Time of emergence of novel climates for North American migratory bird populations

To better understand the ecological implications of global climate change for species that display geographically and seasonally dynamic life‐history strategies, we need to determine where and when novel climates are projected to first emerge. Here, we use a multivariate approach to estimate time of emergence (ToE) of novel climates based on three climate variables (precipitation, minimum and maximum temperature) at a weekly temporal resolution within the Western Hemisphere over a 280‐yr period (2021–2300) under a high emissions scenario (RCP8.5). We intersect ToE estimates with weekly estimates of relative abundance for 77 passerine bird species that migrate between temperate breeding grounds in North America and southern tropical and subtropical wintering grounds using observations from the eBird citizen‐science database. During the non‐breeding season, migrants that winter within the tropics are projected to encounter novel climates during the second half of this century. Migrants that winter in the subtropics are projected to encounter novel climates during the first half of the next century. During the beginning of the breeding season, migrants on their temperate breeding grounds are projected to encounter novel climates during the first half of the next century. During the end of the breeding season, migrants are projected to encounter novel climates during the second half of this century. Thus, novel climates will first emerge ca 40–50 yr earlier during the second half of the breeding season. These results emphasize the large seasonal and spatial variation in the formation of novel climates, and the pronounced challenges migratory birds are likely to encounter during this century, especially on their tropical wintering grounds and during the transition from breeding to migration. When assessing the ecological implications of climate change, our findings emphasize the value of applying a full annual cycle perspective using standardized metrics that promote comparisons across space and time.     

Influence of climate change and postdelisting management on long‐term population viability of the conservation‐reliant Kirtland's Warbler

Rapid global climate change is resulting in novel abiotic and biotic conditions and interactions. Identifying management strategies that maximize probability of long‐term persistence requires an understanding of the vulnerability of species to environmental changes. We sought to quantify the vulnerability of Kirtland's Warbler (Setophaga kirtlandii), a rare Neotropical migratory songbird that breeds almost exclusively in the Lower Peninsula of Michigan and winters in the Bahamian Archipelago, to projected environmental changes on the breeding and wintering grounds. We developed a population‐level simulation model that incorporates the influence of annual environmental conditions on the breeding and wintering grounds, and parameterized the model using empirical relationships. We simulated independent and additive effects of reduced breeding grounds habitat quantity and quality, and wintering grounds habitat quality, on population viability. Our results indicated the Kirtland's Warbler population is stable under current environmental and management conditions. Reduced breeding grounds habitat quantity resulted in reductions of the stable population size, but did not cause extinction under the scenarios we examined. In contrast, projected large reductions in wintering grounds precipitation caused the population to decline, with risk of extinction magnified when breeding habitat quantity or quality also decreased. Our study indicates that probability of long‐term persistence for Kirtland's Warbler will depend on climate change impacts to wintering grounds habitat quality and contributes to the growing literature documenting the importance of considering the full annual cycle for understanding population dynamics of migratory species.     

Carry‐over effects provide linkages across the annual cycle of a Neotropical migratory bird,the Louisiana Waterthrush Parkesia motacilla

Population limitation models of migratory birds have sought to include impacts from events across the full annual cycle. Previous work has shown that events occurring in winter result in some individuals transitioning to the breeding grounds earlier or in better physical condition than others, thereby affecting reproductive success (carry‐over effects). However, evidence for carry‐over effects from breeding to wintering grounds has been shown less often. We used feather corticosterone (CORT_f) levels of the migratory Louisiana Waterthrush Parkesia motacilla as a measure of the physiological state of birds at the time of moult on the breeding territory to investigate whether carry‐over effects provide linkages across the annual cycle of this stream‐obligate bird. We show that birds arriving on wintering grounds with lower CORT_f scores, indicating reduced energetic challenges or stressors at the time of moult, occupied higher quality territories, and that these birds then achieved a better body condition during the overwinter period. Body condition, in turn, was important in determining whether adult birds returned the following winter, with birds in better condition returning at higher rates. Together these data suggest a carry‐over effect from the breeding grounds to the wintering grounds that is further extended with respect to annual return rates. Very few other studies have linked conditions during the previous breeding season with latent effects during the subsequent overwintering period or with annual survival. This study shows that the effects of variation in energetic challenges or stressors can potentially carry over from the natal stream and accumulate over more than one life‐history period before being manifested in reduced survival. This is of particular relevance to models of population limitation in migratory birds.     

Mapping migration in a songbird using high‐resolution genetic markers

Neotropic migratory birds are declining across the Western Hemisphere, but conservation efforts have been hampered by the inability to assess where migrants are most limited—the breeding grounds, migratory stopover sites or wintering areas. A major challenge has been the lack of an efficient, reliable and broadly applicable method for measuring the strength of migratory connections between populations across the annual cycle. Here, we show how high‐resolution genetic markers can be used to identify genetically distinct groups of a migratory bird, the Wilson's warbler (Cardellina pusilla), at fine enough spatial scales to facilitate assessing regional drivers of demographic trends. By screening 1626 samples using 96 highly divergent single nucleotide polymorphisms selected from a large pool of candidates (~450 000), we identify novel region‐specific migratory routes and timetables of migration along the Pacific Flyway. Our results illustrate that high‐resolution genetic markers are more reliable, precise and amenable to high throughput screening than previously described intrinsic marking techniques, making them broadly applicable to large‐scale monitoring and conservation of migratory organisms.     

Global change and the distributional dynamics of migratory bird populations wintering in Central America

Understanding the susceptibility of highly mobile taxa such as migratory birds to global change requires information on geographic patterns of occurrence across the annual cycle. Neotropical migrants that breed in North America and winter in Central America occur in high concentrations on their non‐breeding grounds where they spend the majority of the year and where habitat loss has been associated with population declines. Here, we use eBird data to model weekly patterns of abundance and occurrence for 21 forest passerine species that winter in Central America. We estimate species’ distributional dynamics across the annual cycle, which we use to determine how species are currently associated with public protected areas and projected changes in climate and land‐use. The effects of global change on the non‐breeding grounds is characterized by decreasing precipitation, especially during the summer, and the conversion of forest to cropland, grassland, or peri‐urban. The effects of global change on the breeding grounds are characterized by increasing winter precipitation, higher temperatures, and the conversion of forest to peri‐urban. During spring and autumn migration, species are projected to encounter higher temperatures, forests that have been converted to peri‐urban, and increased precipitation during spring migration. Based on current distributional dynamics, susceptibility to global change is characterized by the loss of forested habitats on the non‐breeding grounds, warming temperatures during migration and on the breeding grounds, and declining summer rainfall on the non‐breeding grounds. Public protected areas with low and medium protection status are more prevalent on the non‐breeding grounds, suggesting that management opportunities currently exist to mitigate near‐term non‐breeding habitat losses. These efforts would affect more individuals of more species during a longer period of the annual cycle, which may create additional opportunities for species to respond to changes in habitat or phenology that are likely to develop under climate change.     

Demographic variation in timing and intensity of feather molt in migratory Fork‐tailed Flycatchers (Tyrannus s. savana)

Understanding the annual cycle of migratory birds is imperative for evaluating the evolution of life‐history strategies and developing effective conservation strategies. Yet, we still know little about the annual cycle of migratory birds that breed at south‐temperate latitudes of South America. We aged, sexed, and determined the progression and intensity of body, remige, and rectrix molt of migratory Fork‐tailed Flycatchers (Tyrannus s. savana) at breeding sites in southern South America and at wintering sites in northern South America. Molt of both body and flight feathers occurred primarily during the winter. In early winter, a similar proportion of young and adult flycatchers molted remiges and rectrices, but remige molt intensity (number of remiges molting) was greater and primary molt progression (mean primary feather molting) more advanced in adults. In late winter, remige molt intensity and primary molt progression did not differ between age groups. We found no difference between males and females either in the proportion of individuals molting in winter or in the intensity or progress of remige molt. Our results suggest that the nominate subspecies of Fork‐tailed Flycatcher undergoes one complete, annual molt on the wintering grounds, and represents the first comprehensive evaluation of molt timing of a migratory New World flycatcher that overwinters in the tropics. Given that breeding, molt, and migration represent three key events in the annual cycle of migratory birds, knowledge of the timing of these events is the first step toward understanding the possible tradeoffs migratory birds face throughout the year.     

Survivorship across the annual cycle of a migratory passerine,the willow flycatcher

Annual survivorship in migratory birds is a product of survival across the different periods of the annual cycle (i.e. breeding, wintering, and migration), and may vary substantially among these periods. Determining which periods have the highest mortality, and thus are potentially limiting a population, is important especially for species of conservation concern. To estimate survival probabilities of the willow flycatcher Empidonax traillii in each of the different periods, we combined demographic data from a 10‐year breeding season study with that from a 5‐year wintering grounds study. Estimates of annual apparent survival for breeding and wintering periods were nearly identical (65–66%), as were estimates of monthly apparent survival for both breeding and wintering stationary periods (98–99%). Because flycatchers spend at least half the year on the wintering grounds, overall apparent survivorship was lower (88%) on the wintering grounds than on the breeding grounds (97%). The migratory period had the highest mortality rate, accounting for 62% of the estimated annual mortality even though it comprises only one quarter or less of the annual cycle. The migratory period in the willow flycatcher and many other neotropical migrants is poorly understood, and further research is needed to identify sources of mortality during this crucial period.     

Migratory patterns and connectivity of two North American grassland bird species

Effective management and conservation of migratory bird populations require knowledge and incorporation of their movement patterns and space use throughout the annual cycle. To investigate the little‐known migratory patterns of two grassland bird species, we deployed 180 light‐level geolocators on Grasshopper Sparrows (Ammodramus savannarum) and 29 Argos‐GPS tags on Eastern Meadowlarks (Sturnella magna) at Konza Prairie, Kansas, USA, and six US Department of Defense (DoD) installations distributed across the species' breeding ranges. We analyzed location data from 34 light‐level geolocators and five Argos‐GPS tags attached for 1 year to Grasshopper Sparrows and Eastern Meadowlarks, respectively. Grasshopper Sparrows were present on the breeding grounds from mid‐April through early October, substantially longer than previously estimated, and migrated on average ~2,500 km over ~30 days. Grasshopper Sparrows exhibited strong migratory connectivity only at a continental scale. The North American Great Lakes region likely serves as a migratory divide for Midwest and East Coast Grasshopper Sparrows; Midwest populations (Kansas, Wisconsin, and North Dakota; n = 13) largely wintered in Texas or Mexico, whereas East Coast populations (Maryland and Massachusetts, n = 20) wintered in the northern Caribbean or Florida. Our data from Eastern Meadowlarks provided evidence for a diversity of stationary and short‐ and long‐distance migration strategies. By providing the most extensive examination of the nonbreeding movement ecology for these two North American grassland bird species to date, we refine information gaps and provide key insight for their management and conservation.     

The large‐scale drivers of population declines in a long‐distance migratory shorebird

Migratory species can travel tens of thousands of kilometers each year, spending different parts of their annual cycle in geographically distinct locations. Understanding the drivers of population change is vital for conserving migratory species, yet the challenge of collecting data over entire geographic ranges has hindered attempts to identify the processes leading to observed population changes. Here, we use remotely sensed environmental data and bird count data to investigate the factors driving variability in abundance in two subspecies of a long‐distance migratory shorebird, the bar‐tailed godwit Limosa lapponica. We compiled a spatially and temporally explicit dataset of three environmental variables to identify the conditions experienced by each subspecies in each stage of their annual cycle (breeding, non‐breeding and staging). We used a Bayesian N‐mixture model to analyze 18 years of monthly count data from 21 sites across Australia and New Zealand in relation to the remote sensing data. We found that the abundance of one subspecies L. l. menzbieri in their non‐breeding range was related to climate conditions in breeding grounds, and detected sustained population declines between 1995 and 2012 in both subspecies (L. l. menzbieri, –6.7% and L. l. baueri, –2.1% year^–1). To investigate the possible causes of the declines, we quantified changes in habitat extent at 22 migratory staging sites in the Yellow Sea, East Asia, over a 25‐year period and found –1.7% and –1.2% year^–1 loss of habitat at staging sites used by L. l. menzbieri and L. l baueri, respectively. Our results highlight the need to identify environmental and anthropogenic drivers of population change across all stages of migration to allow the formulation of effective conservation strategies across entire migratory ranges.     

Complex migration and breeding strategies in an elusive bird species illuminated by genetic and isotopic markers

Unlike the annual bi‐directional movements of over 200 bird species within the Palaearctic–Afrotropical region, irregular movements such as irruptive migration with a low degree of philopatry are reported for a variety of species depending on highly seasonal and unpredictable resources. These flexible movements allow for itinerant breeding – consecutive breeding attempts in two or more geographically different regions during the same annual reproductive cycle. In order to illuminate migratory and breeding strategies of the erratic wetland species Baillon's crake Zapornia pusilla across the W‐Palaearctic–Afrotropical region, we used a set of six DNA microsatellites as well as δ²H_f values of individuals sampled at one African and four European breeding sites. We investigated the degree of genetic population structure within and among different sites and assigned individuals’ feathers of unknown origin to their probable moulting (hence breeding) site using a likelihood approach. We found three genetic clusters, differentiating into one ‘European’ and two ‘African’ populations. Connectivity between the sampling sites was probable as genetic ‘African’ individuals were found in breeding conditions in Europe and vice versa. Likewise, assigned moulting locations based on δ²H isoscapes suggested trans‐continental movements as well as moulting and possibly breeding by the same individual both in African and European breeding grounds. Both isotopic and genetic data reveal the Baillon's crake pursue a complex migration and breeding strategy, allowing as well for irruptive movements and itinerant breeding across the W‐Palaearctic–Afrotropical region. However, a better knowledge about the species’ distribution as well as a more comprehensive data set, including samples from the southern and eastern boundaries of the distribution area would be necessary to improve the spatial resolution to the precision required to unambiguously infer migration directions and extent of exchange between African and European breeding grounds.     

Connecting the dots: Stopover strategies of an intercontinental migratory songbird in the context of the annual cycle

The phases of the annual cycle for migratory species are inextricably linked. Yet, less than five percent of ecological studies examine seasonal interactions. In this study, we utilized stable hydrogen isotopes to geographically link individual black‐and‐white warblers (Mniotilta varia) captured during spring migration with breeding destinations to understand a migrant's stopover strategy in the context of other phases of the annual cycle. We found that stopover strategy is not only a function of a bird's current energetic state, but also the distance remaining to breeding destination and a bird's time‐schedule, which has previously been linked to habitat conditions experienced in the preceding phase of the annual cycle. Birds in close proximity to their breeding destination accumulate additional energy reserves prior to arrival on the breeding grounds, as reflected by higher migratory condition upon arrival, higher refueling rates measured via blood plasma metabolites, and longer stopover durations compared to birds migrating to breeding destinations farther from the stopover site. However, late birds near their breeding destination were more likely to depart on the day of arrival (i.e., transients), and among birds that stopped over at the site, the average duration of stopover was almost half the time of early conspecifics, suggesting late birds are trying to catch‐up with the overall time‐schedule of migration for optimal arrival time on the breeding grounds. In contrast, birds with long distances remaining to breeding destinations were more likely to depart on the day of arrival and primarily used stopover to rest before quickly resuming migration, adopting similar strategies regardless of a bird's time‐schedule. Our study demonstrates that migrants adjust their en route strategies in relation to their time‐schedule and distance remaining to their breeding destination, highlighting that strategies of migration should be examined in the context of other phases of the annual cycle.     

Forage plants of an Arctic‐nesting herbivore show larger warming response in breeding than wintering grounds,potentially disrupting migration phenology

During spring migration, herbivorous waterfowl breeding in the Arctic depend on peaks in the supply of nitrogen‐rich forage plants, following a “green wave” of grass growth along their flyway to fuel migration and reproduction. The effects of climate warming on forage plant growth are expected to be larger at the Arctic breeding grounds than in temperate wintering grounds, potentially disrupting this green wave and causing waterfowl to mistime their arrival on the breeding grounds. We studied the potential effect of climate warming on timing of food peaks along the migratory flyway of the Russian population of barnacle geese using a warming experiment with open‐top chambers. We measured the effect of 1.0–1.7°C experimental warming on forage plant biomass and nitrogen concentration at three sites along the migratory flyway (temperate wintering site, temperate spring stopover site, and Arctic breeding site) during 2 months for two consecutive years. We found that experimental warming increased biomass accumulation and sped up the decline in nitrogen concentration of forage plants at the Arctic breeding site but not at temperate wintering and stop‐over sites. Increasing spring temperatures in the Arctic will thus shorten the food peak of nitrogen‐rich forage at the breeding grounds. Our results further suggest an advance of the local food peak in the Arctic under 1–2°C climate warming, which will likely cause migrating geese to mistime their arrival at the breeding grounds, particularly considering the Arctic warms faster than the temperate regions. The combination of a shorter food peak and mistimed arrival is likely to decrease goose reproductive success under climate warming by reducing growth and survival of goslings after hatching.     

Contrasting consequences of climate change for migratory geese: Predation,density dependence and carryover effects offset benefits of high‐arctic warming

Climate change is most rapid in the Arctic, posing both benefits and challenges for migratory herbivores. However, population‐dynamic responses to climate change are generally difficult to predict, due to concurrent changes in other trophic levels. Migratory species are also exposed to contrasting climate trends and density regimes over the annual cycle. Thus, determining how climate change impacts their population dynamics requires an understanding of how weather directly or indirectly (through trophic interactions and carryover effects) affects reproduction and survival across migratory stages, while accounting for density dependence. Here, we analyse the overall implications of climate change for a local non‐hunted population of high‐arctic Svalbard barnacle geese, Branta leucopsis, using 28 years of individual‐based data. By identifying the main drivers of reproductive stages (egg production, hatching and fledging) and age‐specific survival rates, we quantify their impact on population growth. Recent climate change in Svalbard enhanced egg production and hatching success through positive effects of advanced spring onset (snow melt) and warmer summers (i.e. earlier vegetation green‐up) respectively. Contrastingly, there was a strong temporal decline in fledging probability due to increased local abundance of the Arctic fox, the main predator. While weather during the non‐breeding season influenced geese through a positive effect of temperature (UK wintering grounds) on adult survival and a positive carryover effect of rainfall (spring stopover site in Norway) on egg production, these covariates showed no temporal trends. However, density‐dependent effects occurred throughout the annual cycle, and the steadily increasing total flyway population size caused negative trends in overwinter survival and carryover effects on egg production. The combination of density‐dependent processes and direct and indirect climate change effects across life history stages appeared to stabilize local population size. Our study emphasizes the need for holistic approaches when studying population‐dynamic responses to global change in migratory species.     

Holding steady: Little change in intensity or timing of bird migration over the Gulf of Mexico

Quantifying the timing and intensity of migratory movements is imperative for understanding impacts of changing landscapes and climates on migratory bird populations. Billions of birds migrate in the Western Hemisphere, but accurately estimating the population size of one migratory species, let alone hundreds, presents numerous obstacles. Here, we quantify the timing, intensity, and distribution of bird migration through one of the largest migration corridors in the Western Hemisphere, the Gulf of Mexico (the Gulf). We further assess whether there have been changes in migration timing or intensity through the Gulf. To achieve this, we integrate citizen science (eBird) observations with 21 years of weather surveillance radar data (1995–2015). We predicted no change in migration timing and a decline in migration intensity across the time series. We estimate that an average of 2.1 billion birds pass through this region each spring en route to Nearctic breeding grounds. Annually, half of these individuals pass through the region in just 18 days, between April 19 and May 7. The western region of the Gulf showed a mean rate of passage 5.4 times higher than the central and eastern regions. We did not detect an overall change in the annual numbers of migrants (2007–2015) or the annual timing of peak migration (1995–2015). However, we found that the earliest seasonal movements through the region occurred significantly earlier over time (1.6 days decade^?1). Additionally, body mass and migration distance explained the magnitude of phenological changes, with the most rapid advances occurring with an assemblage of larger‐bodied shorter‐distance migrants. Our results provide baseline information that can be used to advance our understanding of the developing implications of climate change, urbanization, and energy development for migratory bird populations in North America.     

Broad wintering range and intercontinental migratory divide within a core population of the near‐threatened pallid harrier

Aim To identify the migration routes and wintering grounds of the core populations of the near‐threatened pallid harrier, Circus macrourus, and highlight conservation needs associated with these phases of the annual cycle. Location Breeding area: north‐central Kazakhstan; Wintering areas: Sahel belt (Burkina Faso to Ethiopia) and north‐west India. Methods We used ring recovery data from Kazakhstan and satellite tracking data from 2007 to 2008 on six adults breeding in north‐central Kazakhstan to determine migration routes and locate wintering areas. In addition, one first‐year male was tagged in winter 2007–2008 in India. Results Data evidenced an intercontinental migratory divide within the core pallid harrier population, with birds wintering in either Africa or India. The six individuals tagged in north‐central Kazakhstan followed a similar route (west of the Caspian Sea and Middle East) towards east Africa, before spreading along the Sahel belt to winter either in Sudan, Ethiopia, Niger or Burkina Faso. Spring migration followed a shorter, more direct route, with marked interindividual variation. The bird tagged in India spent the summer in central Kazakhstan. Half of the signal losses (either because of failure or bird mortality) occurred on the wintering areas and during migration. Main conclusions Our study shows that birds from one breeding area may winter over a strikingly broad range within and across continents. The intercontinental migratory divide of pallid harriers suggests the coexistence of distinct migratory strategies within the core breeding population, a characteristic most likely shared by a number of threatened species in central Asia. Conservation strategies for species like the pallid harrier, therefore, require considering very large spatial scales with possibly area‐specific conservation issues. We highlight urgent research priorities to effectively inform the conservation of these species.     

Climate and density influence annual survival and movement in a migratory songbird

Assessing the drivers of survival across the annual cycle is important for understanding when and how population limitation occurs in migratory animals. Density‐dependent population regulation can occur during breeding and nonbreeding periods, and large‐scale climate cycles can also affect survival throughout the annual cycle via their effects on local weather and vegetation productivity. Most studies of survival use mark–recapture techniques to estimate apparent survival, but true survival rates remain obscured due to unknown rates of permanent emigration. This is especially problematic when assessing annual survival of migratory birds, whose movement between breeding attempts, or breeding dispersal, can be substantial. We used a multistate approach to examine drivers of annual survival and one component of breeding dispersal (habitat‐specific movements) in a population of American redstarts (Setophaga ruticilla) over 11 years in two adjacent habitat types. Annual survival displayed a curvilinear relation to the Southern Oscillation Index, with lower survival during La Niña and El Niño conditions. Although redstart density had no impact on survival, habitat‐specific density influenced local movements between habitat types, with redstarts being less likely to disperse from their previous year's breeding habitat as density within that habitat increased. This finding was strongest in males and may be explained by conspecific attraction influencing settlement decisions. Survival was lowest in young males, but movement was highest in this group, indicating that apparent survival rates were likely biased low due to permanent emigration. Our findings demonstrate the utility of examining breeding dispersal in mark–recapture studies and complement recent work using spatially explicit models of dispersal probability to obtain greater accuracy in survival estimates.     

The pull of the Central Flyway? Veeries breeding in western Canada migrate using an ancestral eastern route

Understanding non‐breeding season movements and identifying wintering areas of different populations of migratory birds is important for establishing patterns of migratory connectivity over the annual cycle. We analyzed archival solar geolocation (N = 5) and global positioning data (N = 1) to investigate migration routes, stopover sites, and wintering areas of a western‐most breeding population of Veeries (Catharus fuscescens) in the Pemberton Valley, British Columbia, Canada. Geolocation data were analyzed using a Bayesian state‐space model to improve likely position estimates. We compared our results with those from a Veery population located ~250 km east across a mountain chain in the Okanagan Valley, British Columbia, and with an eastern population in Delaware, U.S.A. Migrating Veeries from the Pemberton Valley used an eastern trajectory through the Rocky Mountains to the Great Plains to join a central flyway during fall and spring migration, a route similar to that used by Veeries breeding in the Okanagan Valley. However, wintering destinations of Pemberton Valley birds were more varied, with inter‐individual wintering distances ~1000 km greater than birds from the Okanagan Valley population and ~500 km from the previously known winter range of Veeries. The observed eastern migration path likely follows an ancestral route that evolved following the most recent glacial retreat. Consistent with patterns observed from the Okanagan and Delaware populations, Veeries from the Pemberton Valley undertook an intra‐tropical migration on the wintering grounds, but this winter movement differed from those of previously studied populations. Such winter movements may thus be idiosyncratic or show coarse population associations. Intra‐wintering‐ground movements likely occur either in response to seasonal changes in habitat suitability or as a means of optimizing pre‐migratory fueling prior to long‐distance spring movements to North America.     

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.     

Tracking climate impacts on the migratory monarch butterfly

Understanding the impacts of climate on migratory species is complicated by the fact that these species travel through several climates that may be changing in diverse ways throughout their complete migratory cycle. Most studies are not designed to tease out the direct and indirect effects of climate at various stages along the migration route. We assess the impacts of spring and summer climate conditions on breeding monarch butterflies, a species that completes its annual migration cycle over several generations. No single, broad‐scale climate metric can explain summer breeding phenology or the substantial year‐to‐year fluctuations observed in population abundances. As such, we built a Poisson regression model to help explain annual arrival times and abundances in the Midwestern United States. We incorporated the climate conditions experienced both during a spring migration/breeding phase in Texas as well as during subsequent arrival and breeding during the main recruitment period in Ohio. Using data from a state‐wide butterfly monitoring network in Ohio, our results suggest that climate acts in conflicting ways during the spring and summer seasons. High spring precipitation in Texas is associated with the largest annual population growth in Ohio and the earliest arrival to the summer breeding ground, as are intermediate spring temperatures in Texas. On the other hand, the timing of monarch arrivals to the summer breeding grounds is not affected by climate conditions within Ohio. Once in Ohio for summer breeding, precipitation has minimal impacts on overall abundances, whereas warmer summer temperatures are generally associated with the highest expected abundances, yet this effect is mitigated by the average seasonal temperature of each location in that the warmest sites receive no benefit of above average summer temperatures. Our results highlight the complex relationship between climate and performance for a migrating species and suggest that attempts to understand how monarchs will be affected by future climate conditions will be challenging.     

Flexibility and constraints in the molt schedule of long‐distance migratory shorebirds: causes and consequences

Molt is a major component of the annual cycle of birds, the timing and extent of which can affect body condition, survival, and future reproductive success through carry‐over effects. The way in which molt is fitted into the annual cycle seems to be a somewhat neglected area which is both of interest and of importance. Study of the causes of annual variation in the timing of molt and its potential consequence in long‐distance migratory birds was examined using the Curlew Sandpiper, Calidris ferruginea, as a model species. Using the maximum likelihood molt models of Underhill and Zucchini (1988, Ibis 130:358–372), the relationship between annual variability in the start dates of molt at the population level with conditions on the breeding area was explored. Adult males typically started early in years when temperature in June on the Arctic breeding grounds were high compared to cold years while adult females molted later in years of high breeding success and/or warm July temperature and vice versa. When molt started later, the duration was often shorter, indicating that late completion of molt might have fitness consequences, probably jeopardizing survival. Evidence of this was seen in the low body condition of birds in years when molt was completed late. The results indicate that these migratory shorebirds follow a fine‐tuned annual life cycle, and disturbances at a certain stage can alter next biological events through carry‐over effects.