Junco Hyemalis Research Articles

Experimental traffic noise attracts birds during the breeding season

Abstract Understanding how anthropogenic disturbance affects animal behavior is challenging because observational studies often involve co-occurring disturbances (e.g., noise, lighting, and roadways), and laboratory experiments often lack ecological validity. During the 2016 and 2017 avian breeding seasons, we investigated the effects of anthropogenic noise and light on the singing and spatial behavior of wild birds by independently manipulating the presence of each type of disturbance at 89 sites in an otherwise undisturbed boreal forest in Labrador, Canada. Each treatment was surrounded by an eight-channel microphone array that recorded and localized avian vocalizations throughout the manipulation. We analyzed the effects of noise and light on the timing of the first vocalizations of each species at each array during the dawn chorus, and on the proximity of the vocalizing birds to the disturbance when those songs were produced. We analyzed all species combined, and then conducted separate analyses for the six most common species: boreal chickadee, dark-eyed junco, ruby-crowned kinglet, Swainson’s thrush, white-throated sparrow, and yellow-rumped warbler. When all species were analyzed together, we found that traffic noise attracted vocalizing birds. There was some evidence that light repelled birds, but this evidence was inconsistent. In our species-specific analyses, yellow-rumped warbler sang earlier in response to noise; Swainson’s thrush was attracted to noise and the combination of noise and light but repelled by light alone. Our study provides some of the first experimental evidence of the independent and combined effects of traffic noise and light on the vocal and spatial behavior of wild birds and suggests that breeding birds may be attracted to noisy roads where they could be exposed to additional forms of disturbance.

Migratory strategy explains differences in timing of female reproductive development in seasonally sympatric songbirds

Abstract Divergent migratory strategies among populations can result in population‐level differences in timing of reproduction (allochrony) and local adaptation. However, the mechanisms underlying among‐population variation in timing are insufficiently understood, particularly in females. We studied differences in reproductive development and its related mechanisms along the hypothalamic–pituitary–gonadal axis (HPG) in closely related migratory and sedentary (i.e. resident) female dark‐eyed juncos (Junco hyemalis) living together in sympatry during early spring. Despite exposure to the same environmental cues in early spring, residents initiate breeding prior to the departure of migrants for their breeding grounds. We investigated whether residents would be more reproductively developed than migrants based on their behavioural differences. Alternatively, females could exhibit similar reproductive development in response to the same environmental cues despite differences in migratory behaviour. To compare their degree of reproductive development during seasonal sympatry and the underlying mechanisms of these differences, we collected ovarian and liver tissue in early spring prior to migration and compared transcript abundance of genes associated with reproduction using quantitative PCR. We also used stable hydrogen isotopes to infer relative breeding and wintering latitude of migrants. We found higher transcript abundance of luteinizing hormone receptor and aromatase in the ovary in addition to significantly heavier ovaries in residents than in migrants. Together, these results suggest greater sensitivity and response to upstream endocrine stimulation in resident females. Transcript abundance for other receptors in the ovary and liver associated with reproduction, however, did not differ between populations. When comparing ovarian development within migrants, females with lower hydrogen isotopes (indicating higher breeding latitudes) had smaller ovaries, suggesting that longer‐distance migrations may further delay reproductive development. Based on differences in ovary mass and transcript abundance, we conclude that females that differ in migratory strategy also differ in timing of reproductive development. These results support that divergent migratory behaviour drives allochrony and could enable reproductive isolation between populations; mechanistic differences at the level of gonadal stimulation can explain these differences in timing of reproductive development. A free Plain Language Summary can be found within the Supporting Information of this article.

Onset of Daily Activity in a Female Songbird Is Related to Peak-Induced Estradiol Levels.

Research in captive birds and mammals has demonstrated that circadian (i.e., daily) behavioral rhythms are altered in response to increases in sex-steroid hormones. Recently, we and others have demonstrated a high degree of individual repeatability in peak (gonadotropin-releasing hormone [GnRH]-induced sex) steroid levels, and we have found that these GnRH-induced levels are highly correlated with their daily (night-time) endogenous peak. Whether or not individual variation in organization and activity of the reproductive endocrine axis is related to daily timing in wild animals is not well known. To begin to explore these possible links, we tested the hypothesis that maximal levels of the sex steroid hormone estradiol (E2) and onset of daily activity are related in a female songbird, the dark-eyed junco (Junco hyemalis). We found that females with higher levels of GnRH-induced E2 departed from their nest in the morning significantly earlier than females with lower stimulated levels. We did not observe a relationship between testosterone and this measure of onset of activity. Our findings suggest an interaction between an individual's reproductive endocrine axis and the circadian system and variation observed in an individuals' daily activity onset. We suggest future studies examine the relationship between maximal sex-steroid hormones and timing of daily activity onset.

Elevation affects extra-pair paternity but not a sexually selected plumage trait in dark-eyed juncos

Differences in environmental conditions are expected to generate distinct selective pressures favoring different phenotypes. For example, environmental conditions that affect the timing of breeding may influence opportunities for extra-pair copulations and thus the strength of sexual selection on males. To explore these relationships quantitatively, we compared breeding synchrony, rates of extra-pair paternity, and expression of a sexually selected plumage trait (the amount of white on the tail feathers) in populations of dark-eyed junco (Junco hyemalis) breeding at elevations from 1960 to 2660 m in California. Microsatellite parentage analysis revealed that extra-pair paternity rates varied by elevation, with intermediate elevations having the highest rate. Differences in breeding synchrony could not explain this variation. Extra-pair males had more tail white than the social males they cuckolded, consistent with tail white being a sexually selected trait. Although the observed differences in rates of extra-pair paternity suggested that sexually selected traits should also vary with elevation, there were no differences among elevations in the amount of white on male tails or in the correlation between tail white and proxies for male condition. Multiple factors may have contributed to this result, including persistent gene flow among elevations, which may counter the effects of local differences in selective pressures. These findings demonstrate the complexity of interactions among environmental conditions, selective pressures, and variation in phenotypic traits, and underscore the importance of assessing the impacts of sexual selection in the larger context of population genetic structure. SIGNIFICANCE STATEMENT: Environmental differences, such as those occurring along elevation gradients, can lead to differences in sexual selection. We found that juncos at mid elevations had higher rates of extra-pair paternity than juncos at high and low elevations. Our results also provide evidence that male tail plumage is sexually selected, as females preferred to copulate with males with more white on their tail plumage than the females’ social mates. This suggests that male tail white should differ among elevations, as the reproductive rewards of having an attractive tail would be greater at mid elevations. However, we found no differences in male tail white among elevations. This may be due to the birds breeding freely across elevations, as evidenced by a lack of genetic structure, i.e., gene flow swamping out any differences that might otherwise form.

Within-winter flexibility in muscle and heart lipid transport and catabolism in passerine birds.

Small birds in cold climates may show within-winter metabolic flexibility to match metabolic capacities to prevailing weather conditions. This flexibility may occur over periods of days to weeks, but the underlying mechanisms for such flexibility are not well understood. Because lipids are the primary fuel for sustained thermogenesis, we examined whether lipid transport and catabolism can mediate within-winter metabolic flexibility in two small temperate-zone wintering passerine birds, dark-eyed juncos (Junco hyemalis) and house sparrows (Passer domesticus). We used simple and multiple regression analyses to test for correlations of several lipid transporters in pectoralis muscle (plasma membrane-bound and cytosolic fatty acid-binding proteins, FABP; fatty acyl translocase, FAT/CD36) and regulatory enzymes (carnitine acyl transferase, CPT; β-hydroxyacyl CoA dehydrogenase, HOAD) in pectoralis and heart with short-term (ST, 0-7days), medium-term (MT, 14-30days) and long-term (LT, 30-year mean daily and extreme minimum temperatures, day of winter season) temperature variables. We hypothesized negative correlations between these regulators and temperature variables. Juncos showed negative correlations for FABPs with ST or MT temperature variables, but other lipid transporters and regulatory enzymes showed positive correlations with ST or MT temperatures for juncos, suggesting no consistent pathway-wide response to within-winter temperatures. LT temperature variables showed several significant associations with lipid transporters and enzymes for juncos, but also not in consistent directions. House sparrows showed the expected negative correlations with LT temperatures for FABPpm, but positive correlations with temperature variables for FABPc, CPT and HOAD. Different species-specific patterns of variation and the absence of consistent pathway-wide responses to temperature suggest that the lipid transport and catabolism pathway is not a uniform mediator of within-winter metabolic flexibility among small birds.

Tail-flashing as an anti-predator signal in small wintering birds

Prey species use pursuit-deterrent signals to discourage an attack, by informing a predator either that the latter has been detected, or that the prey is capable of escaping if attacked. These signals tend to be conspicuous behaviors, such as bobbing, stotting, and predator inspection. Dark-eyed juncos (Junco hyemalis) show prominent tail-flashing in social displays during the breeding season, but they continue to tail-flash in winter. We examined whether such tail-flashing functions as a pursuit-deterrent signal by measuring tail-flashing rates in the presence and absence of a taxidermically mounted hawk predator. Our results showed that juncos tail-flash more in the presence of the predator and at higher rates when in direct view of the predator. This suggests that tail-flashing is directed to the predator as a pursuit-deterrent signal. Additionally, juncos reduced tail-flashing when feeding far from cover (low escape probability), suggesting that tail-flashing likely has an attraction cost. We also found a marked group size effect with solitary juncos tail-flashing more than those in large groups, indicating an additional cost to tail-flashing that need not be paid in larger groups. This additional cost is not related to food intake, since we found no negative association between food intake and tail-flashing. We observed tail-flashing in other sparrow species co-occurring with juncos at the study site, suggesting that tail-flashing as a pursuit-deterrent behavior may be widespread in this taxonomic group of birds. Many animals, and especially birds, use tail movements as a signal in various contexts. Dark-eyed juncos make extensive use of tail-flashing as a breeding signal, but they continue to tail-flash during the winter. Our experiment provides evidence that such tail-flashing serves as a pursuit-deterrent signal, a function previously not examined in these birds. Juncos tail-flashed more in the presence of a visible predator, suggesting that the signal is directed towards the predator. Tail-flashing was maximal when close to cover, indicating that tail-flashing incurs a cost of attracting predator attention. A negative relationship between flock size and tail-flashing suggests an additional cost to this behavior, which we demonstrate is not related to food intake rate. We suspect that tail-flashing functions more widely as a pursuit-deterrent signal in the broad phylogenetic group of emberizid sparrows.

New avian breeding records for Kugluktuk, Nunavut

New breeding records for 10 species of tundra and boreal nesting birds were documented near the community of Kugluktuk (Nunavut, Canada) over the course of the 2015 and 2016 breeding seasons and incidentally in 2017 and 2018. These species include American Wigeon (Mareca americana), Mallard (Anas platyrhynchos), Northern Shoveler (Anas clypeata), Green-winged Teal (Anas carolinensis), Greater Scaup (Aythya marila), Say’s Phoebe (Sayornis saya), Canada Jay (Perisoreus canadensis), Greycheeked Thrush (Catharus minimus), Yellow-rumped Warbler (Setophaga coronata), and Dark-eyed Junco (Junco hyemalis). Previously unpublished breeding evidence for Bald Eagle (Haliaeetus leucocephalus) is also discussed along with suspected breeding of Barn Swallow (Hirundo rustica), Bohemian Waxwing (Bombycilla garrulus), White-throated Sparrow (Zonotrichia albicollis), and Pine Grosbeak (Pinicola enucleator). These records represent the first described breeding occurrences for these species in the Kitikmeot region, or new records for the mainland of Nunavut. A lack of historical ornithological survey effort in this area has likely led to the diversity of these previously unrecorded breeding observations. These results highlight the need to increase geographic coverage of bird surveys in northern Canada to more accurately delineate the northern limit of breeding ranges and suggest that further formal survey effort will undoubtedly lead to additional new breeding records.

Corticosterone negative feedback is weaker during spring vs. autumn migration in a songbird (Junco hyemalis)

Birds face many challenges during seasonal migrations and must make important decisions about whether to accelerate, maintain, or delay travel to their final destinations. Spring migration is likely more challenging than autumn migration as spring journeys are completed more quickly and weather conditions are harsher during this time. These differential challenges may be reflected in the endocrine stress response, as the hypothalamic–pituitary-adrenal (HPA) axis is important for both daily energetic needs and coping with stressors. Indeed, most avian studies have found that both baseline and stress-induced corticosterone (CORT) levels tend to be higher in spring migrants than in autumn migrants. We hypothesized that CORT negative feedback efficacy also differs across the season, and is likely weaker during times of year when birds must be most sensitive to stressors. We therefore predicted that CORT negative feedback efficacy would be weaker during spring vs. autumn migration as spring migrants are more likely to encounter situations where they must decide whether to turn back or delay their travel. We examined male dark-eyed juncos (Junco hyemalis) during their spring and autumn stopovers in Fargo, ND, USA. Our prediction was met as we did find that negative feedback efficacy was weaker during spring vs. autumn, although we notably did not find any seasonal differences in baseline and stress-induced CORT. We also found that spring migrants were heavier, had greater subcutaneous fat stores, and had slightly higher hematocrit compared to autumn migrants. These findings suggest that CORT negative feedback sensitivity may help migrating birds effectively cope with the differential challenges of autumn and spring migration.

Density-dependent fitness, not dispersal movements, drives temporal variation in spatial genetic structure in dark-eyed juncos (Junco hyemalis).

Some studies have found that dispersal rates and distances increase with density, indicating that density-dependent dispersal likely affects spatial genetic structure. In an 11-year mark-recapture study on a passerine, the dark-eyed junco, we tested whether density affected dispersal distance and/or fine-scale spatial genetic structure. Contrary to expectations, we found no effect of predispersal density on dispersal distance or the proportion of locally produced juveniles returning to the population from which they hatched. However, even though density did not affect dispersal distance or natal return rates, we found that density still did affect spatial genetic structure. We found significant positive spatial genetic structure at low densities of (postdispersal) adults but not at high densities. In years with high postdispersal (adult) densities that also had high predispersal (juvenile) densities in the previous year, we found negative spatial genetic structure, indicating high levels of dispersal. We found that density also affected fitness of recruits, and fitness of immigrants, potentially linking these population parameters with the spatial genetic structure detected. Immigrants and recruits rarely nested in low postdispersal density years. In contrast, in years with high postdispersal density, recruits were common and immigrants had equal success to local birds, so novel genotypes diluted the gene pool and effectively eliminated positive spatial genetic structure. In relation to fine-scale spatial genetic structure, fitness of immigrants and new recruits is poorly understood compared to dispersal movements, but we conclude that it can have implications for the spatial distribution of genotypes in populations.

Predatory Attack by a Western Terrestrial Garter Snake on a Nestling Dark-Eyed Junco

I observed a Western Terrestrial Garter Snake (Thamnophis elegans) attacking a large (approximately 9-d-old) nestling Dark-eyed Junco (Junco hyemalis) on 7 July 2018 near Rattlesnake Creek, Missoula County, Montana. The snake held the stillliving nestling by its head (the head was mostly enveloped by the snake's jaws) and continued to do so for 3 min before it released the now-dead nestling when disturbed by my presence. The dead nestling exhibited skin wounds on the neck but no other superficial injury. Predation on birds by Western Terrestrial Garter Snakes is infrequently reported, and I found only 2 other reports of predation on juncos by this snake species.

The importance of disturbance and forest structure to bird abundance in the Black Hills

AbstractMany North American birds associated with forest disturbances such as wildfire and mountain pine beetle (Dendroctonus ponderosae) outbreaks are declining in abundance. More information on relationships between avian abundance and forest structure and disturbance is needed to guide conservation and management. Our objective was to determine densities of American Three-toed Woodpecker (Picoides dorsalis), Black-backed Woodpecker (Picoides arcticus), Red-breasted Nuthatch (Sitta canadensis), Brown Creeper (Certhia americana), and White-winged Junco (Junco hyemalis aikeni) in relation to vegetation characteristics and disturbance at the point and landscape level in the Black Hills and Bear Lodge Mountains of South Dakota and Wyoming. We conducted 3 point counts from late March to late June 2015 and 2016 at more than 2,300 locations distributed across a gradient of forest structure and disturbance types. We estimated densities using 3-level hierarchical time-removal models that simultaneously estimated abundance, availability, and detection probability. Black-backed Woodpeckers were positively related to percent area in 1- to 3-year-old wildfires and Brown Creepers were positively associated with percent area in 4- to 5-year-old wildfires; however, Red-breasted Nuthatches were negatively related to percent area in 3- to 5-year-old wildfires. With the exception of American Three-toed Woodpeckers, species were positively related to percent cover of beetle-killed trees. Brown Creepers, White-winged Juncos, and Red-breasted Nuthatches had mixed responses to percent overstory canopy cover. White-winged Juncos also had a positive association with percent ground vegetation at the point and landscape level. Brown Creepers were strongly linked with spruce vegetation type. American Three-toed Woodpeckers, which are thought to occupy spruce forest in the Black Hills, did not show a strong relationship with any covariates. Maintaining some areas of natural disturbances along with heterogeneity of vegetation characteristics within stands and at the landscape scale will benefit the needs of a diverse bird community in the Black Hills.

Influence of the Agricultural Conservation Easement Program wetland practices on winter occupancy of Passerellidae sparrows and avian species richness.

Wetlands enrolled in the Agricultural Conservation Easement Program (ACEP) are established as a means of restoring wetland ecosystems and wildlife habitat on private, agricultural land. In West Virginia, USA, ACEP wetlands have never been evaluated to determine how they function as wildlife habitat in comparison to other available wetland habitat in the state. We measured the wintering occupancy of Passerellidae species and apparent avian species richness on ACEP wetlands and a set of reference wetlands located on public land in West Virginia to evaluate if ACEP wetlands are being used similarly by avian species to other available wetland habitat in the state. Apparent avian species richness and the occupancy probability of four Passerellidae species—song sparrows (Melospiza melodia), dark-eyed juncos (Junco hyemalis), swamp sparrows (Melospiza georgiana), and white-throated sparrows (Zonotrichia albicollis)—did not differ between ACEP and reference sites. In addition to other vegetative and habitat associations for each species, dark-eyed junco occupancy was negatively correlated with wetland size while swamp sparrow occupancy and apparent avian species richness were positively associated with wetland size. These results indicate that ACEP wetlands are providing winter avian habitat as well as another source of wetland habitat in the state. Maintaining and expanding ACEP wetlands in West Virginia would continue to provide wetland systems in areas that are otherwise lacking these habitats.

Experimental evidence that symbiotic bacteria produce chemical cues in a songbird.

Symbiotic microbes that inhabit animal scent glands can produce volatile compounds used as chemical signals by the host animal. Though several studies have demonstrated correlations between scent gland bacterial community structure and host animal odour profiles, none have systematically demonstrated a causal relationship. In birds, volatile compounds in preen oil secreted by the uropygial gland serve as chemical cues and signals. Here, we tested whether manipulating the uropygial gland microbial community affects chemical profiles in the dark-eyed junco (Junco hyemalis). We found an effect of antibiotic treatment targeting the uropygial gland on both bacterial and volatile profiles. In a second experiment, we cultured bacteria from junco preen oil, and found that all of the cultivars produced at least one volatile compound common in junco preen oil, and that most cultivars produced multiple preen oil volatiles. In both experiments, we identified experimentally generated patterns in specific volatile compounds previously shown to predict junco reproductive success. Together, our data provide experimental support for the hypothesis that symbiotic bacteria produce behaviourally relevant volatile compounds within avian chemical cues and signals.

An experimental test of the effect of male attractiveness on maternal investment in Dark-eyed Juncos

AbstractWhile social mate choice is an obvious point where sexual selection acts on males, cryptic selection can occur based on how much females invest in a male’s offspring. I tested 4 hypotheses about the relationship between female investment and social mate attractiveness by manipulating a sexually selected trait, white tail feathers (“tail white”), in male Dark-eyed Juncos (Junco hyemalis thurberi): (1) the differential allocation hypothesis predicts that females optimize lifetime reproductive success by maximizing parental investment in the offspring of ornamented mates and minimizing investment when mated to less attractive males; (2) instead of positive differential allocation, females could show negative differential allocation by investing more in less attractive males to compensate for their lower quality; (3) female care could also be influenced directly by paternal investment, in particular what appears to be positive differential allocation could be compensation for reduced care by attractive males; or (4) females could reduce care when paired with socially attractive males that are “good parents”. To evaluate these hypotheses, I examined the impact of tail white manipulation on clutch size, female incubation and brooding, and female and male provisioning. Females did not differentially allocate in relation to social mate attractiveness, nor were females responsive to patterns of male investment, even although artificially attractive males provisioned offspring significantly more than did controls. Naturally bright males, who were also the largest males, did not provision at higher rates, highlighting the need to experimentally separate the role of specific traits from other correlated factors. Tail white-manipulated males were involved in rare cases of polygyny and at times were actively guarded by their mates. Together, these results suggest that tail white influences reproductive behaviors in this junco population, but because of the potential for extrapair paternity, additional data are needed to accurately investigate the trait’s role in female investment.

Urban residency and leukocyte profiles in a traditionally migratory songbird

Abstract Many animals are shifting migrations in response to human activities. In particular, human-induced changes to climate and habitat (e.g., urbanization) likely facilitate animals becoming year-round residents. Because migration can be energetically expensive, shifts to sedentary behavior could minimize energetic demands incurred and any immunosuppressive effects. Residency in urban habitats could also provide abundant resources and allow sedentary animals to invest more in immunity. However, urban habitats could also expose sedentary animals to novel stressors that counter such benefits. To examine how recent shifts to residency affects physiology in ways that may shape infectious disease dynamics, we analyzed leukocyte profiles of two dark-eyed junco (Junco hyemalis) populations from southern California: the Laguna Mountain population, in which birds breed in high-elevation forests and migrate altitudinally, and the urban University of California San Diego population, which was likely established by overwintering migrants in the 1980s and has since become non-migratory. Over a two-year study of each population’s breeding season, we found no difference in the ratios of heterophils to lymphocytes between populations. However, urban residents had more leukocytes than birds from the altitudinal migrant population. A multivariate analysis suggested urban residents had fewer monocytes, but effect sizes were small. These results suggest no differences in energy demands or stressors between urban resident and altitudinal migrant populations during their breeding season. However, urban residency may confer immunological benefits through anthropogenic resources, which could have important consequences for disease dynamics..

Leukocyte profiles vary with breeding latitude and infection status in a seasonally sympatric songbird

Abstract Billions of animals migrate annually in pursuit of food, safety, and reproduction. Long-distance migration can be energetically expensive, which can force tradeoffs with investment in other physiological systems (e.g., suppressing immunity). Understanding the physiological impacts of migration is important to predict when and where such animals may be vulnerable to parasites and how changes to migration might affect infectious disease risks. To isolate relationships between migration and physiology from energetic tradeoffs and hormonal shifts associated with spring reproduction, we assessed differences in leukocyte profiles between seasonally sympatric resident and recently arrived fall migrant dark-eyed juncos (Junco hyemalis) in the Appalachian Mountains. When examining heterophil:lymphocyte (HL) ratios, which can elevate for long durations after even mild stressors, we found weak associations with migratory strategy (resident or migrant subspecies). In contrast, feather δ2H values showed that HL ratios were highest for juncos breeding at more northern latitudes, and this relationship was strongest for birds that arrived at the overwintering site infected with haemosporidian parasites (Plasmodium and Haemoproteus spp.). These patterns were more pronounced and better indicated hematological responses to stressors when using multivariate analyses. Our findings suggest that short- and long-distance migration may be more energetically costly in the presence of infection and highlight how approximating breeding latitude and using multivariate analyses can help understand host physiology.

Early-breeding females experience greater telomere loss.

Annual reproductive success is often highest in individuals that initiate breeding early, yet relatively few individuals start breeding during this apparently optimal time. This suggests that individuals, particularly females who ultimately dictate when offspring are born, incur costs by initiating reproduction early in the season. We hypothesized that increases in the ageing rate of somatic cells may be one such cost. Telomeres, the repetitive DNA sequences on the ends of chromosomes, may be good proxies of biological wear and tear as they shorten with age and in response to stress. Using historical data from a long-term study population of dark-eyed juncos (Junco hyemalis), we found that telomere loss between years was greater in earlier breeding females, regardless of chronological age. There was no relationship between telomere loss and the annual number of eggs laid or chicks that reached independence. However, telomere loss was greater when temperatures were cooler, and cooler temperatures generally occur early in the season. This suggests that environmental conditions could be the primary cause of accelerated telomere loss in early breeders.

Genome‐wide signals of drift and local adaptation during rapid lineage divergence in a songbird

The formation of independent evolutionary lineages involves neutral and selective factors, and understanding their relative roles in population divergence is a fundamental goal of speciation research. Correlations between allele frequencies and environmental variability can reveal the role of selection, yet the relative contribution of drift can be difficult to establish. Recently diversified taxa like the Oregon junco (Aves, Passerellidae, Junco hyemalis oreganus) of western North America provide ideal scenarios to apply genetic-environment association analyses (GEA) while controlling for population structure. Analysis of genome-wide SNP loci revealed marked genetic structure consisting of differentiated populations in isolated, dry southern mountain ranges, and less divergent, recently expanded populations in humid northern latitudes. We used correlations between genomic and environmental variance to test for three specific modes of evolutionary divergence: (a) drift in geographic isolation, (b) differentiation along continuous selective gradients and (c) isolation-by-adaptation. We found evidence of strong drift in southern mountains, but also signals of local adaptation driven by temperature, precipitation, elevation and vegetation, especially when controlling for population history. We identified numerous variants under selection scattered across the genome, suggesting that local adaptation can promote rapid differentiation when acting over multiple independent loci.

Male courtship preference during seasonal sympatry may maintain population divergence.

Animal migration can lead to a population distribution known as seasonal sympatry, in which closely‐related migrant and resident populations of the same species co‐occur in sympatry during part of the year, but are otherwise allopatric. During seasonal sympatry in early spring, residents may initiate reproduction before migrants depart, presenting an opportunity for gene flow. Differences in reproductive timing between migrant and resident populations may favor residents that exhibit preferences for potential mates of similar migratory behavior and reproductive timing, thus maintaining population divergence. We studied dark‐eyed juncos (Junco hyemalis), a songbird that exhibits seasonal sympatry. We conducted simulated courtship interactions in which we presented free‐living resident males with either a caged migrant or resident female and quantified courtship behavior prior to the departure of the migrants. We found that resident males preferred to court resident females: they sang more short‐range songs and exhibited more visual displays associated with courtship when presented with resident females. We conclude that males distinguish between migrant and resident females during seasonal sympatry when the risk of interacting with non‐reproductive, migrant females is high. Male mate choice in seasonal sympatry is likely adaptive for male reproductive success. As a secondary effect, male mating preference could act to maintain or promote divergence between populations that differ in migratory strategy.

Wax Ester Composition of Songbird Preen Oil Varies Seasonally and Differs between Sexes, Ages, and Populations.

Chemical signaling has been well studied in invertebrates and mammals but less so in birds, due to the longstanding misconception that olfaction is unimportant or even non-existent in this taxon. However, recent findings suggest that olfaction plays an important role in avian mate choice and reproductive behavior, similar to other taxa. The leading candidate source for compounds involved in avian chemical communication is preen oil, a complex mixture secreted from the uropygial gland. Preen oil contains volatile compounds and their potential wax ester precursors, and may act as a reproductive chemosignal. Reproductive signals are generally sexually dimorphic, age-specific, seasonally variable, and may also vary geographically. We tested whether preen oil meets these expectations by using gas chromatography to examine the wax ester composition of preen oil in song sparrows (Melospiza melodia). We found that the wax ester composition of preen oil was significantly different between sexes, age classes, seasons, and populations. Collectively, our results suggest that song sparrow preen oil meets the criteria of a chemical cue that may influence mate choice and reproduction. Our findings in song sparrows, which are sexually monomorphic in plumage, also parallel patterns described for dark-eyed juncos (Junco hyemalis), a closely related songbird with sexually dimorphic plumage. Behavioral tests are needed to confirm that song sparrows attend to the cues present in preen oil, but our findings support the increasingly accepted idea that chemical communication is common and widespread in birds as it is in other taxa.