Research Paper

INTRODUCTION

Migratory birds experience an extraordinary variety of habitats and climates across their complex annual cycles: from the breeding grounds, throughout the migratory route, to the non-breeding grounds. In addition to making these regular seasonal movements, many individuals also breed in different geographic regions from their natal origins or from previous breeding sites, a phenomenon known as dispersal (Greenwood and Harvey 1982). While migration in avian species is typically defined as cyclical trips to and from the breeding area (Salewski and Bruderer 2007), dispersal is a one-way movement an individual makes from either its natal origin to its first breeding site (natal dispersal) or between successive breeding sites (breeding dispersal; Clobert et al. 2012). Although movements from one geographically distinct breeding population to another is typically not as common as dispersal events across shorter distances (Paradis et al. 1998), long-distance dispersal is a major driver of many important ecological and evolutionary processes, including changes in genetic structure among populations (Monti et al. 2018), source-sink dynamics (Brawn and Robinson 1996), and shifting species ranges due to climate change (Travis et al. 2013, Arevall et al. 2018). Therefore, understanding long-distance dispersal patterns is especially important in making management decisions to conserve threatened species.

There are many apparent benefits of remaining faithful to the same breeding site, including a familiarity with the resource distribution (Rappole and Jones 2003) and social environment (Hansson et al. 2004) at that location. Individuals that have experienced the particular habitat and conspecific members at a breeding site may be at an advantage compared to immigrants that would be unfamiliar with the region. On the other hand, dispersal from natal or breeding sites may be driven by conditions experienced the previous year, such as intraspecific competition (Matthysen 2005), a lack of resources (Bowler and Benton 2005), reproductive failure (Schaub and Von Hirschheydt 2009), or presence of conspecifics (Betts et al. 2008). For females, failure to mate with high-quality males may also drive dispersal (Forero et al. 2002). Studies have reported that dispersers show reduced reproductive success compared to non-dispersers. For example, a study on Great Reed Warblers (Acrocephalus arundinaceus) discovered that philopatric males showed higher lifetime fitness compared to males that had dispersed (Hansson et al. 2004). In contrast, Rushing et al. (2016) found that American Redstart (Setophaga ruticilla) reproductive performance was not affected by dispersal, and dispersal distance in Tree Swallows (Tachycineta bicolor) did not impact subsequent breeding success (Shutler and Clark 2003). These contrasting findings on how dispersal impacts individual fecundity demonstrate a need for additional research on the reproductive consequences of dispersal events, especially for species of conservation concern.

Rates of long-distance dispersal in avian species often vary between juveniles and adults, with natal dispersal events typically occurring more frequently than breeding dispersal events (Greenwood and Harvey 1982, Hansson et al. 2002, Hobson et al. 2004). Several hypotheses attempt to explain why natal dispersal is usually more common than breeding dispersal in migratory birds. First, inexperienced, juvenile males returning from the non-breeding grounds may select their first breeding sites in regions in which there are other males, using presence of conspecifics as an indicator of habitat quality (Nocera et al. 2006, Betts et al. 2008). A second explanation for higher rates of natal dispersal is that experienced adults with established breeding sites may outcompete juveniles, who are forced to disperse from their natal area in search of available breeding sites (Greenwood and Harvey 1982). Natal dispersal may be especially common in regions where habitat availability is limited and, therefore, juvenile birds must travel farther in search of suitable habitat (Hansson et al. 2002). However, a few studies on songbirds have reported infrequent natal dispersal (Girvan et al. 2007, Haché et al. 2014) or longer dispersal distances of adults compared to juveniles (Dale et al. 2005). Therefore, the effects of age on long-distance dispersal patterns should continue to be investigated across species.

There are many challenges associated with studying dispersal of animals, particularly in tracking the movements of small songbirds. Until recent decades, following the annual movements of avian species was most commonly conducted through mark-recapture techniques, which are not reliable in differentiating between rates of mortality and dispersal and can underestimate rates of survival in populations (Jones et al. 2004). Recapturing marked birds is also reliant upon high site fidelity, a characteristic that most migratory species lack, as migrants typically disperse over longer distances compared to resident species (Paradis et al. 1998). Automated radio-tracking technology (Brown and Taylor 2015) and geolocators (Delancey et al. 2020) can allow researchers to follow avian movement across long distances, however, these tracking techniques can be limited by cost (Fiedler 2009) and the potential negative effects that these extrinsic markers may have on certain species (Raybuck et al. 2017). One of the most effective methods of tracking large-scale movements of small songbirds is by analyzing the stable-hydrogen isotope (δ²H) ratios found in their feathers (Hobson 2005). Many Neotropical migrants molt their flight feathers following each breeding season prior to migrating south to the non-breeding grounds (Pyle 1997, Pyle et al. 2018). When a bird undergoes molt, the new feathers retain unique isotopic signatures that are specific to the environment in which they were grown. In North America, these signatures vary spatially based on predictable growing-season precipitation patterns (δ²H_p). These geographic variations in precipitation δ²H values can be mapped in an “isoscape” from which the approximate origins of the feathers can then be assigned (Hobson et al. 2012). Therefore, researchers can infer the region where a feather was grown based on δ²H values found in that feather sample (δ²H_f) and can thus collect information on avian movement and dispersal (Hobson and Wassenaar 1996).

The Cerulean Warbler is a Nearctic-Neotropical migratory songbird that breeds in deciduous forests of eastern North America and spends the non-breeding season in the northern Andes mountains of South America. The species has experienced severe population declines over the last several decades, largely due to habitat loss across the breeding grounds, and is listed as “Near Threatened” by the IUCN (BirdLife International 2021), and “Endangered” in Canada (COSEWIC 2010) and the state of Indiana, USA (Indiana Department of Natural Resources 2020). Its dependence upon large, continuous tracts of mature forest in conjunction with deforestation and forest fragmentation is likely one of the main reasons for the Cerulean Warblers’ decline (Buehler et al. 2020) and, as a result, the species has a patchy distribution across eastern North America. Although long-distance dispersal in Cerulean Warblers has not been studied extensively, findings suggest that dispersal rates are high in this species (Veit et al. 2005, Girvan et al. 2007, Deane et al. 2013, Connare et al. 2020, Raybuck et al. 2022). Two studies examined the genetic structure of Cerulean Warblers across their breeding range and reported very minimal differences among populations, suggesting strong gene flow within this species (Veit et al. 2005, Deane et al. 2013). A study investigating the site fidelity of a population of Cerulean Warblers in southern Indiana discovered very low return rates of color-banded individuals (10.2%), and the authors proposed that long-distance dispersal may be common within this population (Connare et al. 2020). As climate change alters North American landscapes and threatens many migratory bird species, including the Cerulean Warbler (Culp et al. 2017), there is a need to improve our understanding of this species’ long-distance dispersal patterns throughout its breeding range to determine whether and how the Cerulean Warbler may adapt to projected changes in climate and land use.

A previous study, which sampled five geographically distinct populations of Cerulean Warblers across their breeding range using stable-hydrogen isotopes in feathers, found that breeding dispersal by after-second year (ASY) males occurred more frequently than natal dispersal of second year (SY) males, at a rate of 28.2% compared to 9.4% (Girvan et al. 2007). This finding, where natal dispersal is less common, contradicts many other studies of avian dispersal, in which dispersal of juvenile individuals occurs more frequently than that of adults (Greenwood and Harvey 1982, Hansson et al. 2002, Hobson et al. 2004). In contrast to the study by Girvan et al. (2007), Connare et al. (2020) did not find different return rates for juvenile and adult Cerulean Warblers in southern Indiana, which suggests that age may not affect dispersal in this population.

The objectives of this study were to determine long-distance dispersal rates of Cerulean Warblers in a population in southern Indiana and to investigate whether long-distance dispersal patterns differ based on age and nesting success in this species. Using a nine-year dataset of Cerulean Warbler banding and reproductive data, we used feather δ²H values to categorize each bird as either a disperser or a non-disperser based on its most likely region of origin. We focus solely on long-distance rather than short-distance dispersal, as analyses from this study estimating dispersal distances ranged from ~800–1100 km (Appendix 1 for estimated distances of Cerulean Warblers categorized as dispersers). We predicted that immigration rates into our study area would be high due to reported low philopatry at our study sites (Connare et al. 2020), as well as apparent frequent long-distance dispersal of this species across the breeding range (Girvan et al. 2007). We also predicted that juveniles would disperse more often than adults and that philopatric individuals would have higher nesting success compared to immigrants.

METHODS

Study sites

This study took place as part of the Hardwood Ecosystem Experiment (HEE) within Yellowwood and Morgan-Monroe state forests in south-central Indiana, USA, from May to July 2013–2021. The HEE is a 100-year collaborative study that has been investigating the impacts of forest management practices on the ecology of plants and wildlife in the Central Hardwood Forest since 2006 (Kalb and Mycroft 2013). Data for this study were collected within nine 225-ha HEE units.

Reproductive monitoring

Throughout the breeding season from 2013–2021, we intensively searched for and monitored Cerulean Warbler nests to quantify reproductive success. Each of the nine units was visited approximately once every 2–3 days to search for nests. Once a nest was located, we used a spotting scope to monitor it for 30 minutes once every 3 days. We documented the nest stage (building, laying, incubating, feeding, or fledge/fail) during each 30-minute observation period. The nestling period for Cerulean Warblers is typically 10–11 days (Buehler et al. 2020). We observed nests every day when the expected fledging date was within 3 days to increase the chances of viewing a fledging event and, therefore, more accurately determine whether the nest successfully fledged. If the nestlings fledged, we attempted to locate and count the total number of fledglings by following the parents feeding their young. A nest was considered to be successful if at least one nestling fledged.

Bird capture and sampling

To capture Cerulean Warblers for banding and tissue sampling, we set up a 6-meter mist net within a paired male’s territory and used a speaker to broadcast either male Cerulean Warbler songs, call notes, or fledgling calls to lure individuals into the net. Once captured, each bird was banded with a USGS aluminum band and a unique combination of three color bands so that we could identify individuals in the field with binoculars. We took feather samples from all adult Cerulean Warblers captured to infer molt origin locations, and, therefore, dispersal status, from stable-hydrogen isotope values. Sex was determined based on plumage and the presence of a cloacal protuberance and/or brood patch, and we classified each bird into one of two age groups based on molt limits: second year (SY) or after-second year (ASY; Pyle 1997). The second outermost tail feather (rectrix 5) was extracted from each bird and stored in a paper envelope. We did not collect any feathers after mid-July to ensure that all tail feathers had been grown the previous year. Our total sample size from 2013–2021 was 148 tail feathers from 132 males and 16 females (Table 1).

Stable isotope analysis

We sent feather samples to the Central Appalachians Stable Isotope Facility in Frostburg, Maryland, USA, for preparation and analysis. Samples were cleaned in a 1:200 Triton x-100 solution, rinsed in dH²O, soaked in ethanol, and left it to air dry in a fume hood at room temperature. A sizable portion of the hydrogen in feather samples may exchange with ambient water vapor in the environment and affect the total H isotopic composition (Chamberlain et al. 1997); therefore, samples were allowed to equilibrate with the laboratory atmosphere for at least three days prior to analysis to ensure that samples had adequate time to exchange with the local atmosphere (Soto et al. 2017). Next, a microbalance was used to weigh a portion of the tip of each feather sample into silver capsules, which were combusted and analyzed using a Thermo Scientific high temperature conversion analyzer (TC/EA) interfaced with a Thermo Delta V+ isotope ratio mass spectrometer (IRMS). Four standard keratin calibration reference materials (USGS 42, USGS 43, CBS, and KHS) had typical mean δ²H ± SD values of -72.3 ± 1.6‰ (n = 3), -44.2 ± 1.4‰ (n = 3), -158 ± 2‰ (n = 3), and -37.9 ± 1.9‰ (n = 3), respectively. One in-house quality control standard, powdered porcine keratin, was also used in each run. The long-term mean δ²H ± SD of the in-house standard is -57.4 ± 2.3‰ with a typical within-run SD of 1.4‰ (n = 8). Isotope ratios are presented in delta (δ) form relative to the Vienna Standard Mean Ocean Water (VSMOW) international reference scale.

Statistical analysis

All analyses were performed in R version 4.1.1 (R Core Team 2021). To estimate region of origin for each feather sample, we used R package “assignR”, which uses a continuous-surface assignment framework based on the relationship between a modeled isoscape and known-origin tissue to determine most probable regions of origin for individuals (Ma et al. 2020). To fit the linear relationship between δ²H_f and δ²H_p, we first regressed δ²H_f values from known-origin samples against δ²H_p values from a long-term growing-season precipitation isoscape (http://waterisotopes.org). This precipitation isoscape raster was clipped to the extent of the Cerulean Warblers’ breeding range of eastern North America.

We obtained known-origin samples from the “knownOrig” database within the “assignR” package. “Known-origin” refers to feathers that were collected from individuals that were known to have been at that same breeding site the previous year and, therefore, the isotopic values of those feathers can be assumed to represent that specific geographic location. This database includes stable-hydrogen isotope data from samples of known origin across various avian species and locations compiled from public and private sources. Due to the lack of known-origin δ²H_f data on Cerulean Warblers, we selected two avian passerine species, American Redstart (Setophaga ruticilla) and Golden-winged Warbler (Vermivora chrysoptera), whose migratory guild, foraging guild, and foraging substrate matched those of the Cerulean Warbler, as these variables have been shown to influence δ²H values in feathers (Hobson et al. 2012). We also chose these species based on their large sample sizes and wide geographic spread of sampling locations across the entire Cerulean Warbler breeding range to improve the fit of known-origin δ²H_f values with precipitation δ²H values. Consistent with our tissue sampling methodology, outer rectrices were used for the American Redstart and Golden-winged Warbler samples, which were collected between 1999–2009 (Hobson et al. 2012, Hobson and Koehler 2015). We also included nine Cerulean Warbler samples of known origin from our own study sites in southern Indiana for a total sample size of known δ²H_f values of n = 47 from 31 different locations (Fig. 1). Known-origin isotope values were transformed to the same reference scale (VSMOW) to account for differences in stable isotope analysis procedures used across the different studies (Magozzi et al. 2021).

Next, we regressed δ²H_f values of the known-origin samples against the δ²H_p values from the precipitation isoscape to transform the precipitation isoscape into a feather δ²H isoscape (Fig. 2). We then created posterior probability density maps for each unknown Cerulean Warbler to assign each individual a likely region of origin based on the fit of known-origin isotope values with precipitation isotope values (Appendix 2a–c for assignment maps of all individuals). Cell values in these posterior probability maps represent the probability that each cell reflects the true origin of the feather sample (Wunder 2010). In a comparison of four different methods of estimating dispersal status, López-Calderón et al. (2019) determined that using likely regions of origin to assign individuals as either immigrants or locals based on odds ratios resulted in the lowest rates of classification error. Therefore, we used this method to classify each Cerulean Warbler as either a disperser or non-disperser. We used the “qtlRaster” function to create binary rasters from the previously generated probability maps to include only the top 67% of the most likely cells representing regions of origin, resulting in an assignment region with 2:1 odds of being correct geographic origins versus incorrect. We chose to use a 2:1 odds ratio as done by several previous studies (Hobson et al. 2009, Procházka et al. 2013, Haché et al. 2014, Hobson et al. 2015) and based on findings that these odds were likely to correctly identify regions of feather origin (Chabot et al. 2012, Hobson et al. 2012). As a result, each individual bird had a raster layer (resolution = 0.08 x 0.08 degrees) with cells coded as 1 for most likely regions of origin, and all other cells outside the most likely region of origin coded as 0 (Fig. 3). We then used the “raster” package (Hijmans 2021) to extract these binary values at our study site GPS coordinates (39°10’N, 86°22’W) to assign each individual as either a disperser (0) or non-disperser (1) based on whether or not our study site location was within the individual’s most likely region of origin. We also performed a Chi-square (χ²) test to determine whether the proportion of birds assigned as dispersers differed from that expected based on chance given our 2:1 odds ratio (López-Calderón et al. 2019) and found a marginally significant χ² value (p = 0.08). Given the evidence from several previous studies (Chabot et al. 2012, Hobson et al. 2012) that these odds are likely to correctly identify regions of feather origin, and this nearly significant χ² value, we proceeded with using the 2:1 odds ratio to assign each unknown individual a dispersal status.

Several previous studies that used δ²H values in feathers to assign geographic origins of passerines applied a correction factor to isotope values from SY individuals to account for potential age-specific differences in hydrogen isotopes (Studds et al. 2012, Haché et al. 2014). However, Rushing et al. (2015) did not find any differences in δ²H_f for SY and ASY American Redstarts with known origins, and López-Calderón et al. (2019) did not find that δ²H_f varied between ages classes in Golden-winged Warblers. Therefore, due to the similarities in foraging, migratory, and habitat guilds between these two study species and Cerulean Warblers, we chose not to apply a correction factor to isotope values of SY birds.

One Cerulean Warbler individual had an outlier δ²H_f value of -97‰ and was assigned a prior probability region of far northern Canada, which is well outside the documented range of this species (Buehler et al. 2020). This individual was, therefore, removed from analyses, as this outlier isotope value was likely a result of individual variation, a difference in local hydrology, or lab error. Shapiro-Wilk tests were performed to determine normality of data. There was no effect of sex on δ²H_f (t = 0.39, p = 0.70) or dispersal status (β = 0.94, z = 1.53, p = 0.13), therefore, we combined males and females for all analyses to maximize sample size. We used a one-way ANOVA to test the effect of year on δ²H feather values for birds of unknown origin and followed up with a Tukey HSD post hoc test to determine which years had significantly different isotopic values. We created a linear mixed-effects model to test the effect of age on δ²H_f values, and a generalized linear mixed-effects model (GLMM) fit to a binomial distribution to test whether δ²H_f values affected nest success. A one-way ANOVA was used to determine whether δ²H values of known-origin Cerulean Warbler feathers at our study sites (local δ²H_f values) were significantly different from δ²H_f values of SY and ASY birds. We also conducted a one-way ANOVA to test whether local, known-origin Cerulean Warbler δ²H_f values differed between years. Given the smaller sample sizes in 2013, 2015, and 2016, we used Fisher’s exact test to test whether the number of dispersers differed significantly by year. We created a GLMM fit to a binomial distribution to test whether age affected dispersal, with dispersal status (dispersing = 0, non-dispersing = 1) as the response variable and age as the predictor. To determine whether non-dispersers had higher reproductive success than dispersers, we used a GLMM fit to a binomial distribution with dispersal status as the predictor variable and reproductive success (successful = 1, non-successful = 0) as the response variable. In all mixed-effects models, we included individual birds and sampling year as random effects to account for several birds being sampled more than once between years and to account for variations in δ²H_f values across years. An alpha value of 0.05 was used to test for significance in all statistical analyses.

RESULTS

Stable-hydrogen isotope patterns

Mean δ²H ± standard deviation of Cerulean Warbler feathers was -50.13 ± 8.07‰, and isotope values were normally distributed (W = 0.99, p = 0.57). Reproductive data were obtained for 85/147 individuals, and 55.3% (47/85) of these individuals successfully nested, while 44.7% (38/85) were unsuccessful. There was no effect of age class on nest success (β = -0.54, z = -1.05, p = 0.29). Feather δ²H values differed significantly across several years: birds in 2013 had lower isotopic values than those in 2014, 2015, 2016, 2018, 2019, and 2021, and birds in 2017 and 2020 had higher values than those in 2019 (Fig. 4). Feather δ²H values were not significantly affected by age class (t = -1.54, p = 0.16), and nest success was not significantly impacted by δ²H_f values (β = -0.02, z = -0.79, p = 0.43). Mean δ²H_f of known-origin Cerulean Warblers at our local study sites in southern Indiana was -45.95 ± 6.6‰. There were no significant differences in δ²H_f values of local Cerulean Warblers among sampling years (F_4,4 = 1.62, p = 0.32). There was a marginally significant difference (p < 0.1) in δ²H_f values among SY birds, ASY birds, and local known-origin Cerulean Warblers, with juveniles on average appearing to have more depleted isotopic values compared to the local δ²H_f values (Fig. 5).

Dispersal patterns

Overall, 26.5% (39/147) of Cerulean Warblers at our study sites in southern Indiana were classified as long-distance dispersers. The distribution of δ²H values of these dispersers was not normal (W = 0.83, p < 0.001) and skewed left, suggesting that the majority of immigrants had higher isotope values than the mean of known-origin Cerulean Warblers from our study sites (Fig. 6). Consistent with differences in δ²H_f among years, the number of individuals that had dispersed differed significantly by year (p = 0.007). Of the ASY birds at our study sites, 26.3% (30/114) were dispersers, and 27.3% (9/33) of SY birds were dispersers (Table 2). Out of birds that had nested successfully, 23.4% (11/47) were dispersers, and 34.2% (13/38) of individuals that had failed to nest were dispersers. There were no significant relationships between dispersal status and age (β = -0.35, z = -0.72, p = 0.47) or nest success (β = 0.54, z = 1.1, p = 0.28).

DISCUSSION

We found that 26.5% of Cerulean Warblers at our study sites in Indiana were long-distance dispersers, which is a similar, but slightly higher, rate compared to a study by Girvan et al. (2007) who reported a dispersal rate of 22.3% including both adults and juveniles. This rate of over one quarter of individuals originating from outside of our study area is quite high compared to dispersal rates of other similar Nearctic-Neotropical wood-warblers. In contrast, Rushing et al. (2015) found a long-distance dispersal rate of approximately 14% in a population of American Redstarts using comparable methods to this study. Our findings support previous reports that the breeding population of Cerulean Warblers in southern Indiana has low site fidelity compared to populations in other regions of the breeding range (Connare et al. 2020). While previous studies have been unable to distinguish whether this apparent low philopatry is due to factors such as mortality rates or short-distance dispersal, our findings indicate that long-distance dispersal is likely a very prominent cause for the low site fidelity of this population.

Both δ²H_f values and dispersal rates differed by year throughout this study, suggesting that long-distance dispersal of Cerulean Warblers into our study region in southern Indiana shifts over time. This variation could be due to yearly differences in large-scale environmental conditions experienced at some point during the annual cycle, such as during spring migration. For example, Rushing et al. (2015) found that the timing of spring phenology consistently impacted both natal and breeding dispersal of American Redstarts: During years when onset of spring phenology occurred earlier than normal, birds were more likely to disperse north. The authors hypothesized that this northward dispersal during years of early spring phenology was driven by individuals attempting to settle at breeding sites at higher latitudes that would more closely match the timing of raising nestlings with the peak of food resources. It is possible that timing of spring phenology caused the variations in the proportion of immigrants into our study sites and δ²H feather values during certain years; however, this hypothesis needs to be further tested in Cerulean Warblers. Another potential cause of the inter-annual differences in feather δ²H values may be variations in precipitation isotope values during the period of tissue growth. Feather isotope values were calibrated from long-term growing season average precipitation δ²H measurements, and deviations from mean rainfall δ²H values may consequently affect tissue isotope values. Vander Zanden et al. (2014), however, found that using short-term precipitation δ²H isoscapes representing only the period of tissue growth to model the relationship between rainfall and tissue isotope values did not increase accuracy of origin assignments for monarch butterflies (Danaus plexippus) or Great Reed Warblers (Coulton et al. 2009). Furthermore, we found that δ²H_f values of local known-origin Cerulean Warblers did not differ among sampling years. Therefore, these yearly differences in Cerulean Warbler δ²H_f values are likely driven more by inter-annual variations in rate and direction of dispersal of birds rather than short-term variations in precipitation δ²H values.

Our finding, that yearling birds had very similar dispersal rates compared to adults, contradicts previous reports that natal dispersal is typically more common than breeding dispersal in avian species (Greenwood and Harvey 1982, Hansson et al. 2002, Hobson et al. 2004, Rushing et al. 2015). Girvan et al. (2007) reported extremely low rates of long-distance dispersal in SY Cerulean Warblers across the breeding range; however, we found that the dispersal rates of adults (26%) and yearlings (27%) were nearly identical in our population in southern Indiana. This is supported by previous reports of similar rates of site fidelity between SY and ASY birds in this population (Connare et al. 2020). It is possible that breeding dispersal may be relatively common due to reported low reproductive success in this Cerulean Warbler population (Buehler et al. 2008), as long-distance dispersal has been associated with nesting failure in avian species (Harvey et al. 1979). Snowy Plovers (Charadrius nivosus), for example, were more likely to return to the same breeding site if they nested successfully the previous year and dispersed over longer distances after nest failure (Pearson and Colwell 2014). Furthermore, Hoover (2003) found that in a Nearctic-Neotropical migratory songbird, the Prothonotary Warbler (Protonotaria citrea), the probability of remaining faithful to a breeding site increased with the number of successful broods raised. If Cerulean Warblers in southern Indiana have low nest success compared to populations in other regions (Buehler et al. 2008), dispersal of adults may occur more frequently than expected in this population.

Although the proportions of SY and ASY Cerulean Warbler dispersers were very similar in this study, our finding, that younger birds appear to have more depleted δ²H_f values compared to isotopic values of known-origin birds originating from our study sites, supports our prediction that natal dispersal patterns may differ from breeding dispersal patterns. Specifically, these lower δ²H_f values suggest that juveniles may have originated from regions north of Indiana. Both Studds et al. (2008) and Rushing et al. (2015) discovered that natal dispersal in American Redstarts was mainly influenced by habitat quality occupied during the non-breeding season. Birds that spent their first winters in high-quality habitat were more likely to undertake shorter spring migrations and disperse south of their natal origins, while those that overwintered in low quality habitat had longer spring migrations and dispersed north of their natal regions. In contrast, dispersal of adult birds was less likely to be influenced by wintering conditions. While there have been very limited studies to date on carry-over effects from the non-breeding season in Cerulean Warblers (Boves et al. 2016, Jones 2022), it is possible that the difference we found between breeding and natal δ²H_f values is due to age-biased habitat use during the winter. Boves et al. (2016) found that corticosterone levels in Cerulean Warblers from the non-breeding grounds were more elevated in younger birds than in adults, suggesting that juveniles may be experiencing more stressful environmental conditions in the winter. Another possibility is that juvenile birds are gathering social information from cues such as conspecific male song and fledgling calls during the post-breeding season to determine suitable breeding sites (Nocera et al. 2006, Betts et al. 2008). Betts et al. (2008) found that in Black-throated Blue Warblers (Setophaga caerulescens), SY birds were more likely than ASY birds to use social information gathered after the previous breeding season to settle at breeding sites. Although this behavior has not yet been documented in Cerulean Warblers, inexperienced juvenile birds may depend more upon cues from conspecifics to determine where to breed. Additional studies, examining natal and breeding long-distance dispersal in this species, should also test whether the presence of social information or carry-over effects from the non-breeding season are potential causes of the difference in dispersal direction between juvenile and adult birds. While the sample size of SY birds was considerably lower than that of ASY birds in this study, future research on Cerulean Warbler dispersal that includes juveniles would provide further insight into the differences between natal and breeding dispersal in this species.

Nest success of Cerulean Warblers was not significantly impacted by long-distance dispersal in this study, although a higher proportion of unsuccessful nesters were classified as dispersers (34%) compared to individuals that had fledged offspring (23%). While some studies have found that dispersal can result in a decrease in reproductive performance (Pärt 1994, Hansson et al. 2004, Germain et al. 2017), there is no general consensus on this topic (Doligez and Pärt 2008). Some research, conversely, has shown that dispersal can result in increased fitness in certain avian species (Linkhart and Reynolds 2007). Our finding that immigrants into our study sites did not have significantly reduced nesting success compared to non-dispersers may be positive for this declining species, as long-distance dispersal in Cerulean Warblers is likely common enough that decreases in reproductive success due to dispersal events would be considerably detrimental to population growth.

Long-distance dispersal rates in this study are likely conservative estimates, as stable-hydrogen isotope values vary mostly along a latitudinal gradient, and movements along similar latitudes may have been less likely to be detected using these methods. The left-skewed distribution of δ²H_f values in individuals that had dispersed to our study sites suggests that the majority of immigrants originated from latitudes south of Indiana rather than from the north. This finding is not surprising considering that the relative abundance of Cerulean Warblers is higher in states south of our study site, such as Tennessee and Kentucky, compared to more northern populations in Michigan and Ontario, Canada (Breeding Bird Survey: https://www.mbr-pwrc.usgs.gov/). Regions that can support higher breeding densities and, therefore, contain more potential dispersers, are generally more likely to act as source populations than regions with lower breeding densities, according to source-sink theory (Pulliam 1988, Dias 1996). However, the high rate of immigration into the southern Indiana population combined with low fecundity of Cerulean Warblers within this region (Buehler et al. 2008) suggests that this population may serve as an ecological trap. On the other hand, high immigration rates have been shown to play a significant role in preventing populations from declining (Ward 2005, Schaub et al. 2013), which is promising for the persistence of Cerulean Warblers within this region.

An additional aspect of dispersal that has yet to be studied in many Neotropical migrants is differences between the sexes. Female songbirds are severely underrepresented in research on migratory birds, despite the common occurrence of sexual segregation found across these species’ annual cycles (Bennett et al. 2019). Dispersal in avian species is typically believed to be sex-biased, in which females will disperse more often due to the higher costs of leaving a familiar area for territory-defending males (Greenwood 1980). While we were not able to formally test for sex-biased differences in δ²H_f or dispersal rates due to our low sample size of females, the observed proportion of females categorized as immigrants (6/16, 38%) was notably higher than that of males (33/131, 25%). In general, population-level data on female Cerulean Warblers is largely lacking, although one previous study found that females showed some population genetic structure, suggesting that long-distance dispersal may in fact be sex-biased in this species (Deane et al. 2013). Female Cerulean Warblers are notoriously difficult to capture due to their secretive nature, inaccessible nests, and lack of responsiveness to conspecific playback (L. E. Jones, personal observation). However, it is critical for researchers to continue attempting to capture and collect data on females of this species to understand whether dispersal differs between the two sexes and how this may impact population dynamics.

When possible, assigning individuals to regions of origin using isotope values of known-origin tissue to model the relationship between precipitation δ²H values and feather δ²H values is done using known-origin samples from the same species as the individuals being assigned. Different avian species occupying the same geographic area are likely to have slightly different tissue isotope values due to interspecific variations in microhabitat use, diet, and metabolic processes (Hobson et al. 2004, Fraser 2011, Hobson 2011). Although it is possible that using known-origin samples from other similar species may have introduced some variance into our calibration function, including known-origin samples from multiple sites spanning the entirety of our study species’ breeding range ensured a higher sample size, as well as more accurate calibration in potential regions of origin other than our study site (Voigt and Lehnert 2019, Wunder and Norris 2019). Additionally, previous research has shown that passerine species that use the same migratory strategy and foraging substrate, such as Cerulean Warblers, American Redstarts, and Golden-winged Warblers, have very similar δ²H_f values (Hobson et al. 2012). Nevertheless, it would be ideal for future stable isotope studies on Cerulean Warblers to obtain same-species known-origin samples to improve the accuracy of assignment models for this species. Previous stable isotope studies have also incorporated breeding abundance data into prior probability models to improve the accuracy of origin assignments (Royle and Rubenstein 2004, Rushing et al. 2017). This technique may be beneficial in assigning regions of origin for Cerulean Warblers, as this species has a very patchy distribution across its breeding range and, therefore, some geographic regions may have a much lower probability of producing dispersers compared to others. Acquiring a more detailed picture of dispersal direction using same-species calibration samples and species density data would be beneficial for determining priority conservation regions for the Cerulean Warbler.

CONCLUSION

This is the first study to examine long-distance dispersal patterns of the Cerulean Warbler population in southern Indiana, and it is also the first to investigate how immigration events may impact reproductive success in this species. In addition, we provide a small sample of known-origin Cerulean Warbler stable-hydrogen isotope values from across several years, which may improve future isotope-based studies for this species. Disentangling the causes of the low site fidelity of a state-endangered bird, including rates of mortality, detection, and dispersal, is key to effectively managing for and conserving this population. In a species with a relatively wide geographic range but specific breeding habitat requirements, understanding and tracking the movements of individuals across the breeding range is also necessary to fulfill these conservation goals.

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