocorticóides sejam elevados em resposta a uma variedade de estressores em vertebrados, as funções primárias de glicocorticóides são a de não responder aos causadores do stress e eles são apenas um componente dentro de um complexo conjunto de respostas fisiológicas e comportamentais aos estressores. Assim, o uso da frase curta "hormônio do estresse" pode ser errôneo. Além disso, simplesmente medir glicocorticóides não é equivalente a medir uma resposta ao estresse, nem a manipulação de glicocorticoides é equivalente a expor um animal a um estressor. Neste comentário, destacamos os problemas com o uso de nomes funcionais para hormônios e o tratamento de cortisol ou corticosterona como sinônimos de estresse. Nós provemos recomendações para adicionar clareza à apresentação de pesquisas deste tópico e para evitar a o tratamento direto de glicocorticoides como estressores e a resposta ao estresse no planejamento de experimentos. Translated to Portuguese by Diego Vaz (dbistonvaz@vims.edu).

Alternative reproductive strategies in white-throated sparrows are associated with differences in parasite load following experimental infection.

Immune defences often trade off with other life-history components. Within species, optimal allocation to immunity may differ between the sexes or between alternative life-history strategies. White-throated sparrows (Zonotrichia albicollis) are unusual in having two discrete plumage morphs, white-striped and tan-striped. Within each sex, white-striped individuals are more aggressive and provide less parental care than tan-striped individuals. We extended immunocompetence handicap models, which predict sex differences in immunity and parasitism, to hypothesize that infection susceptibility should be greater in white-striped than tan-striped birds. We inoculated birds of both morphs with malarial parasites. Contrary to our prediction, among birds that became infected, parasite loads were higher in tan-striped than white-striped individuals and did not differ between the sexes. Circulating androgen levels did not differ between morphs but were higher in males than females. Our findings are not consistent with androgen-mediated immunosuppression. Instead, morph differences in immunity could reflect social interactions or life-history-related differences in risk of injury, and/or genetic factors. Although plumage and behavioural morphs of white-throated sparrow may differ in disease resistance, these differences do not parallel sex differences that have been reported in animals, and do not appear to be mediated by differences in androgen levels.

Effects of nutritional stress during different developmental periods on song and the hypothalamic-pituitary-adrenal axis in zebra finches.

In songbirds, developmental stress affects song learning and production. Altered hypothalamic-pituitary-adrenal (HPA) axis function resulting in elevated corticosterone (CORT) may contribute to this effect. We examined whether developmental conditions affected the association between adult song and HPA axis function, and whether nutritional stress before and after nutritional independence has distinct effects on song learning and/or vocal performance. Zebra finches (Taeniopygia guttata) were raised in consistently high (HH) or low (LL) food conditions until post-hatch day (PHD) 62, or were switched from high to low conditions (HL) or vice versa (LH) at PHD 34. Song was recorded in adulthood. We assessed the response of CORT to handling during development and to dexamethasone (DEX) and adrenocorticotropic hormone (ACTH) challenges during adulthood. Song learning and vocal performance were not affected by nutritional stress at either developmental stage. Nutritional stress elevated baseline CORT during development. Nutritional stress also increased rate of CORT secretion in birds that experienced stress only in the juvenile phase (HL group). Birds in the LL group had lower CORT levels after injection of ACTH compared to the other groups, however there was no effect of nutritional stress on the response to DEX. Thus, our findings indicate that developmental stress can affect HPA function without concurrently affecting song.

Gonadotropin-releasing hormone plasticity: a comparative perspective.

Gonadotropin-releasing hormone 1 (GnRH1) is a key regulator of the reproductive neuroendocrine system in vertebrates. Recent developments have suggested that GnRH1 neurons exhibit far greater plasticity at the cellular and molecular levels than previously thought. Furthermore, there is growing evidence that sub-populations of GnRH1 neurons in the preoptic area are highly responsive to specific environmental and hormonal conditions. In this paper we discuss findings that reveal large variation in GnRH1 mRNA and protein expression that are regulated by social cues, photoperiod, and hormonal feedback. We draw upon studies using histochemistry and immediate early genes (e.g., c-FOS/ZENK) to illustrate that specific groups of GnRH1 neurons are topographically organized. Based on data from diverse vertebrate species, we suggest that GnRH1 expression within individuals is temporally dynamic and this plasticity may be evolutionarily conserved. We suggest that the plasticity observed in other neuropeptide systems (i.e. kisspeptin) may have evolved in a similar manner.

Stress, song and survival in sparrows.

The stress response--increases in circulating glucocorticoids following a stressor--is typically considered adaptive, but few studies address the fitness consequences of individual variation in stress response. Generally, due to negative consequences of prolonged elevation of glucocorticoids, animals should have a transient stress response just sufficient to cope with the stressor. In rodents, stress responsiveness is affected by early developmental experience, and hyper-responsiveness to stress is linked to morbidity and mortality. We assessed individual variation in stress responses in free-living song sparrows, Melospiza melodia, in relation to fitness-related measures including song and overwinter survival. Birds with greater increases in corticosterone 30 min following restraint stress were less likely to return to breed the following year. Stress responsiveness was also correlated with song complexity: males with fewer syllables in their song repertoires had greater stress reactivity. Our findings support the hypothesis that developmental stressors both impair song development and affect the adult stress response. Thus, individual variation in the stress response may relate to variation in fitness.

Immediate early gene response to hearing song correlates with receptive behavior and depends on dialect in a female songbird.

Stimulus-induced expression of the immediate early gene ZENK (egr-1) in the songbird's auditory forebrain presumably depends on the behavioral significance of the stimulus. Few studies, however, have quantified both the ZENK and behavioral responses to a stimulus in the same individuals. We played conspecific male song of either hatch (local) or foreign dialect to female white-crowned sparrows (Zonotrichia leucophrys oriantha) and quantified both the auditory ZENK response and their behavioral response, which is known to depend on dialect. Birds hearing hatch dialect showed greater ZENK induction in the caudomedial hyperstriatum ventrale and the dorsal portion of the caudomedial neostriatum than birds hearing foreign dialect, supporting previous work showing a relationship between ZENK and salience of the stimulus. In the dorsal portion of the caudomedial neostriatum, ZENK induction was correlated with the amount of non-vocal courtship behavior; however, in the caudomedial hyperstriatum ventrale, ZENK induction was more highly correlated with the females' own vocal behavior and thus may have been partly self-induced. Some females sang and showed a male-like pattern of ZENK induction in their song systems. This study provides the first evidence that the ZENK response in a sensory area to a social stimulus is proportional to the animal's preference for the stimulus.

Sex differences in songbirds 25 years later: what have we learned and where do we go?

About 25 years ago, Nottebohm and Arnold reported that there are profound male-biased sex differences in volume in selected nuclei in telencephalic portions of the song control system. This review focuses on issues related to the cellular bases of these sex differences in volume and comparative studies that might elucidate the function of this variation between the sexes. Studies utilizing a variety of neurohistological methods in several different species to define the boundaries of two key telencephalic song nuclei HVc and the robust nucleus of the archistriatum (RA) all tend to find a sex difference in volume in agreement with Nissl-defined boundaries. Sex differences in volume in nuclei such as HVc and RA are associated with differences in cell size and cell number. Other attributes of the phenotype of cells in these nuclei are also different in males and females such as the number of cells expressing androgen receptors. Comparative studies have been employed to understand the function of these sex differences in the brain. In some songbird species, females sing rarely or not at all, and the brain nuclei that control song are many times larger volume in males than females. In other species, males and females sing approximately equally, and the brain nuclei that control song are approximately equal between the sexes. Recently, statistical methods have been employed to control for phylogenetic effects while comparing the co-evolution of traits. This analysis indicates that the evolution of sex differences in song has co-evolved with the evolution of sex differences in singing behavior in songbird species. Future studies should focus on the function of the smaller song control nuclei of females and investigate the role these nuclei might play in perception as well as in production.

Cultural and genetic evolution in mountain white-crowned sparrows: song dialects are associated with population structure.

Bird song often varies geographically within a species; when this geographic variation has distinct boundaries, the shared song types are referred to as song dialects. How dialects are produced and their adaptive significance are longstanding problems in biology, with implications for the role of culture in the evolution and ecology of diverse organisms, including humans. Here we test the hypothesis that song dialect, a culturally transmitted trait, is related to the population genetic structure of mountain white-crowned sparrows (Zonotrichia leucophrys oriantha). To address this, we compared microsatellite allele frequencies from 18 sample sites representing eight dialect regions in the Sierra Nevada. Pairwise genetic distances were not significantly correlated with geographic distances either within or between dialects, nor did dialect groups form distinct genetic groups according to neighbor-joining or UPGMA analysis, and most variation in allele frequencies occurred among individuals rather than at higher levels. However, most of the remaining variation was attributable to differences among, rather than within, dialect regions, and this among-dialect component of variance was statistically significant. Moreover, when controlling for the effect of geographic distance, song dissimilarity and genetic distance between site pairs were significantly correlated. Thus, song dialects appear to be associated with reductions in, but not strict barriers to, gene flow among dialect regions.

Seasonal changes in brain GnRH immunoreactivity and song-control nuclei volumes in an opportunistically breeding songbird.

White-winged crossbills (Loxia leucoptera) are opportunistic breeders that can nest at almost any time of year if there is sufficient food. Other cardueline finches that have been shown to breed on a strictly seasonal schedule become absolutely refractory to the stimulatory effects of long-day photoperiod, dramatically down-regulate hypothalamic gonadotropin-releasing hormone (GnRH), and reduce the volume of several song-control nuclei in autumn. This study examined whether changes in photoperiod modify the GnRH and song-control systems in white-winged crossbills. Adult male and female crossbills were captured and held on a naturally changing photoperiod. Brains of male and female birds were collected in May, October, and January. GnRH content was assessed by immunocytochemistry and the volumes of Nissl-defined song-control nuclei (HVc, Area X, and the robust nucleus of the archistriatum) were reconstructed. In contrast to other cardueline finches, GnRH immunoreactivity was relatively stable across the year, exhibiting only modest seasonal variation. The song control system, on the other hand, exhibited large seasonal changes as well as sex differences. Thus, crossbills appear to maintain hypothalamic GnRH content year round, perhaps to facilitate a rapid response to favorable breeding conditions, even on short days. However, song control nuclei are dramatically affected by photoperiod. Future work should examine these systems in crossbills breeding on short days to compare photoperiod-dependent and -independent effects on neural plasticity.

Photoperiodic regulation of the reproductive axis in male zebra finches, Taeniopygia guttata.

In arid central Australia, breeding of zebra finches (Taeniopygia guttata) tends to take place immediately after the aperiodic rainfall-this being irrespective of the time of year. As a result, zebra finches have traditionally been considered nonphotoperiodic. Despite this, there are some published reports of photoperiodic effects on behavior in zebra finches. Therefore, we transferred singly housed male zebra finches from a photoperiod of 14 h light and 10 h dark per day (14L:10D) to either 20L:4D or 8L:16D. Control birds remained exposed to 14L:10D. Plasma LH, testicular volume, and body mass were assessed at the start of the experiment and at intervals for a period of 56 days. Testicular mass was measured at the end of the 56-day period. Plasma LH increased significantly in the 20L:4D group after 14 days, but decreased again by 56 days, presumably an effect of increased gonadal steroid negative feedback. Plasma LH did not change significantly in the other two groups. Testicular volume increased steadily in the 20L:4D group during the treatment period and it was significantly higher than that of the 8L:16D group at the end of the experiment. After 56 days of treatment, combined testicular mass in the 20L:4D group was much greater than that of the 8L:16D group, but not quite statistically different from the 14L:10D group. There was no statistical difference in testicular mass between the 14L and 8L groups. Body mass did not differ between any of the groups at any time in the experiment. Our results are consistent with zebra finches being photoperiodic to some degree, despite their opportunistic breeding strategy. When considered in conjunction with recent reports of photoperiodic responses in tropical avian species, these data suggest that the ability to respond to changing photoperiod is more common among avian species than previously hypothesized.

Comparative studies of sex differences in the song-control system of songbirds.

Songbirds exhibit some of the most extreme sex differences in the brain of all vertebrates. Understanding the function of these sex differences has relied on making interspecies comparisons. In some species, females sing rarely or not at all, and the brain nuclei that control song are many times larger in volume in males than in females. In other species, males and females sing approximately equally, and the sizes of the brain nuclei that control song are approximately equal between the sexes. This article reviews sex differences in the song-control system of songbirds, and introduces statistical comparative methods developed by evolutionary biologists. These methods control for phylogenetic effects while comparing the co-evolution of traits. The extreme sex differences in song seem to have co-evolved with the extreme sex differences in singing behavior in songbird species.

Neural bases of song preferences in female zebra finches (Taeniopygia guttata).

We examined the neural bases of song preferences in female zebra finches (Taeniopygia guttata). Females performed more courtship displays in response to conspecific songs than to heterospecific songs. Following electrolytic lesion to the HVc (sometimes referred to as high vocal center), females maintained normal song preferences. However, following lesion to cHV (caudal hyperstriatum ventrale, an auditory area) females performed courtship displays at high rates in response to both conspecific and heterospecific song. Thus cHV, but not HVc, must be intact for female zebra finches to exhibit normal song preferences. Differences between this study and those showing HVc lesions disrupting song preferences in female canaries (Serinus canaria) indicate interspecific variation in the function of HVc in female songbirds.

Neural correlates of singing behavior in male zebra finches (Taeniopygia guttata).

This study examined the relationship between the volumes of four song control nuclei: the high vocal center (HVC), the lateral part of the magnocellular nucleus of the anterior neostriatum (IMAN), Area X, and the robust nucleus of the archistriatum (RA), as well as syrinx mass, with several measures of song output and song complexity in male zebra finches (Taeniopygia guttata). Male zebra finches' songs were recorded in standardized recording sessions. The syrinx and brain were subsequently collected from each bird. Volumes of the song control nuclei were reconstructed by measuring the cross-sectional area of serial sections. Syrinx mass was positively correlated with RA volume. The volume of IMAN was negatively related to element repertoire size and the number of elements per phrase. We found no other correlations between brain and behavioral measures. This study, combined with others, indicates that the evidence for a general relationship among songbirds between HVC volume and song complexity is equivocal. There are clear species differences in this brain-behavior correlation.

Auditory scene analysis by European starlings (Sturnus vulgaris): perceptual segregation of tone sequences.

Like humans, animals that use acoustic stimuli to perceive their world ought to be able to parse the auditory scene into functionally significant sounds. The ability to do so ought to have significant adaptive value when, for example, an animal can identify the sounds of a predator among other natural noises. In earlier work it was shown that a species of songbird, the European starling, can identify excerpts of both its own song and songs from other avian species when the songs are mixed concurrently with other natural signals. In this experiment it is demonstrated that starlings can segregate two synthetic pure-tone sequences when the sequences differ in frequency. Taken together, the experiments show that at least one nonhuman species is capable of auditory scene analysis both for natural and for non-natural acoustic stimuli. This suggests in turn that auditory scene analysis may be a general perceptual process that occurs in many species that make use of acoustic information.

Auditory scene analysis by songbirds: stream segregation of birdsong by European starlings (Sturnus vulgaris).

Three experiments examined the capacity of European starlings to segregate perceptually 2 superimposed, intermixed auditory stimuli. The stimuli were 10-s song samples from 2 of 4 songbird species: European starling, brown thrasher, mockingbird, and nightingale. The birds first learned a discrimination between the intermixed song pairs. Then, they maintained the discrimination with novel song exemplars in the mixtures and when song stimuli for each species were presented alone. Performance fell, but remained above chance, when song pairs were mixed with the dawn chorus of bird song. The results show that starlings were identifying the songs of individual species within the baseline superimposed song pairs, a process of auditory stream segregation and scene analysis (A. S. Bregman, 1990).