Setophaga Ruticilla Research Articles

Beyond tracking: Advancing animal ecology through the longitudinal sampling of individuals

Abstract Rarely, is there an opportunity to study and directly observe the same migratory animal across both breeding and nonbreeding seasons. In a rare, documented case, we highlight a live reencounter of a female Setophaga ruticilla (American Redstart) on its nest during the breeding season—an individual that was originally captured and tagged on its nonbreeding territory in Jamaica more than 3,250 km away. Advances in tracking technology allow researchers to track the movements of smaller organisms at higher resolutions. While tracking data alone have provided critical advances in the ecology of migratory animals, especially small migratory birds, there are exciting opportunities to move beyond tracking and to deeply explore biological questions about migratory animals by locating and directly studying (e.g., direct observation and physical sampling) the same individuals across the year. Although we have had the ability to precisely track large-bodied migratory birds (>100 g) for decades, researchers have rarely used these data to localize and directly observe or sample them across the year. Ultimately, we must consider the potential advances to be made in avian ecology through this endeavor. To this end, we highlight several specific areas of research that we believe move beyond tracking and can only be addressed by integrating direct longitudinal studies of individuals.

The role of tropical rainfall in driving range dynamics for a long-distance migratory bird.

Predicting how the range dynamics of migratory species will respond to climate change requires a mechanistic understanding of the factors that operate across the annual cycle to control the distribution and abundance of a species. Here, we use multiple lines of evidence to reveal that environmental conditions during the nonbreeding season influence range dynamics across the life cycle of a migratory songbird, the American redstart (Setophaga ruticilla). Using long-term data from the nonbreeding grounds and breeding origins estimated from stable hydrogen isotopes in tail feathers, we found that the relationship between annual survival and migration distance is mediated by precipitation, but only during dry years. A long-term drying trend throughout the Caribbean is associated with higher mortality for individuals from the northern portion of the species' breeding range, resulting in an approximate 500 km southward shift in breeding origins of this Jamaican population over the past 30 y. This shift in connectivity is mirrored by changes in the redstart's breeding distribution and abundance. These results demonstrate that the climatic effects on demographic processes originating during the tropical nonbreeding season are actively shaping range dynamics in a migratory bird.

Low-coverage whole genome sequencing for highly accurate population assignment: Mapping migratory connectivity in the American Redstart (Setophaga ruticilla).

Understanding the geographic linkages among populations across the annual cycle is an essential component for understanding the ecology and evolution of migratory species and for facilitating their effective conservation. While genetic markers have been widely applied to describe migratory connections, the rapid development of new sequencing methods, such as low-coverage whole genome sequencing (lcWGS), provides new opportunities for improved estimates of migratory connectivity. Here, we use lcWGS to identify fine-scale population structure in a widespread songbird, the American Redstart (Setophaga ruticilla), and accurately assign individuals to genetically distinct breeding populations. Assignment of individuals from the nonbreeding range reveals population-specific patterns of varying migratory connectivity. By combining migratory connectivity results with demographic analysis of population abundance and trends, we consider full annual cycle conservation strategies for preserving numbers of individuals and genetic diversity. Notably, we highlight the importance of the Northern Temperate-Greater Antilles migratory population as containing the largest proportion of individuals in the species. Finally, we highlight valuable considerations for other population assignment studies aimed at using lcWGS. Our results have broad implications for improving our understanding of the ecology and evolution of migratory species through conservation genomics approaches.

Migratory birds with delayed spring departure migrate faster but pay the costs.

Migratory birds that experience poor overwintering conditions are often late to arrive at the breeding grounds, which is known to depress individual fitness. Despite the importance of this carryover effect, few studies have investigated how individuals can modify migratory behaviors en route to reduce delays on arrival and whether accelerating migration incurs survival costs. To examine this, we used Motus Wildlife Tracking System to track individual American redstarts (Setophaga ruticilla) as they migrated from wintering grounds in Southwest Jamaica through Florida en route to their breeding areas. We leveraged long-term data on spring departure timing and breeding latitude to quantify the relative departure dates (early vs. delayed) of tagged individuals, which we then related to individual migration rates and apparent annual survival. Compared to those initiating migration earlier, individuals that departed relatively late (10-day delay) migrated at a 43% faster rate, which decreased their annual survival by 6.3%. Our results are consistent with the hypothesis that spring migrants use speed to compensate for departure delays despite incurring survival costs. This compensatory behavior may potentially underly differential survival during spring migration and may be particularly widespread across short-lived migratory birds generally considered time-constrained.

Winter Territoriality of the American Redstart in Oil Palm Plantations

Aspects of territorial behavior of Nearctic-neotropical migratory birds during the nonbreeding period are poorly studied. Information about territoriality, site persistence, between-year site fidelity, and territory sizes are not available for most birds, especially in tropical agroecosystems. Given the rapid expansion of oil palm in the neotropics, determining how oil palm affects the territorial behaviors of overwintering migratory birds is an important line of inquiry with conservation implications. The American Redstart (Setophaga ruticilla) is considered a model species for the study of population dynamics in the neotropics; however, territory size for American Redstart has only been assessed in native habitats. In this study, we outfitted individual redstarts with radio tags, across two winter seasons, to determine variation in territory sizes across oil palm plantations and native forest patches in the State of Tabasco, Mexico. Average redstart territory size was 0.29 ha in oil palm plantations and 0.17 ha in native forest. Albeit presenting larger territories in oil palm plantations, which could indicate poorer habitat quality, the difference between both habitats was not statistically significant. Our results demonstrate, for the first time, that American Redstarts hold territories in oil palm plantations and that territory size may serve as an important indicator of relative habitat quality for redstart populations in tropical working landscapes.

A tail of plumage colouration: disentangling geographic, seasonal and dietary effects on plumage colour in a migratory songbird

Plumage ornamentation in birds serves critical inter‐ and intra‐sexual signaling functions. While carotenoid‐based plumage colouration is often viewed as a classic condition‐dependent sexually selected trait, plumage colouration can be influenced by a wide array of both intrinsic and extrinsic factors. Understanding the mechanisms underlying variation in colouration is especially important for species where the signaling function of ornamental traits is complex or when the literature is conflicting. Here, we examined variation in the yellow/orange tail feathers of American redstarts Setophaga ruticilla passing through two migratory stopover sites in eastern North America during both spring and fall migration to assess the role of geographic variation and seasonality in influencing feather colouration. In addition, we investigated whether diet during moult (inferred via stable isotope analysis of feather δ15N and δ13C) influenced plumage colouration. Our findings indicate that geographic variation, season and diet all influence individual differences in American redstart colouration, represented by both traditional and tetrahedral colour variables. The extent to which these factors influence colour expression however is largely dependent on the colour metric under study, likely because different colour metrics reflect different attributes of the feather (e.g. structural components versus pigment deposition). The effects of diet (δ15N) and season were pronounced for brightness, suggesting a strong effect of diet and feather wear/degradation on feather structure. Though hue, a metric that should strongly reflect pigment deposition, also changed from spring to fall, that effect was dependent on age, with only adults experiencing a reduction in ornamentation. Taken together, our results highlight the numerous sources of variation behind plumage coloration and underscores the difficulty of unraveling complex visual signaling systems, such as those in American redstarts.

Flight call response is lower at dusk than during daytime in two parulid species

Many passerines regularly use flight calls to communicate during migration. However, flight-calling is inconsistent throughout the day, with a markedly lower degree of calling by free-flying individuals during the early evening hours and with peaks in calling typically around dawn. Based on these observations, we hypothesized that individual flight call response would vary by time of day, and the likelihood to respond would be lower at dusk than during the daytime. We tested daytime and dusk flight call responsiveness of captive American Redstarts (Setophaga ruticilla) and Magnolia Warblers (S. magnolia) during spring migration at the Braddock Bay Bird Observatory (near Rochester, New York, USA). We played a conspecific flight call sound stimulus for individual birds placed inside a soundproof recording studio and recorded their acoustic responses. In our experiment, American Redstarts and Magnolia Warblers were significantly less likely to give flight call responses at dusk than the individuals tested during the daytime. The almost absent responsiveness to flight calls at dusk suggests that these species either minimally communicate with one another during take-off or do not use flight calls as a source of communication at this time. These results have important implications for detection and quantitative bioacoustics migration monitoring. They also demonstrate the need for additional studies in temporal flight-calling behavior.

High resource overlap and small dietary differences are widespread in food‐limited warbler (Parulidae) communities

Although both interspecific competition and coexistence mechanisms are central to ecological and evolutionary theory, past empirical studies have generally focused on simple (two‐species) communities over short time periods. Experimental tests of these species interactions are challenging in complex study systems. Moreover, several studies of ‘imperfect generalists’, consistent with Liem's Paradox, raise questions about the ability of evolved species differences to partition niche space effectively when resources vary considerably across the annual cycle. Here we used a recently developed theoretical framework to combine past research on population‐level processes with observational data on resource use to test for ongoing interspecific competition and understand the nature of resource overlap. We compared species diet overlaps and differences in several distinctive communities centred on a focal species, the American Redstart Setophaga ruticilla replicated both spatially and seasonally, in combination with documentation of population regulation to assess the ability of similar species to partition dietary niche space and limit interspecific competition. Our results document high dietary overlap in most of the communities studied, with only subtle differentiation consistent with known species differences in foraging behaviour and morphology. These findings are largely consistent with species foraging as imperfect generalists. However, in contrast to past studies, the high diet overlaps observed here during times of inferred resource scarcity were driven by low‐value prey taxa (e.g. small ants) and did not involve truly ‘private’ resources. All of these factors increase the potential negative impacts of interspecific competition, and limit the ability of these birds to avoid competition if food availability deteriorates further than observed in our study, either seasonally or at longer intervals.

Strong differences in migratory connectivity patterns among species of Neotropical‐Nearctic migratory birds revealed by combining stable isotopes and abundance in a Bayesian assignment analysis

AbstractAimConservation planning for migratory species requires knowing how populations are connected throughout the annual cycle. We measured migratory connectivity for five migratory songbird species by combining stable‐hydrogen isotopes in feathers (δ2HF) with citizen science data on breeding range abundance.LocationCaribbean Basin and North America.TaxaNeotropical migratory birds: Mniotilta varia, Setophaga ruticilla, Seiurus aurocapilla, Setophaga americana, and Setophaga discolor.MethodsWe analyzed δ2HF in feathers grown on North American breeding grounds and sampled at 20 Caribbean Basin sites across most of the non‐breeding range of each species. We made Bayesian assignments to breeding origin by combining δ2HF values and North American Breeding Bird Survey abundance data. For each species, we used cluster analysis to group Caribbean sites into non‐breeding regions that shared birds with similar breeding ground assignment probabilities. We then mapped assignment probabilities for the non‐breeding regions of each species onto breeding ground δ2HF surfaces and estimated the strength of migratory connectivity.ResultsApart from some general similarities, the five species demonstrated different patterns of migratory connectivity. Species with larger non‐breeding ranges exhibited stronger migratory connectivity compared to those with smaller non‐breeding ranges. Despite geographical overlap, birds from each non‐breeding region had mostly unique breeding ground origins.Main conclusionsThe contrasting patterns of migratory connectivity shown by the five species suggest that geographic distribution during the non‐breeding period is a poor indicator of breeding ground origin. Our results indicate that full annual cycle conservation planning, which requires migratory connectivity information, should proceed on a species‐specific basis.

Two cases of a previously undocumented New World warbler hybrid (Setophaga magnolia × S. ruticilla) in eastern North America

This study confirms 2 independent accounts of previously undocumented hybridization between Magnolia Warbler (Setophaga magnolia) and American Redstart (S. ruticilla) through morphological and genetic analyses. Hybrids were discovered in 2 locations in eastern North America: Ottawa County, Ohio, USA, and Laval, Québec, Canada. The hybrids, one male and one female, showed plumage intermediate to both parental species. White eye arcs, yellow wash on the belly, narrow white wing bars, and black lores reflect the Magnolia Warbler parentage, while dark yellow-orange coloration on the sides of the breast and under the wing, yellow on the base of most of the secondaries, and pale yellow patches on the outer tail feathers reflect American Redstart. Sequences of the mitochondrial ND2 gene, the Z-linked muscle-specific kinase (MUSK) gene, and the myoglobin intron-2 (MYO2) confirmed maternal ancestry from Magnolia Warblers and paternal ancestry in American Redstarts for both hybrids. There is ample opportunity for this species pair to hybridize; they are congeners with broad range overlap, similar habitat preferences, song structure similarities, and plumage similarities in some age categories. Despite these conditions and the unusually high rate of hybridization within the Parulidae, this hybrid combination has never been documented before, and we suggest that this is a rare event that may be expected anywhere in their broad, overlapping ranges.

An inexpensive, 3D‐printable breast muscle meter for field ornithologists

The size of the pectoral muscle is an important component of body condition in birds and has been linked to indices of fitness and migratory performance. Bauchinger et al. (2011. Journal of Ornithology 152: 507–514) developed, calibrated, and validated an aluminum “muscle meter” device that estimates the size of pectoral muscles noninvasively. To make this tool more widely available, we created a CAD model from 3D‐scan data of the aluminum muscle meter that can be 3D‐printed in durable plastic for ~ $30 USD. We tested this device on seven species of songbirds in Jamaica, The Bahamas, Cameroon, Equatorial Guinea, and Michigan. We demonstrate that the breast muscle meter measurements are (1) repeatable among users, (2) correlated with a four‐category visual breast muscle scoring system, and (3) correlated with scaled mass index (an index of body condition). Muscle scores from our device outperformed the traditional four‐category muscle scoring system in predicting scaled mass index. Finally, with our device, we quantified the increasing breast muscle size of American Redstarts (Setophaga ruticilla) from March through May as they prepared for spring migration. Given the precision of the 3D‐scanning hardware used to generate our 3D image for printing, we produced a plastic muscle meter that is as precise and useful as the aluminum original, but more cost‐effective and widely available.

Interspecific competition between resident and wintering birds: experimental evidence and consequences of coexistence.

The contribution of interspecific competition to structuring population and community dynamics remains controversial and poorly tested. Interspecific competition has long been thought to influence the structure of migrant-resident bird communities in winter, yet experimental evidence remains elusive. The arrival of billions of songbirds into Neotropical habitats, where they co-exist with residents, provides a unique opportunity to assess interspecific competition and its consequences. Working in 15ha of Jamaican black mangrove forest, we used removal experiments to test whether dominant resident Yellow Warblers compete interspecifically with subordinate wintering American Redstarts; we also used observational evidence (interspecific territorial overlap) to understand whether this coexistence influences physical condition, spring departure dates or annual return rates. Consistent with interspecific competition, after experimental removal of the resident, yearling male Redstarts (but not females or adult males) immediately moved into vacated Yellow Warbler territories, increasing their overlap with the space by 7.3%. Yearling Redstarts also appeared to adjust their territorial space use by actively avoiding Yellow Warblers; for example, Redstarts departing the wintering grounds as yearlings and returning the following winter shifted such that their territories overlapped 32% less with those of Yellow Warblers. Adult Redstarts showed no such territorial flexibility. Adult male Redstarts also showed evidence supporting the consequences of coexistence: territorial overlap with Yellow Warblers was negatively correlated with body condition and annual return rates. Adult male Redstarts with <25% territorial overlap with Yellow Warblers were more than three times as likely to return between seasons than those with 100% overlap. We propose that the territorial inflexibility of adult male Redstarts produces these consequences, which may be due to their years-long investment in that particular territory. More generally, the temporary nature of migrant-resident interspecific competition is likely what allows coexistence during winter, the most resource-poor time of year. Interspecific competition and the consequences of coexistence are likely age- and sex-specific and the product of intraspecific dominance hierarchy in Redstarts. Our observations suggest that interspecific coexistence has measurable consequences, and our experiments support the long-held, but previously untested belief that resident birds compete interspecifically with wintering migrants.

Earlier and slower or later and faster: Spring migration pace linked to departure time in a Neotropical migrant songbird.

Migratory birds travel vast distances and the timing of migratory flights can affect survival and the ability to reproduce. For Neotropical migrant songbirds, early spring departure from wintering sites, early arrival to the breeding grounds and higher reproductive success have been related to the use of suitable habitats and environmental conditions during the non-breeding season. However, how migratory strategies are shaped by winter habitat choice is largely unknown due to the general inability to track birds from specific wintering habitats to stopovers or breeding destinations. We assessed how winter habitat (native forest vs. shade-grown coffee plantations) relates to spring departure date and migration pace in Swainson's Thrush Catharus ustulatus. We also determined the effect of departure date and total migration duration on the arrival date of birds detected near or within their breeding range. We used a novel application of Motus radiotelemetry arrays to track individuals from their wintering grounds in the Andes of South America along their migratory journey to North America. We found variation in migratory strategies between habitats, with birds wintering in native forest departing later than birds in coffee. We present isotopic evidence for native forest being of higher quality than shade-coffee for Swainson's Thrush and hypothesize that moister conditions in forest, as shown by stable isotope (δ13 C) analysis of thrush whole blood, provides favourable pre-migratory conditions allowing birds to delay departure from wintering grounds. Habitat, between-site and -year variation in departure date, suggests that birds made facultative adjustments to winter habitat quality and environmental conditions. Independent of habitat, birds that departed later migrated faster and this pattern was maintained along the migration route (n=44). Migrating earlier and slower or later and faster was unlikely to result in significant differences in arrival time to breeding destinations. Our findings reveal underappreciated complexity in migratory decisions by long-distance migrants that contrast with the current paradigm of earlier departures and arrival from optimal habitats. The next step is to understand the relative fitness benefits of early versus late schedules or whether each strategy is an equally good response to experienced conditions.

Predicting effects of selection cutting on avian diversity and abundance in shade-tolerant hardwood forests of central Ontario

Emulating natural disturbance (END) patterns to conserve biodiversity is often a goal of forest management but the impact of forest cutting is still controversial. We examined effects on avian assemblages of partially-cut forest stands of 1–10, 11–20 and > 45 years (cut classes) post-harvest differing in the % basal area retained as wildlife species’ habitat in 85 shade-tolerant hardwood stands in Algonquin Park, Ontario. Permutational multivariate analysis of variance indicated significant compositional differences among cut classes, after controlling for year and township (spatial variation in surficial geology and topography), both of which were also significant. Pairwise comparisons indicated significant compositional differences only between young and old stands. The difference in beta diversity (homogenization/differentiation) among cut classes was marginally significant (P = 0.066). Pairwise differences were significant between old versus mid-aged cut classes, marginally significant for young versus old cuts, and non-significant for young versus mid-aged cut classes. Six species and cavity users as a group were more abundant in > 45 years post-harvest stands than in the other two cut classes. Six species had higher abundances in younger cuts than > 45 years post-harvest cuts. Red-eyed vireo (Vireo olivaceus) showed a nonlinear response and was least abundant in 11–20 year cuts. Blackburnian warblers Setophaga fusca, black-throated green warblers S. virens and pileated woodpeckers Dryocopus pileatus increased with the density of supercanopy trees (≥ 60 cm diameter at breast height - dbh) and in the case of pileated woodpeckers more large snags (≥ 40 cm dbh). Brown creeper Certhia americana abundance increased with greater basal area of trees ≥ 25 cm dbh and ovenbird numbers were significantly related to overall basal area. Chestnut-sided warbler S. castanea abundance decreased with an increase in overall basal area and was higher in the 1–10 year cuts and American redstarts S. ruticilla decreased with an increase in basal area. Our modelling suggests that effects of selection cutting systems on the avian community may be reduced as tolerant hardwood stands become regulated and have higher residual basal areas. Current guidelines in Ontario for the retention of large, scattered conifer trees appear to meet the requirements for most, but not all, species; however, guidelines for supercanopy trees are too low. These results indicate that the END paradigm for single-tree selection cutting in tolerant hardwoods may be partially effective but requires modifications, such as retaining higher densities of supercanopy trees that would be beneficial for birds.

Can patterns of habitat use by western Nearctic–Neotropical migratory landbirds in winter inform conservation priorities?

I use point-count survey data collected from 171 locations across 11 vegetation conditions in western Mexico to illustrate common patterns of winter habitat use by 97 Nearctic–Neotropical migratory landbird species. A number of bird species are relatively restricted in their habitat use, with some (e.g., Northern Waterthrush [Parkesia noveboracensis], American Redstart [Setophaga ruticilla]) occurring only in relatively undisturbed habitats, and others (e.g., Say's Phoebe [Sayornis saya], Horned Lark [Eremophila alpestris]) occurring only in relatively disturbed lands associated with agriculture. A large number of bird species (e.g., Cassin's Vireo [Vireo cassinii], MacGillivray's Warbler [Geothlypis tolmiei]) use every one of the vegetation types considered, from low-elevation tropical deciduous forests to high-elevation conifer forests. Bird species showing patterns of restricted habitat use deserve conservation attention, but even the more broadly distributed species might become significantly less abundant in human-altered portions of habitats. Identifying the latter will require the inclusion of a wider spectrum of altered vegetation types/conditions than what I included here, or than what is typically considered in wildlife–habitat relationship programs.

Speciation Associated with Shifts in Migratory Behavior in an Avian Radiation.

Migratory animals move up to thousands of kilometers every year [1]. Losses of migration (i.e., migratory drop-offs) occur when individuals of a migratory species stop migrating and establish founder sedentary populations, a phenomenon documented in birds [2-5] and butterflies [6]. In theory, losses-and also gains-of migration might promote speciation if sedentary and migratory populations become reproductively isolated [7-9]. Because migratory and sedentary strategies involve alternative physiological, behavioral, and morphological traits [10-13], divergence along multiple axes of organismal function is expected to accompany switches in migratory behavior, potentially accelerating speciation. We present evidence of speciation driven by a migratory drop-off in the fork-tailed flycatcher (Tyrannus savana) resulting in reproductive isolation likely driven by changes in breeding schedules (allochronic speciation [13-15]) and geographic isolation of breeding grounds (allopatric speciation [16]). Phylogenetic analyses across New World flycatchers (Tyrannidae) showed that an association between speciation and drop-offs is also observable at a macroevolutionary scale. Loss of migration was significantly more frequent than its gain, and speciation rates of migratory and partially migratory lineages (i.e., species having both migratory and sedentary populations) exceeded those of sedentary lineages. Models of trait evolution indicated that partial migration is an intermediate step between migratory and sedentary states in this family. Given that partial migration is widespread across migratory animals (e.g., of all migratory birds, ca. 51% are partially migratory [5]), speciation via switches in migratory behavior might be an important yet overlooked mechanism of animal diversification.

Extra‐pair offspring are less heterozygous than within‐pair offspring in American redstarts Setophaga ruticilla

The vast majority of bird species are socially monogamous; however, extra‐pair paternity is nearly ubiquitous and a number of theories have been proposed to explain the prevalence of this mixed mating strategy. Here, we test the genetic compatibility hypothesis – the idea that females seek extra‐pair copulations with males whose genes are more compatible with her own. For this study, we examined eight years of paternity data (2004–2011) from a Nearctic‐Neotropical migratory bird, the American redstart Setophaga ruticilla, breeding in southeastern Ontario, Canada. We predicted that females paired with genetically similar males (higher relatedness) would be more likely to produce extra‐pair offspring and that extra‐pair offspring would have higher levels of heterozygosity than within‐pair offspring. Alternatively, because this population experiences high levels of immigration, females may produce extra‐pair offspring with more genetically similar males because of the potential for outbreeding depression. Using five highly variable microsatellite markers, we examined patterns of relatedness among social pairs as well as measures of offspring heterozygosity. In contrast to our predictions, we found no difference in relatedness between social pairs where the females produced extra‐pair offspring and social pairs where the females produced only within‐pair offspring. However, extra‐pair offspring were significantly less heterozygous than within‐pair offspring. Together, these findings suggest that females a) are not engaging in extra‐pair fertilizations based on relatedness to their social mate and b) appear to be mating with extra‐pair males that are more genetically similar to themselves. We suggest there may be benefits for females to mate with genetically similar extra‐pair males in highly outbred populations with high rates of immigration, such as for maintaining co‐adapted gene complexes or genes coding for local adaptations.

The strength of migratory connectivity for birds en route to breeding through the Gulf of Mexico

The strength of migratory connectivity is a measure of the cohesion of populations among phases of the annual cycle, including breeding, migration, and wintering. Many Nearctic‐Neotropical species have strong migratory connectivity between breeding and wintering phases of the annual cycle. It is less clear if this strength persists during migration when multiple endogenous and exogenous factors may decrease the cohesion of populations among routes or through time along the same routes. We sampled three bird species, American redstart Setophaga ruticilla, ovenbird Seiurus aurocapilla, and wood thrush Hylocichla mustelina, during spring migration through the Gulf of Mexico region to test if breeding populations differentiate spatially among migration routes or temporally along the same migration routes and the extent to which within‐population timing is a function of sex, age, and carry‐over from winter habitat, as measured by stable carbon isotope values in claws (δ13C). To make quantitative comparisons of migratory connectivity possible, we developed and used new methodology to estimate the strength of migratory connectivity (MC) from probabilistic origin assignments identified using stable hydrogen isotopes in feathers (δ2H). We found support for spatial differentiation among routes by American redstarts and ovenbirds and temporal differentiation along routes by American redstarts. After controlling for breeding origin, the timing of American redstart migration differed among ages and sexes and ovenbird migration timing was influenced by carry‐over from winter habitat. The strength of migratory connectivity did not differ among the three species, with each showing weak breeding‐to‐spring migration MC relative to prior assessments of breeding‐wintering connectivity. Our work begins to fill an essential gap in methodology and understanding of the extent to which populations remain together during migration, information critical for a full annual cycle perspective on the population dynamics and conservation of migratory animals.

American Redstarts (Setophaga ruticilla) usurp Cerulean Warbler (Setophaga cerulea) nest in Southern Indiana

ABSTRACT Documented observations of interspecific nest usurpation are rare and have been observed mostly in cavity-nesting species, where cavities are limiting and competition for them can be strong. We report one of the few known instances of interspecific nest usurpation in the family Parulidae (North American wood-warblers) between the Cerulean Warbler (Setophaga cerulea) and American Redstart (Setophaga ruticilla). During May and June 2016, we observed a female American Redstart usurp and successfully fledge at least one of its young from a nest built by a female Cerulean Warbler in Yellowwood State Forest, Brown County, Indiana. Based on our field observations, we provide information on nest chronology and describe American Redstart adult behaviors at the usurped nest. We hypothesize that this instance of interspecific nest usurpation may have been a result of high local breeding densities of both Cerulean Warblers and American Redstarts in our study area.

Evidence of negative seasonal carry‐over effects of breeding ground mercury exposure on survival of migratory songbirds

Mercury (Hg) is a well‐known global contaminant that persists in the environment. The organic form, methylmercury (MeHg) has been shown to adversely affect bird immune function, foraging behavior, navigation, and flight ability, which individually or together could reduce migration performance, and ultimately survival. Nestlings grow feathers at their natal site, and in North America many adult passerines undergo a complete feather molt prior to autumn migration at or near their breeding location. Body Hg is redistributed into growing feathers, and remains stable following feather growth. As flight feathers are retained in most species over the non‐breeding season until molt in the following summer, tail feathers can be used at other times and places as indicators of Hg body burden on the breeding grounds. In five migratory passerine species, we compared Hg concentrations in tail feathers that were grown prior to autumn migration and retained until the following spring. We predicted that we would observe a shift in the distribution of species‐specific feather Hg values towards lower means in the spring if Hg reduced survival over the migration and winter periods. We found reductions in mean feather Hg between autumn and spring in two long‐distance migratory insectivores (blackpoll warbler Setophaga striata; American redstart Setophaga ruticilla). Most significantly, spring‐returning blackpoll warblers, a species that undertakes long non‐stop flights to South America during autumn migration, had nearly 50 percent lower Hg concentrations than those that departed in the autumn. Our finding suggests that Hg exposure on the breeding areas could have a carry‐over effect to influence migration success and survival of insectivorous songbirds that undergo extensive and demanding migratory journeys. More investigation is needed to fully understand the relationships among Hg exposure, migration performance, and survival of songbirds.