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Long-distance migrations influence the physiology, behavior, and fitness of migratory animals throughout their annual cycles, and fundamentally alter their interactions with parasites. Several hypotheses relating migratory behavior to the likelihood of parasitism have entered the literature, making conflicting, testable predictions. To assess how migratory behavior of hosts is associated with parasitism, we compared haemosporidian parasite infections between two closely related populations of a common North American sparrow, the dark-eyed junco, that co-occur in shared habitats during the non-breeding season. One population is sedentary and winters and breeds in the Appalachian Mountains. The other population is migratory and is found in seasonal sympatry with the sedentary population from October through April, but then flies (≥ 900 km) northwards to breed. The populations were sampled in the wild on the shared montane habitat at the beginning of winter and again after confining them in a captive common environment until the spring. We found significantly higher prevalence of haemosporidian parasite infections in the sedentary population. Among infected juncos, we found no difference in parasite densities (parasitemias) between the sedentary and migrant populations and no evidence for winter dormancy of the parasites. Our results suggest that long-distance migration may reduce the prevalence of parasite infections at the population level. Our results are inconsistent with the migratory exposure hypothesis, which posits that long-distance migration increases exposure of hosts to diverse parasites, and with the migratory susceptibility hypothesis, which posits that trade-offs between immune function and migration increase host susceptibility to parasites. However, our results are consistent with the migratory culling hypothesis, which posits that heavily infected animals are less likely to survive long-distance migration, and with the migratory escape hypothesis, which posits that long-distance migration allows host populations to seasonally escape areas of high infection risk.

American robins and dark-eyed juncos migrate across North America and have been found to be competent hosts for some bacterial and viral pathogens, but their contributions to arthropod-borne diseases more broadly remain poorly characterized. Here, we sampled robins and juncos in multiple sites across North America for arthropod-borne bacterial pathogens of public health significance. We identified two novel Rickettsia spp. in one wintering migrant per bird species related to bellii, transitional, and spotted rickettsiae fever groups. Stable isotope analyses of feathers suggested spring migration of these common songbirds could disperse these novel rickettsiae hundreds-to-thousands of kilometers to host breeding grounds. Further work is needed to characterize zoonotic potential of these rickettsiae and host reservoir competence.

Hormones coordinate the co-expression of behavioral, physiological, and morphological traits, giving rise to correlations among traits and organisms whose parts work well together. This article considers the implications of these hormonal correlations with respect to the evolution of hormone-mediated traits. Such traits can evolve owing to changes in hormone secretion, hormonal affinity for carrier proteins, rates of degradation and conversion, and interaction with target tissues to name a few. Critically, however, we know very little about whether these changes occur independently or in tandem, and thus whether hormones promote the evolution of tight phenotypic integration or readily allow the parts of the phenotype to evolve independently. For example, when selection favors a change in expression of hormonally mediated characters, is that alteration likely to come about through changes in hormone secretion (signal strength), changes in response to a fixed level of secretion (sensitivity of target tissues), or both? At one extreme, if the phenotype is tightly integrated and only the signal responds via selection's action on one or more hormonally mediated traits, adaptive modification may be constrained by past selection for phenotypic integration. Alternatively, response to selection may be facilitated if multivariate selection favors new combinations that can be easily achieved by a change in signal strength. On the other hand, if individual target tissues readily \"unplug\" from a hormone signal in response to selection, then the phenotype may be seen as a loose confederation that responds on a trait-by-trait basis, easily allowing adaptive modification, although perhaps more slowly than if signal variation were the primary mode of evolutionary response. Studies reviewed here and questions for future research address the relative importance of integration and independence by comparing sexes, individuals, and populations. Most attention is devoted to the hormone testosterone (T) and a songbird species, the dark-eyed junco (Junco hyemalis).

Climate change, habitat alteration, range expansions, and biological invasions are all predicted to require rapid shifts in multiple traits including behavior and life history, both for initial population establishment and subsequent adaptation. Hormonal mechanisms likely play a key role in facilitating or constraining plastic and genetic responses for suites of traits, but few studies have evaluated their role in shaping contemporary adaptation or diversification. We examined multiple phenotypic adjustments and associated hormonal changes following a recent (early 1980s) colonization event, in which a temperate-breeding songbird, the dark-eyed junco (Junco hyemalis), became established in the Mediterranean climate of San Diego, California. The milder climate has led to an extended breeding season and year-round residency, and we document shifts in multiple sexually selected behaviors and plumage traits. Testosterone titers in San Diego were elevated for longer but with a lower peak value compared to a nearby native-range population, and correlations between testosterone and related traits were similar within and among populations. A common garden study indicated that changes in testosterone likely represent plastic responses to the less seasonal environment of the city, providing the context against which subsequent genetic changes in morphology likely occurred. We argue that correlated shifts in multiple traits, organized by underlying physiology, may be a generally important element of many successful adjustments to changing environments.

Reproductive allochrony presents a potential barrier to gene flow and is common in seasonally sympatric migratory and sedentary birds. Mechanisms mediating reproductive allochrony can influence population divergence and the capacity of populations to respond to environmental change. We asked whether reproductive allochrony in seasonally sympatric birds results from a difference in response to supplementary or photoperiodic cues and whether the response varies in relation to the distance separating breeding and wintering locations as measured by stable isotopes. We held seasonally sympatric migratory and sedentary male dark-eyed juncos (Junco hyemalis) in a common garden in early spring under simulated natural changes in photoperiod and made measurements of reproductive and migratory physiology. On the same dates and photoperiods, sedentary juncos had higher testosterone (initial and gonadotropin-releasing hormone induced), more developed cloacal protuberances, and larger testes than migrants. In contrast, migratory juncos had larger fat reserves (fuel for migration). We found a negative relationship between testis mass and feather hydrogen isotope ratios, indicating that testis growth was more delayed in migrants making longer migrations. We conclude that reproductive allochrony in seasonally sympatric migratory and sedentary birds can result from a differential response to photoperiodic cues in a common garden, and as a result, gene flow between migrants and residents may be reduced by photoperiodic control of reproductive development. Further, earlier breeding in response to future climate change may currently be constrained by differential response to photoperiodic cues.

Evolutionary approaches to culture remain contentious. A source of contention is that cultural mutation may be substantial and, if it drives cultural change, then current evolutionary models are not adequate. But we lack studies quantifying the contribution of mutations to directional cultural change. We estimated the contribution of one type of cultural mutations—modification of memes—to directional cultural change using an amenable study system: learned birdsongs in a species that recently entered an urban habitat. Many songbirds have higher minimum song frequency in cities, to alleviate masking by low-frequency noise. We estimated that the input of meme modifications in an urban songbird population explains about half the extent of the population divergence in song frequency. This contribution of cultural mutations is large, but insufficient to explain the entire population divergence. The remaining divergence is due to selection of memes or creation of new memes. We conclude that the input of cultural mutations can be quantitatively important, unlike in genetic evolution, and that it operates together with other mechanisms of cultural evolution. For this and other traits, in which the input of cultural mutations might be important, quantitative studies of cultural mutation are necessary to calibrate realistic models of cultural evolution.

Allochrony, the mismatch of reproductive schedules, is one mechanism that can mediate sympatric speciation and diversification. In songbirds, the transition into breeding condition and gonadal growth is regulated by the hypothalamic–pituitary–gonadal (HPG) axis at multiple levels. We investigated whether the difference in reproductive timing between two seasonally sympatric subspecies of dark-eyed juncos ( Junco hyemalis ) was related to gene expression along the HPG axis. During the sympatric pre-breeding stage, we measured hypothalamic and testicular mRNA expression of candidate genes via qPCR in captive male juncos. For hypothalamic mRNA, we found our earlier breeding subspecies had increased expression of gonadotropin-releasing hormone ( GnRH ) and decreased expression of androgen receptor, oestrogen receptor alpha and mineralocorticoid receptor ( MR ). Subspecies did not differ in expression of hypothalamic gonadotropin-inhibitory hormone ( GnIH ) and glucocorticoid receptor ( GR ). While our earlier breeding subspecies had higher mRNA expression of testicular GR , subspecies did not differ in testicular luteinizing hormone receptor, follicle-stimulating hormone receptor or MR mRNA expression levels. Our findings indicate increased GnRH production and decreased hypothalamic sensitivity to sex steroid negative feedback as factors promoting differences in the timing of gonadal recrudescence between recently diverged populations. Differential gene expression along the HPG axis may facilitate species diversification under seasonal sympatry.

Recent studies exploring the molecular genetic basis for migratory variation in animals have identified polymorphisms in two genes ( CLOCK and ADCYAP1) that are linked to circadian rhythms and correlate with migratory propensity and phenology among individuals and populations. Results from these initial studies are mixed, however, and additional data are needed to assess the generality and diversity of the molecular mechanisms that regulate the biology of migration. We sequenced CLOCK and ADCYAP1 in 15 populations across the two species of the avian genus Junco, a North American lineage in which multiple recently diverged subspecies and populations range from sedentary to long-distance migrants. We found no consistent associations between allele length and migratory status across the genus for either CLOCK or ADCYAP1. However, within two subspecies groups, populations that migrate longer distances have longer CLOCK alleles on average. Additionally, there was a positive relationship between ADCYAP1 allele length and migratory restlessness (zugunruhe) among individuals within one of two captive populations studied-a result similar to those reported previously within captive blackcaps ( Sylvia atricapilla). We conclude that, while both ADCYAP1 and CLOCK may correlate with migratory propensity within or among certain populations or species, previously identified relationships between migratory behavior and sequence variants cannot be easily generalized across taxa.

Migratory birds generally divide the annual cycle between discrete breeding and nonbreeding ranges. Itinerant breeders, however, reproduce twice at different geographic locations, migrating between them. This unusual flexibility in movement ecology and breeding biology suggests that some species can rapidly modulate the conflicting physiological and behavioral traits required for migration and reproduction. The Phainopepla (Phainopepla nitens), a songbird of the southwestern USA, has long been suspected to breed first in desert habitats in spring, then migrate to woodland habitats to breed again in summer. However, direct evaluation of movement and gene flow among individuals breeding in different locations has previously been logistically intractable. We deployed GPS tags on free-flying Phainopeplas in southern California, all of which migrated to hypothesized woodland breeding habitats after desert breeding (an average distance of 232 km). GPS data also revealed previously unknown fall and spring stopover sites. Population genomic analyses revealed no genetic differentiation among desert and woodland breeding populations, indicating significant movement and gene flow across the region. Finally, we used random forest analyses to quantify substantial environmental differences among temporal stages. Our results provide direct evidence that individual Phainopeplas do indeed move between 2 drastically different breeding habitats in the same year, representing a rare and extreme example of life-history flexibility.

Premise of research. Selection on floral traits by pollinators is expected to play a key role in the diversification of floral phenotypes and the maintenance of species boundaries in sympatry. However, relatively few studies have tested the generality of these assumptions in natural systems, and fewer still have examined multiple phenotypes to better understand which ones are most important to pollinators. Methodology. We created F2hybrids between two sympatric species of dioeciousSileneto segregate groups of floral characteristics from their associated pollination syndromes. Arrays of hybrid female plants were placed among natural field populations, where we observed native pollinators (diurnal and crepuscular) in order to address which individual traits or groups of traits influenced pollinator visitation, seed set, or predation. Pollinator visits included bees, bee flies, butterflies, and moths, including the mutualist nursery pollinator/seed predatorHadena bicruris. Pivotal results. Regardless of time of day, large flowers and tall plants had higher visitation rates. However, large flowers and tall plants were also more likely to be preyed on byH. bicruris. In all treatments, pollinators showed no preferences for flower color, despite the hypothesized general importance of color for pollinator attraction. Conclusions. Floral phenotypes are the result of both floral preferences by pollinators and antagonistic interactions with floral predators in this system. These findings provide further empirical evidence that floral traits may be important for maintaining species boundaries.