Integrating data types to estimate spatial patterns of avian migration across the Western Hemisphere.

For many avian species, spatial migration patterns remain largely undescribed, especially across hemispheric extents. Recent advancements in tracking technologies and high-resolution species distribution models (i.e., eBird Status and Trends products) provide new insights into migratory bird movements and offer a promising opportunity for integrating independent data sources to describe avian migration. Here, we present a three-stage modeling framework for estimating spatial patterns of avian migration. First, we integrate tracking and band re-encounter data to quantify migratory connectivity, defined as the relative proportions of individuals migrating between breeding and nonbreeding regions. Next, we use estimated connectivity proportions along with eBird occurrence probabilities to produce probabilistic least-cost path (LCP) indices. In a final step, we use generalized additive mixed models (GAMMs) both to evaluate the ability of LCP indices to accurately predict (i.e., as a covariate) observed locations derived from tracking and band re-encounter data sets versus pseudo-absence locations during migratory periods and to create a fully integrated (i.e., eBird occurrence, LCP, and tracking/band re-encounter data) spatial prediction index for mapping species-specific seasonal migrations. To illustrate this approach, we apply this framework to describe seasonal migrations of 12 bird species across the Western Hemisphere during pre- and postbreeding migratory periods (i.e., spring and fall, respectively). We found that including LCP indices with eBird occurrence in GAMMs generally improved the ability to accurately predict observed migratory locations compared to models with eBird occurrence alone. Using three performance metrics, the eBird + LCP model demonstrated equivalent or superior fit relative to the eBird-only model for 22 of 24 species-season GAMMs. In particular, the integrated index filled in spatial gaps for species with over-water movements and those that migrated over land where there were few eBird sightings and, thus, low predictive ability of eBird occurrence probabilities (e.g., Amazonian rainforest in South America). This methodology of combining individual-based seasonal movement data with temporally dynamic species distribution models provides a comprehensive approach to integrating multiple data types to describe broad-scale spatial patterns of animal movement. Further development and customization of this approach will continue to advance knowledge about the full annual cycle and conservation of migratory birds.

Early-life experience shapes patterns of senescence in a food-caching passerine.

For many species, breeding performance increases through early adulthood followed by declines later in life. Although patterns of age-specific decline have been shown to vary between individuals, the factors that lead to this individual variation in the intensity of reproductive senescence are yet to be fully understood. We investigated whether early-life social status influenced age-related trends in the breeding performance of male Canada jays (Perisoreus canadensis), year-round residents of North America's boreal and sub-alpine forests. Shortly after young become nutritionally independent, intra-brood dominance struggles lead to one juvenile (Dominant Juvenile) remaining on the natal territory after expelling its subordinate siblings (Ejectees). First, we show via radio tracking that in our declining range-edge population Ejectees either join an unrelated pair (67%), form a breeding pair with another bird (28%) or occupy a territory alone (5%). Second, using 39 years of breeding data, we demonstrate that Ejectee males advanced laying dates and increased the annual number of nestlings until 6 years of age before declining, whereas Dominant Juvenile males advanced laying dates until 11 years and increased annual number of nestlings until 12 years of age before declining. This study documents clear variation in ageing patterns between dominant and expelled young, with implications for the role of early-life experiences and phenotypic quality in determining patterns of ageing.

A mixed methodology for evaluating use of evidence in conservation planning.

Conservation practitioners widely recognize the importance of making decisions based on the best available evidence. However, the effectiveness of evidence use in conservation planning is rarely assessed, which limits opportunities to improve evidence-based practice. We devised a mixed methodology for empirically evaluating use of evidence that applies social science tools to systematically appraise what kinds of evidence are used in conservation planning, to what effect, and under what limitations. We applied our approach in a case study of the Nature Conservancy of Canada (NCC), a leading land conservation organization. We conducted qualitative and quantitative analyses of 65 NCC planning documents (n = 13 in-depth) to identify patterns in evidence use, and surveyed 35 conservation planners to examine experiences of and barriers to using evidence. Although claims in plans contained a wide range of evidence types, 26% of claims were not referenced or associated with an identifiable source. Lack of evidence use was particularly apparent in claims associated with direct threats, particularly those identified as low (71% coded as insufficient or lacking evidence) or medium (45%) threats. Survey participants described relying heavily on practitioner experience and highlighted capacity limitations and disciplinary gaps in expertise among planning teams as barriers to using evidence effectively. We found that although time-intensive, this approach yielded actionable recommendations for improving evidence use in NCC conservation plans. Similar mixed-method assessments may streamline the process by including interviews and refining the document analysis frames to target issues or sections of concern. We suggest our method provides an accessible and robust point of departure for conservation practitioners to evaluate whether the use of conservation planning reflects in-house standards and more broadly recognized best practices.

RESUMEN: Los practicantes de la conservación reconocen ampliamente la importancia de tomar decisiones con base en la mejor evidencia disponible. Sin embargo, pocas veces se evalúa la efectividad del uso de evidencias en la planeación de la conservación, lo que limita las oportunidades para mejorar la práctica basada en evidencias. Diseñamos una metodología mixta para evaluar empíricamente el uso de evidencias que aplica herramientas de las ciencias sociales para estimar cuáles son los tipos de evidencia que se usan en la planeación de la conservación, con cuál efecto y con cuáles limitaciones. Aplicamos nuestra estrategia a un estudio de caso de la Nature Conservancy of Canada (NCC), una organización puntera en la conservación del suelo. Realizamos el análisis cualitativo y cuantitativo de 65 documentos de planeación de NCC (n = 13 a fondo) para identificar patrones en el uso de evidencias y encuestamos a 35 planeadores de la conservación para examinar las barreras y las experiencias del uso de evidencias. Aunque las afirmaciones en los planes contenían una amplia gama de tipos de evidencia, el 26% de éstas no estuvo referenciado o asociado con una fuente identificable. La falta del uso de evidencias fue particularmente evidente en las afirmaciones asociadas con amenazas directas, particularmente aquellas identificadas como amenaza menor (71% codificado como insuficiente o falto de evidencias) o media (45%). Los participantes de la encuesta describieron una fuerte dependencia de la experiencia de los practicantes y resaltaron las limitaciones de capacidades y brechas disciplinarias en la experiencia entre los equipos de planeación como barreras para el uso efectivo de las evidencias. Descubrimos que, aunque lleva tiempo, esta estrategia produjo recomendaciones viables para mejorar el uso de evidencias en los planes de conservación de la NCC. Las evaluaciones similares de métodos mixtos pueden simplificar el proceso al incluir entrevistas y refinar los marcos de análisis documental para enfocarse en temas o secciones de interés. Sugerimos que nuestro método proporciona un punto de partida accesible y sólido para que los practicantes de la conservación evalúen si el uso de la planeación de la conservación refleja los estándares internos y las mejores prácticas reconocidas más ampliamente. Una Metodología Mixta para la Evaluación del Uso de Evidencias en la Planeación de la Conservación.

Effects of early-life exposure to sublethal levels of a common neonicotinoid insecticide on the orientation and migration of monarch butterflies (Danaus plexippus).

Migratory insects use a variety of innate mechanisms to determine their orientation and maintain correct bearing. For long-distance migrants, such as the monarch butterfly (Danaus plexippus), these journeys could be affected by exposure to environmental contaminants. Neonicotinoids are synthetic insecticides that work by affecting the nervous system of insects, resulting in impairment of their mobility, cognitive performance, and other physiological and behavioural functions. To examine how neonicotinoids might affect the ability of monarch butterflies to maintain a proper directional orientation on their ∼4000 km migration, we grew swamp milkweed (Asclepias incarnata) in soil that was either untreated (0 ng g-1: control) or mixed with low (15 ng g-1 of soil) or high (25 ng g-1 of soil) levels of the neonicotinoid clothianidin. Monarch caterpillars were raised on control or clothianidin-treated milkweed and, after pupation, either tested for orientation in a static flight simulator or radio-tracked in the wild during the autumn migration period. Despite clothianidin being detectable in milkweed tissue consumed by caterpillars, there was no evidence that clothianidin influenced the orientation, vector strength (i.e. concentration of direction data around the mean) or rate of travel of adult butterflies, nor was there evidence that morphological traits (i.e. mass and forewing length), testing time, wind speed or temperature impacted directionality. Although sample sizes for both flight simulator and radio-tracking tests were limited, our preliminary results suggest that clothianidin exposure during early caterpillar development does not affect the directed flight of adult migratory monarch butterflies or influence their orientation at the beginning of migration.

Early warning indicators of population collapse in a seasonal environment.

Recent studies have demonstrated that generic statistical signals derived from time series of population abundance and fitness-related traits of individuals can provide reliable indicators of impending shifts in population dynamics. However, how the seasonal timing of environmental stressors influences these early warning indicators is not well understood. The goal of this study was to experimentally assess whether the timing of stressors influences the production, detection and sensitivity of abundance- and trait-based early warning indicators derived from declining populations. In a multi-generation, season-specific habitat loss experiment, we exposed replicate populations of Drosophila melanogaster to one of two rates of chronic habitat loss (10% or 20% per generation) in either the breeding or the non-breeding period. We counted population abundance at the beginning of each season, and measured body mass and activity levels in a sample of individuals at the end of each generation. When habitat was lost during the breeding period, declining populations produced signals consistent with those documented in previous studies. Inclusion of trait-based indicators generally improved the detection of impending population collapse. However, when habitat was lost during the non-breeding period, the predictive capacity of these indicators was comparatively diminished. Our results have important implications for interpreting signals in the wild because they suggest that the production and detection of early warning indicators depends on the season in which stressors occur, and that this is likely related to the capacity of populations to respond numerically the following season.

Experimental field evidence shows milkweed contaminated with a common neonicotinoid decreases larval survival of monarch butterflies.

Neonicotinoid insecticides are the most widely used class of insecticides in the world and can have both lethal and sub-lethal effects on non-target organisms in agricultural areas. Monarch butterflies Danaus plexippus have experienced dramatic declines in recent decades and, given that a large proportion of milkweed on the landscape grows in agricultural areas, there is concern about the negative effects of neonicotinoids on this non-target insect. In the field, we exposed common milkweed Asclepias syriaca, an obligate host plant of monarch butterflies, to agriculturally realistic levels of clothianidin, a widely used neonicotinoid insecticide. We tested whether this treatment influenced the number of eggs laid and larval survival over 2 years. Milkweeds were transplanted into 60 experimental plots alongside a corn crop planted with a clothianidin seed coat and 60 control plots alongside an untreated corn crop. The number of eggs, larvae at each stage (first to fifth instar), and the presence of other arthropods were recorded weekly from June to the end of August and survival from egg to fifth instar was estimated using a Bayesian state-space statistical model. We counted more eggs in treated plots compared to control plots, suggesting a preference for treated milkweed. The number of plots with arthropods did not differ between treatments, but within treated plots, there was a greater decrease in the number of arthropods throughout the season. There was no evidence that monarchs selected plots with fewer arthropods for oviposition. Larval survival was lower in clothianidin-treated plots compared to control plots. Our results suggest milkweed near clothianidin-treated crops can reduce larval survival of monarch butterflies. While we provide some evidence that clothianidin could also act as an ecological trap for this species, further work is needed to identify additional components of fitness, including individual egg-laying rates and survival beyond the pupal stage. Our findings add to a growing body of evidence that neonicotinoids can negatively affect non-target organisms. ​.

Climate-driven carry-over effects negatively influence population growth rate in a food-caching boreal passerine.

Understanding how events throughout the annual cycle are linked is important for predicting variation in individual fitness, but whether and how carry-over effects scale up to influence population dynamics is poorly understood. Using 38 years of demographic data from Algonquin Provincial Park, Ontario, and a full annual cycle integrated population model, we examined the influence of environmental conditions and density on the population growth rate of Canada jays (Perisoreus canadensis), a resident boreal passerine that relies on perishable cached food for over-winter survival and late-winter breeding. Our results demonstrate that fall environmental variables, most notably the number of freeze-thaw events, carried over to influence late-winter fecundity, which, in turn, was the main vital rate driving population growth. These results are consistent with the hypothesis that warmer and more variable fall conditions accelerate the degradation of perishable stored food that is relied upon for successful reproduction. Future warming during the fall and winter may compromise the viability of cached food that requires consistent subzero temperatures for effective preservation, potentially exacerbating climate-driven carry-over effects that impact long-term population dynamics.

Simple signals indicate which period of the annual cycle drives declines in seasonal populations.

For declining wild populations, a critical aspect of effective conservation is understanding when and where the causes of decline occur. The primary drivers of decline in migratory and seasonal populations can often be attributed to a specific period of the year. However, generic, broadly applicable indicators of these season-specific drivers of population decline remain elusive. We used a multi-generation experiment to investigate whether habitat loss in either the breeding or non-breeding period generated distinct signatures of population decline. When breeding habitat was reduced, population size remained relatively stable for several generations, before declining precipitously. When non-breeding habitat was reduced, between-season variation in population counts increased relative to control populations, and non-breeding population size declined steadily. Changes in seasonal vital rates and other indicators were predicted by the season in which habitat loss treatment occurred. Per capita reproductive output increased when non-breeding habitat was reduced and decreased with breeding habitat reduction, whereas per capita non-breeding survival showed the opposite trends. Our results reveal how simple signals inherent in counts and demographics of declining populations can indicate which period of the annual cycle is driving declines.

A range-wide domino effect and resetting of the annual cycle in a migratory songbird.

Latitudinal differences in timing of breeding are well documented but how such differences carry over to influence timing of events in the annual cycle of migratory birds is not well understood. We examined geographical variation in timing of events throughout the year using light-level geolocator tracking data from 133 migratory tree swallows ( Tachycineta bicolor) originating from 12 North American breeding populations. A swallow's breeding latitude influenced timing of breeding, which then carried over to affect breeding ground departure. This resulted in subsequent effects on the arrival and departure schedules at autumn stopover locations and timing of arrival at non-breeding locations. This 'domino effect' between timing events was no longer apparent by the time individuals departed for spring migration. Our range-wide analysis demonstrates the lasting impact breeding latitude can have on migration schedules but also highlights how such timing relationships can reset when individuals reside at non-breeding sites for extended periods of time.

Radio-tracking reveals how wind and temperature influence the pace of daytime insect migration.

Insects represent the most diverse and functionally important group of flying migratory animals around the globe, yet their small size makes tracking even large migratory species challenging. We attached miniaturized radio transmitters (less than 300 mg) to monarch butterflies ( Danaus plexippus) and common green darner dragonflies ( Anax junius) and tracked their autumn migratory movements through southern Ontario, Canada and into the United States using an automated array of over 100 telemetry towers. The farthest estimated distance a monarch travelled in a single day was 143 km at a wind-assisted groundspeed of 31 km h-1 (8.7 m s-1) and the farthest estimated distance a green darner travelled in a single day was 122 km with a wind-assisted groundspeed of up to 77 km h-1 (21.5 m s-1). For both species, increased temperature and wind assistance positively influenced the pace of migration, but there was no effect of precipitation. While limitations to tracking such small animals remain, our approach and results represent a fundamental advance in understanding the natural history of insect migration and environmental factors that govern their movements.

Migratory monarchs that encounter resident monarchs show life-history differences and higher rates of parasite infection.

Environmental change induces some wildlife populations to shift from migratory to resident behaviours. Newly formed resident populations could influence the health and behaviour of remaining migrants. We investigated migrant-resident interactions among monarch butterflies and consequences for life history and parasitism. Eastern North American monarchs migrate annually to Mexico, but some now breed year-round on exotic milkweed in the southern US and experience high infection prevalence of protozoan parasites. Using stable isotopes (δ2 H, δ13 C) and cardenolide profiles to estimate natal origins, we show that migrant and resident monarchs overlap during fall and spring migration. Migrants at sites with residents were 13 times more likely to have infections and three times more likely to be reproductive (outside normal breeding season) compared to other migrants. Exotic milkweed might either attract migrants that are already infected or reproductive, or alternatively, induce these states. Increased migrant-resident interactions could affect monarch parasitism, migratory success and long-term conservation.

Hot temperatures during the dry season reduce survival of a resident tropical bird.

Understanding how climate change will shape species distributions in the future requires a functional understanding of the demographic responses of animals to their environment. For birds, most of our knowledge of how climate influences population vital rates stems from research in temperate environments, even though most of Earth's avian diversity is concentrated in the tropics. We evaluated effects of Southern Oscillation Index (SOI) and local temperature and rainfall at multiple temporal scales on sex-specific survival of a resident tropical bird, the rufous-and-white wren Thryophilus rufalbus, studied over 15 years in the dry forests of northwestern Costa Rica. We found that annual apparent survival of males was 8% higher than females, more variable over time, and responded more strongly to environmental variation than female survival, which did not vary strongly with SOI or local weather. For males, mean and maximum local temperatures were better predictors of survival than either rainfall or SOI, with high temperatures during the dry season and early wet season negatively influencing survival. These results suggest that, even for species adapted to hot environments, further temperature increases may threaten the persistence of local populations in the absence of distributional shifts.

Experimental evidence that density mediates negative frequency-dependent selection on aggression.

Aggression can be beneficial in competitive environments if aggressive individuals are more likely to access resources than non-aggressive individuals. However, variation in aggressive behaviour persists within populations, suggesting that high levels of aggression might not always be favoured. The goal of this study was to experimentally assess the effects of population density and phenotypic frequency on selection on aggression in a competitive environment. We compared survival of two strains of Drosophila melanogaster that differ in aggression across three density treatments and five frequency treatments (single strain groups, equal numbers of each strain and strains mixed at 3:1 and 1:3 ratios) during a period of limited resources. While there was no difference in survival across single-strain treatments, survival was strongly density dependent, with declining survival as density increased. Furthermore, at medium and high densities, there was evidence of negative frequency-dependent selection, where rare strains experienced greater survival than common strains. However, there was no evidence of negative frequency-dependent selection at low density. Our results indicate that the benefits of aggression during periods of limited resources can depend on the interaction between the phenotypic composition of populations and population density, both of which are mechanisms that could maintain variation in aggressive behaviours within natural populations.

Quantitative tools for implementing the new definition of significant portion of the range in the U.S. Endangered Species Act.

In 2014, the Fish and Wildlife Service (FWS) and National Marine Fisheries Service announced a new policy interpretation for the U.S. Endangered Species Act (ESA). According to the act, a species must be listed as threatened or endangered if it is determined to be threatened or endangered in a significant portion of its range (SPR). The 2014 policy seeks to provide consistency by establishing that a portion of the range should be considered significant if the associated individuals' "removal would cause the entire species to become endangered or threatened." We reviewed 20 quantitative techniques used to assess whether a portion of a species' range is significant according to the new guidance. Our assessments are based on the 3R criteria-redundancy (i.e., buffering from catastrophe), resiliency (i.e., ability to withstand stochasticity), and representation (i.e., ability to evolve)-that the FWS uses to determine if a species merits listing. We identified data needs for each quantitative technique and considered which methods could be implemented given the data limitations typical of rare species. We also identified proxies for the 3Rs that may be used with limited data. To assess potential data availability, we evaluated 7 example species by accessing data in their species status assessments, which document all the information used during a listing decision. In all species, an SPR could be evaluated with at least one metric for each of the 3Rs robustly or with substantial assumptions. Resiliency assessments appeared most constrained by limited data, and many species lacked information on connectivity between subpopulations, genetic variation, and spatial variability in vital rates. These data gaps will likely make SPR assessments for species with complex life histories or that cross national boundaries difficult. Although we reviewed techniques for the ESA, other countries require identification of significant areas and could benefit from this research.

Fear creates an Allee effect: experimental evidence from seasonal populations.

Allee effects driven by predation can play a strong role in the decline of small populations but are conventionally thought to occur when generalist predators target specific prey (i.e. type II functional response). However, aside from direct consumption, fear of predators could also increase vigilance and reduce time spent foraging as population size decreases, as has been observed in wild mammals living in social groups. To investigate the role of fear on fitness in relation to population density in a species with limited sociality, we exposed varying densities of Drosophila melanogaster to mantid predators either during an experimental breeding season or non-breeding season. The presence of mantids in either season decreased the reproductive performance of individuals but only at low breeding densities, providing evidence for an Allee effect. We then used our experimental results to parametrize a mathematical model to examine the population consequences of fear at low densities. Fear tended to destabilize population dynamics and increase the risk of extinction up to sevenfold. Our study provides unique experimental evidence that the indirect effects of the presence of predators can cause an Allee effect and has important consequences for our understanding of the dynamics of small populations.

Local density regulates migratory songbird reproductive success through effects on double-brooding and nest predation.

Knowledge of the density-dependent processes that regulate animal populations is key to understanding, predicting, and conserving populations. In migratory birds, density-dependence is most often studied during the breeding season, yet we still lack a robust understanding of the reproductive traits through which density influences individual reproductive success. We used 27-yr of detailed, individual-level productivity data from an island-breeding population of Savannah sparrows Passerculus sandwichensis to evaluate effects of local and total annual population density on female reproductive success. Local density (number of neighbors within 50 m of a female's nest) had stronger effects on the number of young fledged than did total annual population density. Females nesting in areas of high local density were more likely to suffer nest predation and less likely to initiate and fledge a second clutch, which led to fewer young fledged in a season. Fledging fewer young subsequently decreased the likelihood of a female recruiting offspring into the breeding population in a subsequent year. Collectively, these results provide insight into the scale and reproductive mechanisms mediating density-dependent reproductive success and fitness in songbirds.

Regional climate on the breeding grounds predicts variation in the natal origin of monarch butterflies overwintering in Mexico over 38 years.

Addressing population declines of migratory insects requires linking populations across different portions of the annual cycle and understanding the effects of variation in weather and climate on productivity, recruitment, and patterns of long-distance movement. We used stable H and C isotopes and geospatial modeling to estimate the natal origin of monarch butterflies (Danaus plexippus) in eastern North America using over 1000 monarchs collected over almost four decades at Mexican overwintering colonies. Multinomial regression was used to ascertain which climate-related factors best-predicted temporal variation in natal origin across six breeding regions. The region producing the largest proportion of overwintering monarchs was the US Midwest (mean annual proportion = 0.38; 95% CI: 0.36-0.41) followed by the north-central (0.17; 0.14-0.18), northeast (0.15; 0.11-0.16), northwest (0.12; 0.12-0.16), southwest (0.11; 0.08-0.12), and southeast (0.08; 0.07-0.11) regions. There was no evidence of directional shifts in the relative contributions of different natal regions over time, which suggests these regions are comprising the same relative proportion of the overwintering population in recent years as in the mid-1970s. Instead, interannual variation in the proportion of monarchs from each region covaried with climate, as measured by the Southern Oscillation Index and regional-specific daily maximum temperature and precipitation, which together likely dictate larval development rates and food plant condition. Our results provide the first robust long-term analysis of predictors of the natal origins of monarchs overwintering in Mexico. Conservation efforts on the breeding grounds focused on the Midwest region will likely have the greatest benefit to eastern North American migratory monarchs, but the population will likely remain sensitive to regional and stochastic weather patterns.

Time as tyrant: The minute, hour and day make a difference for corticosterone concentrations in wild nestlings.

The hypothalamic-pituitary-adrenal (HPA) axis has been studied extensively in adults, but the HPA axis in early life is not well characterized, and there is an immense amount of unexplained variation in glucocorticoid levels during early life, especially in wild animals. To characterize population-wide natural variation in early-life HPA axis function, we compared plasma corticosterone levels (at baseline and after 30min acute restraint-stress) from seven-day-old nestlings (n=123) from a free-living, marked population of Savannah sparrows (Passerculus sandwichensis). We found a surprising sensitivity of the HPA axis to timing of sample collection across time scales. Even within the accepted 3-min framework to collect baseline samples, time to collect blood had a significant effect on baseline corticosterone concentrations. Daily rhythms also influenced baseline levels, which increased significantly during the relatively short window of sample collection (1100 and 1600). On a broader timeframe, there was a strong effect of hatch date (over a 2month period) on HPA axis responsiveness, where nestlings hatched later in the breeding season had lower stress-induced corticosterone levels than those hatched earlier. The ecophysiological mechanisms and implications of these patterns warrant future investigation; meanwhile this study highlights the critical need to consider, and potentially restrict, time across scales when collecting blood samples from wild birds to assess stress physiology.

Experimental evidence for within- and cross-seasonal effects of fear on survival and reproduction.

Fear of predation can have non-lethal effects on individuals within a season but whether, and to what extent, these effects carry over into subsequent seasons is not known. Using a replicated seasonal population of the common fruit fly, Drosophila melanogaster, we examined both within- and cross-seasonal effects of fear on survival and reproductive output. Compared to controls, flies exposed to the scent of mantid (Tenodera sinensis) predators in the non-breeding season had 64% higher mortality, and lost 60% more mass by the end of the non-breeding season and, in the subsequent breeding season, produced 20% fewer offspring that weighed 9% less at maturity. Flies exposed to the scent of mantids in the breeding season did not produce fewer offspring, but their offspring developed faster and weighed less as adults compared to the controls. Our results demonstrate how effects of fear can be manifested both within and across seasons and emphasize the importance of understanding how events throughout the annual cycle influence individual success of animals living in seasonal environments.

Differential migration and the link between winter latitude, timing of migration, and breeding in a songbird.

Patterns of connectivity between breeding and wintering grounds can have important implications for individual fitness and population dynamics. Using light-level geolocators and stable hydrogen isotopes (δ(2)H) in feathers, we evaluated differential migration of Savannah sparrows (Passerculus sandwichensis) breeding on Kent Island in the Bay of Fundy, New Brunswick, Canada in relation to sex, age, and body size. Based on geolocators recovered from 38 individuals between 2012 and 2014, the winter distribution was centered in North Carolina (median latitude 34°, range 26°-41°), with males overwintering, on average, approximately 275 km further north than females. Based on analyses of tail feather samples collected from 106 individuals from the same population between 2008 and 2012, males and adults had more negative δ(2)H values than females and juveniles, respectively, providing additional evidence that males wintered north of females and that adults wintered north of juveniles. Winter latitude and δ(2)H values within each sex were not found to be related to body size. From geolocator data, males returned to the breeding grounds, on average, 14 days earlier than females. For males, there was some evidence that arrival date on the breeding grounds was negatively correlated with winter latitude and that individuals which arrived earlier tended to breed earlier. Thus, benefits for males of early arrival on the breeding grounds may have contributed to their wintering farther north than females. Social dominance may also have contributed to age and sex differences in winter latitude, whereby dominant males and adults forced subordinate females and juveniles further south.