Quantifying full phenological event distributions reveals simultaneous advances, temporal stability and delays in spring and autumn migration timing in long‐distance migratory birds

Phenological changes in key seasonally expressed life-history traits occurring across periods of climatic and environmental change can cause temporal mismatches between interacting species, and thereby impact population and community dynamics. However, studies quantifying long-term phenological changes have commonly only measured variation occurring in spring, measured as the first or mean dates on which focal traits or events were observed. Few studies have considered seasonally paired events spanning spring and autumn or tested the key assumption that single convenient metrics accurately capture entire event distributions. We used 60 years (1955-2014) of daily bird migration census data from Fair Isle, Scotland, to comprehensively quantify the degree to which the full distributions of spring and autumn migration timing of 13 species of long-distance migratory bird changed across a period of substantial climatic and environmental change. In most species, mean spring and autumn migration dates changed little. However, the early migration phase (≤10th percentile date) commonly got earlier, while the late migration phase (≥90th percentile date) commonly got later. Consequently, species' total migration durations typically lengthened across years. Spring and autumn migration phenologies were not consistently correlated within or between years within species and hence were not tightly coupled. Furthermore, different metrics quantifying different aspects of migration phenology within seasons were not strongly cross-correlated, meaning that no single metric adequately described the full pattern of phenological change. These analyses therefore reveal complex patterns of simultaneous advancement, temporal stability and delay in spring and autumn migration phenologies, altering species' life-history structures. Additionally, they demonstrate that this complexity is only revealed if multiple metrics encompassing entire seasonal event distributions, rather than single metrics, are used to quantify phenological change. Existing evidence of long-term phenological changes detected using only one or two metrics should consequently be interpreted cautiously because divergent changes occurring simultaneously could potentially have remained undetected.

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title: The Migration Ecology of Birds
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title: Challenging claims in the study of migratory birds and climate change
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title: Long-distance migration: evolution and determinants
abstract: Long distance migration has evolved in many organisms moving through different media and using various modes of locomotion and transport. Migration continues to evolve or become suppressed as shown by ongoing dynamic and rapid changes of migration patterns. This great evolutionary flexibility may seem surprising for such a complex attribute as migration. Even if migration in most cases has evolved basically as a strategy to maximise fitness in a seasonal environment, its occurrence and extent depend on a multitude of factors. We give a brief overview of different factors (e.g. physical, geographical, historical, ecological) likely to facilitate and/or constrain the evolution of long distance migration and discuss how they are likely to affect migration. The basic driving forces for migration are ecological and biogeographic factors like seasonality, spatiotemporal distributions of resources, habitats, predation and competition. The benefit of increased resource availability will be balanced by costs associated with the migratory process in terms of time (incl. losses of prior occupancy advantages), energy and mortality (incl. increased exposure to parasites). Furthermore, migration requires genetic instructions (allowing substantial room for learning in some of the traits) about timing, duration and distance of migration as well as about behavioural and physiological adaptations (fuelling, organ flexibility, locomotion, use of environmental transport etc) and control of orientation and navigation. To what degree these costs and requirements put constraints on migration often depends on body size according to different scaling relationships. From this expos it is clear that research on migration warrants a multitude of techniques and approaches for a complete as possible understanding of a very complex evolutionary syndrome. In addition, we also present examples of migratory distances in a variety of taxons. In recent years new techniques, especially satellite radio telemetry, provide new information of unprecedented accuracy about journeys of individual animals, allowing re-evaluation of migration, locomotion and navigation theories.

title: Travelling through a warming world: climate change and migratory species.
abstract: Long-distance migrations are among the wonders of the natural world, but this multi- taxon review shows that the characteristics of species that undertake such movements appear to make them particularly vulnerable to detrimental impacts of climate change. Migrants are key com- ponents of biological systems in high latitude regions, where the speed and magnitude of climate change impacts are greatest. They also rely on highly productive seasonal habitats, including wet- lands and ocean upwellings that, with climate change, may become less food-rich and predictable in space and time. While migrants are adapted to adjust their behaviour with annual changes in the weather, the decoupling of climatic variables between geographically separate breeding and non- breeding grounds is beginning to result in mistimed migration. Furthermore, human land-use and activity patterns will constrain the ability of many species to modify their migratory routes and may increase the stress induced by climate change. Adapting conservation strategies for migrants in the light of climate change will require substantial shifts in site designation policies, flexibility of management strategies and the integration of forward planning for both people and wildlife. While adaptation to changes may be feasible for some terrestrial systems, wildlife in the marine ecosystem may be more dependent on the degree of climate change mitigation that is achievable.

title: Population limitation in migrants
abstract: Unlike resident bird species, the population sizes of migratory species can be influenced by conditions in more than one part of the world. Changes in the numbers of migrant birds, either long-term or year-to-year, may be caused by changes in conditions in the breeding or wintering areas or both. The strongest driver of numerical change is provided in whichever area the per capita effects of adverse factors on survival or fecundity are greatest. Examples are given of some species whose numbers have changed in association with conditions in breeding areas, and of others whose numbers have changed in association with conditions in wintering areas. In a few such species, the effects of potential limiting factors have been confirmed locally by experiment. In theory, population sizes might also be limited by severe competition at restricted stopover sites, where bird densities are often high and food supplies heavily depleted, but (with one striking exception) the evidence is as yet no more than suggestive. In some species, habitats occupied in wintering and migration areas, and their associated food supplies, can influence the body condition, migration dates and subsequent breeding success of migrants. Body reserves accumulated in spring by large waterfowl serve for migration and for subsequent breeding, and females with the largest reserves are most likely to produce young. Hence, the conditions experienced by individuals in winter in one region can affect their subsequent breeding success in another region. Such effects are apparent at the level of the individual and at the level of the population. Similarly, the numbers of young produced in one region could, through density-dependent processes, affect subsequent overall mortality in another region. Events in breeding, migration and wintering areas are thus interlinked in their effects on bird numbers. Although in the last 30–40 years the numbers of some tropical wintering birds have declined in western Europe and others in eastern North America, the causes seem to differ. In Europe, declines have mainly involved species that winter in the arid savannas of tropical Africa, which have suffered from the effects of drought and increasing desertification. In several species, annual fluctuations in numbers and adult survival rates were correlated with annual fluctuations in rainfall, and by implication in winter food supplies. In North America, by contrast, numerical declines have affected many species that breed and winter in forest, especially those eastern species favouring the forest interior. Declines have been attributed ultimately to human-induced changes in the breeding range, particularly forest fragmentation, which have led to increases in the densities of nest predators and parasitic cowbirds. These in turn are thought to have caused declines in the breeding success of some neotropical migrants, which is now too low to offset the usual adult mortality, but as yet convincing evidence is available for only a minority of species. The breeding rates and population changes of some migratory species have been influenced by natural changes in the availability of defoliating caterpillars. In other species, tropical deforestation is likely to have played the major role in population decline, and if recent rates of tropical deforestation continue, it is likely to affect an increasing range of migratory species in the future. Not all such species are likely to be affected adversely by deforestation, however, and some may benefit from the resulting habitat changes.

title: Animal migration: is there a common migratory syndrome?
abstract: Ornithologists, and especially northern hemisphere ornithologists, have traditionally thought of migration as an annual return movement of populations between regular breeding and non-breeding grounds. Problems arise because selection does not ordinarily act on populations and because organisms of many taxa (including birds) are clearly migrants, but fail to undertake movements of the kind described. There are also extensive return movements that are not migratory. I propose that it is more useful to think of migration as a syndrome of behavioral and other traits that function together within individuals, and that such a syndrome provides a common ground across taxa from aphids to albatrosses. Large-scale return movements of populations are one outcome of the syndrome. Similar behavioral and physiological traits serve both to define migration and to provide a test for it. I use two insect (Hemipteran) examples to illustrate migratory syndromes and to demonstrate that, in many migrants, behavior and physiology correlate with life history and morphological traits to form syndromes at two levels. I then compare the two Hemipterans with migration in birds, butterflies, and fish to assess the question of whether there are migratory syndromes in common between these diverse migrants. Syndromes are more similar at the level of behavior than when morphology and life history traits are included. Recognizing syndromes leads to important evolutionary questions concerning migration strategies, trade-offs, the maintenance of genetic variance and the responses of migratory syndromes to both similar and different selective regimes.

title: Avian migration phenology and global climate change
abstract: There is mounting evidence that global climate change has extended growing seasons, changed distribution patterns, and altered the phenology of flowering, breeding, and migration. For migratory birds, the timing of arrival on breeding territories and over-wintering grounds is a key determinant of reproductive success, survivorship, and fitness. But we know little of the factors controlling earlier passage in long-distance migrants. Over the past 30 years in Oxfordshire, U.K., the average arrival and departure dates of 20 migrant bird species have both advanced by 8 days; consequently, the overall residence time in Oxfordshire has remained unchanged. The timing of arrival has advanced in relation to increasing winter temperatures in sub-Saharan Africa, whereas the timing of departure has advanced after elevated summer temperatures in Oxfordshire. This finding demonstrates that migratory phenology is quite likely to be affected by global climate change and links events in tropical winter quarters with those in temperate breeding areas.

title: Land–atmosphere coupling and climate change in Europe
abstract: Increasing greenhouse gas concentrations are expected to enhance the interannual variability of summer climate in Europe and other mid-latitude regions, potentially causing more frequent heatwaves. Climate models consistently predict an increase in the variability of summer temperatures in these areas, but the underlying mechanisms responsible for this increase remain uncertain. Here we explore these mechanisms using regional simulations of recent and future climatic conditions with and without land–atmosphere interactions. Our results indicate that the increase in summer temperature variability predicted in central and eastern Europe is mainly due to feedbacks between the land surface and the atmosphere. Furthermore, they suggest that land–atmosphere interactions increase climate variability in this region because climatic regimes in Europe shift northwards in response to increasing greenhouse gas concentrations, creating a new transitional climate zone with strong land–atmosphere coupling in central and eastern Europe. These findings emphasize the importance of soil-moisture–temperature feedbacks (in addition to soil-moisture–precipitation feedbacks) in influencing summer climate variability and the potential migration of climate zones with strong land–atmosphere coupling as a consequence of global warming. This highlights the crucial role of land–atmosphere interactions in future climate change.

title: Climate change threatens European conservation areas
abstract: Europe has the world's most extensive network of conservation areas. Conservation areas are selected without taking into account the effects of climate change. How effectively would such areas conserve biodiversity under climate change? We assess the effectiveness of protected areas and the Natura 2000 network in conserving a large proportion of European plant and terrestrial vertebrate species under climate change. We found that by 2080, 58 ± 2.6% of the species would lose suitable climate in protected areas, whereas losses affected 63 ± 2.1% of the species of European concern occurring in Natura 2000 areas. Protected areas are expected to retain climatic suitability for species better than unprotected areas (P<0.001), but Natura 2000 areas retain climate suitability for species no better and sometimes less effectively than unprotected areas. The risk is high that ongoing efforts to conserve Europe's biodiversity are jeopardized by climate change. New policies are required to avert this risk.

title: Rapid Advance of Spring Arrival Dates in Long-Distance Migratory Birds
abstract: Several bird species have advanced the timing of their spring migration in response to recent climate change. European short-distance migrants, wintering in temperate areas, have been assumed to be more affected by change in the European climate than long-distance migrants wintering in the tropics. However, we show that long-distance migrants have advanced their spring arrival in Scandinavia more than short-distance migrants. By analyzing a long-term data set from southern Italy, we show that long-distance migrants also pass through the Mediterranean region earlier. We argue that this may reflect a climate-driven evolutionary change in the timing of spring migration.

title: Spring arrival response to climate change in birds: a case study from eastern Europe
abstract: This paper analyses the dependence of the first spring arrival dates of short/medium- and long-distance migrant bird species on climate warming in eastern Europe. The timing of arrival of the selected species at the observation site correlates with the North Atlantic Oscillation (NAO) index, air temperature, atmospheric pressure, precipitation and wind characteristics. A positive correlation of fluctuations in winter and spring air temperatures with variations in the NAO index has been established in eastern Europe. Positive winter NAO index values are related to earlier spring arrival of birds in the eastern Baltic region and vice versa—arrival is late when the NAO index is negative. The impact of climate warming on the bird’s life cycle depends on local or regional climate characteristics. We tested the hypothesis that differences in climate indices between North Africa and Europe can influence the timing of spring arrival. Our results support the hypothesis that differences in first spring arrival dates between European populations occur after individuals cross the Sahara. We assume that the endogenous programme of migration control in short/medium-distance migrants synchronises with the changing environment on their wintering grounds and along their migration routes, whereas in long-distance migrants it is rather with environmental changes in the second part of their migratory route in Europe. Our results strongly indicate that the mechanism of dynamic balance in the interaction between the endogenous regulatory programme and environmental factors determines the pattern of spring arrival, as well as migration timing.

title: The impact of climate change on birds: Impact of climate change on birds
abstract: Weather is of major importance for the population dynamics of birds, but the implications of climate change have only recently begun to be addressed. There is already compelling evidence that birds have been affected by recent climate changes. This review suggests that although there is a substantial body of evidence for changes in the phenology of birds, particularly of the timing of migration and of nesting, the consequences of these responses for a species’ population dynamics is still an area requiring in-depth research. The potential for phenological miscuing (responding inappropriately to climate change, including a lack of response) and for phenological disjunction (in which a bird species becomes out of synchrony with its environment) are beginning to be demonstrated, and are also important areas for further research. The study of climatically induced distributional change is currently at a predictive modelling stage, and will need to develop methods for testing these predictions. Overall, there is a range of intrinsic and extrinsic factors that could potentially inhibit adaptation to climate change and these are a high priority for research.

title: Quantifying the robustness of first arrival dates as a measure of avian migratory phenology
abstract: As the climate changes, many long-term studies have shown that the timing of bird migration is shifting, increasing the need for reliable measures of migratory phenology. Ideally, daily counts of birds at a site are used to calculate the mean arrival date (MAD) but, as this approach is not always possible and is very labour-intensive, simpler metrics such as first arrival date (FAD) have commonly been used. Here, we examine the relationship between FAD and MAD in 28 summer migrant bird species over a 42-year period (1970–2011) at Portland Bird Observatory, UK. Although significant correlations between FAD and MAD were detected, relationships were weak, particularly in long-distance migrants. We suggest that FAD, although a simple and straightforward measure, is not particularly robust as a proxy for overall migratory phenology at a population level.

title: Variable shifts in spring and autumn migration phenology in North American songbirds associated with climate change.
abstract: Monitoring studies find that the timing of spring bird migration has advanced in recent decades, especially in Europe. Results for autumn migration have been mixed. Using data from Powdermill Nature Reserve, a banding station in western Pennsylvania, USA, we report an analysis of migratory timing in 78 songbird species from 1961 to 2006. Spring migration became significantly earlier over the 46-year period, and autumn migration showed no overall change. There was much variation among species in phenological change, especially in autumn. Change in timing was unrelated to summer range (local vs. northern breeders) or the number of broods per year, but autumn migration became earlier in neotropical migrants and later in short-distance migrants. The migratory period for many species lengthened because late phases of migration remained unchanged or grew later as early phases became earlier. There was a negative correlation between spring and autumn in long-term change, and this caused dramatic adjustments in the amount of time between migrations: the intermigratory periods of 10 species increased or decreased by > 15 days. Year-to-year changes in timing were correlated with local temperature (detrended) and, in autumn, with a regional climate index (detrended North Atlantic Oscillation). These results illustrate a complex and dynamic annual cycle in songbirds, with responses to climate change differing among species and migration seasons.

title: Do changes in climate patterns in wintering areas affect the timing of the spring arrival of trans‐Saharan migrant birds?
abstract: The life cycles of plants and animals are changing around the world in line with the predictions originated from hypotheses concerning the impact of global warming and climate change on biological systems. Commonly, the search for ecological mechanisms behind the observed changes in bird phenology has focused on the analysis of climatic patterns from the species breeding grounds. However, the ecology of bird migration suggests that the spring arrival of long-distance migrants (such as trans-Saharan birds) is more likely to be influenced by climate conditions in wintering areas given their direct impact on the onset of migration and its progression. We tested this hypothesis by analysing the first arrival dates (FADs) of six trans-Saharan migrants (cuckoo Cuculus canorus ,s wiftApus apus, hoopoe Upupa epops ,s wallowHirundo rustica, house martin Delichon urbica and nightingale Luscinia megarhynchos), in a western Mediterranean area since from 1952 to 2003. By means of multiple regression analyses, FADs were analysed in relation to the monthly temperature and precipitation patterns of five African climatic regions south of the Sahara where species are thought to overwinter and from the European site from where FADs were collected. We obtained significant models for five species explaining 9‐41% of the variation in FADs. The interpretation of the models suggests that: (1) The climate in wintering quarters, especially the precipitation, has a stronger influence on FADs than that in the species’ potential European breeding grounds. (2) The accumulative effects of climate patterns prior to migration onset may be of considerable importance since those climate variables that served to summarize climate patterns 12 months prior to the onset of migration were selected by final models. (3) Temperature and precipitation in African regions are likely to affect departure decision in the species studied through their indirect effects on food availability and the build-up of reserves for migration. Our results concerning the factors that affect the arrival times of trans-Saharan migrants indicate that the effects of climate change are more complex than previously suggested, and that these effects might have an interacting impact on species ecology, for example by reversing ecological pressures during species’ life cycles.

title: Climatic connectivity between Africa and Europe may serve as a basis for phenotypic adjustment of migration schedules of trans-Saharan migratory birds
abstract: Most European migratory birds wintering in sub-Saharan Africa have anticipated arrival to the breeding areas over the past decades. This phenological change may be ultimately caused by warming of the Northern Hemisphere via evolutionary changes or phenotypic plasticity in migration behavior. First arrival dates are negatively predicted by temperatures upon arrival to the breeding grounds. This seems puzzling, because migrants should be unable to predict weather conditions at long range. Migrants can enjoy diverse fitness benefits from early arriving. However, if weather conditions at destination cannot be predicted, early arrival can also entail severe costs. If meteorological conditions in Europe during breeding covary with those in sub-Saharan Africa during late winter, long-distance migrants may have a clue to predict meteorological conditions in their breeding areas while they are still in Africa and adjust their migration schedule consequently, an idea that has never been tested. We analyzed the correlation between March–April temperature anomalies (Tan) in Europe and February Tan in the Sahel and sub-Sahel, where long-distance migrants winter or stop-over. Tan in Africa negatively predicted Tan in Europe, the association being particularly strong (unsigned effect size, zr>0.35) for eastern Sahel and northern and eastern Europe, where the risks of early arrival may be larger. However, the strength of the correlations between Tan in the two continents has declined during the last 25 years; thus, possibly, partly compromising adaptive mechanisms of adjustment of migration. The existence of such climatic connectivity leads to several predictions, including that positive Tan in Africa should delay arrival. Consistent with this prediction, we found that first arrival dates of seven long-distance migratory species positively covaried with February Tan in Africa. Thus, while wintering, migrants might be able to predict meteorological conditions at the beginning of the breeding season, and phenotypically adjust migration schedules to optimally tune arrival date.

title: The North Atlantic Oscillation: Past, present, and future
abstract: The climate of the Atlantic sector exhibits considerable variability on a wide range of time scales. A substantial portion is associated with the North Atlantic Oscillation (NAO), a hemispheric meridional oscillation in atmospheric mass with centers of action near Iceland and over the subtropical Atlantic. NAO-related impacts on winter climate extend from Florida to Greenland and from northwestern Africa over Europe far into northern Asia. Over the last 3 decades, the phase of the NAO has been shifting from mostly negative to mostly positive index values. Much remains to be learned about the mechanisms that produce such low frequency changes in the North Atlantic climate, but it seems increasingly likely that human activities are playing a significant role.

title: Assortative Mating as a Mechanism for Rapid Evolution of a Migratory Divide
abstract: There have been numerous recent observations of changes in the behavior and dynamics of migratory bird populations, but the plasticity of the migratory trait and our inability to track small animals over large distances have hindered investigation of the mechanisms behind migratory change. We used habitat-specific stable isotope signatures to show that recently evolved allopatric wintering populations of European blackcaps Sylvia atricapilla pair assortatively on their sympatric breeding grounds. Birds wintering further north also produce larger clutches and fledge more young. These findings describe an important process in the evolution of migratory divides, new migration routes, and wintering quarters. Temporal segregation of breeding is a way in which subpopulations of vertebrates may become isolated in sympatry.

title: THE TIMING OF BIRDS‘ BREEDING SEASONS
abstract: Examination of survival rdtes of nestlings and fledglings of some species show that there is a strong tendency for those young which are hatched earliest in the season to have the greatest chance of surviving to breed. Since natural selection so strongly favours parents who leave many surviving young, the question arises as to why other birds breed later than the date at which they could most successfully raise their young. 
 
It is suggested that the food supply for the breeding females immediately prior to the breeding season may limit their ability to form eggs and the females may thus not be able to lay at the time which would result in young being in the nest at the best time for raising them, but as soon after this time as the female is able to produce her eggs. Not all species are likely to be prevented, by food shortage, from breeding at the best time for raising young and the groups of birds most likely to be affected are discussed.

title: Heritability of arrival date in a migratory bird.
abstract: The behaviour of long-distance migratory birds is assumed to partly be under the influence of genes, as demonstrated by selection experiments. Furthermore, competition for early arrival among males may lead to condition-dependent migration associated with fitness benefits of early arrival achieved by individuals in prime condition. Here I present field data on the repeatability and the heritability of arrival date in a trans-equatorial migratory bird, the barn swallow Hirundo rustica, and I test for a genetic correlation between arrival date and the expression of a condition-dependent secondary sexual character. The repeatability was statistically significant and the heritability of arrival date was estimated to be 0.54 (s.e. = 0.15). There was no significant evidence of this estimate being inflated by environmental or maternal condition during rearing. Arrival date and migration are condition dependent in the barn swallow, with males with the most exaggerated secondary sexual characters also arriving the earliest. There was a significant genetic correlation between arrival date and tail length in male barn swallows, providing indirect evidence for a genetic basis of this condition dependence. Given the high level of heritability, arrival date could readily respond to selection caused by environmental change.

title: Patterns of spring arrival dates differ in two hirundines
abstract: We examined the first arrival dates in the UK over a 56 yr period of 2 hirundines, sand martin Riparia riparia and barn swallow Hirundo rustica, in relation to temperatures along migration routes and at destination. Changes in arrival dates have been much greater for sand martin than for barn swallow, and the arrival order of the 2 species now appears to have reversed. There appear to be 2 reasons for this: a greater response to temperatures in SW Europe and a different response to temperature in recent years, leading to earlier arrival at the same temperature than formerly. We argue that this may involve evolutionary change, but conclude that it is difficult to be definite about this without information on individual animals.

title: Timing of Autumn Migration and Its Relation to Winter Distribution in Dark‐Eyed Juncos
abstract: Migratory Dark-eyed Juncos in eastern North America migrate southward in autumn and tend to segregate in the winter range according to sex and age. North to south, the most abundant classes are young males, adult males, young females, and adult females. Because adults tend to dominate young at winter feeding sites, this distribution appears at first to conflict with the view that dominance interactions are responsible for differential avian migrations and that dominant individuals remain nearest the breeding range. However, if young juncos establish winter residency at earlier dates than adults, a prior residence effect might make them dominant. This hypothesis requires that young arrive at wintering sites earlier than adults and that residents of all classes arrive earlier than their transient counterparts. We tested these predictions by comparing first-capture dates ofjuncos at Bloomington, Indiana, which is north of the latitudinal midline of the junco's winter range. During 13 autumns, individuals were classed as local residents or as transients and were sexed and aged. Contrary to predictions, residents were not caught earlier than transients. Further, adults were caught earlier, not later, than young among transients and probably also among residents. Thus, settlement of the winter range does not proceed from north to south, and dominance established through prior residence cannot account for the concentration of 1st-yr males in the northern part of the winter range. The fact that some sex-age classes tend to winter south of others predicts that at a northern capture site the classes that migrate farthest should be commoner among transients than among residents. This expectation was fulfilled. In addition, the median capture dates of the sex-age classes were arranged approximately according to the north-to-south order of their distribution, indicating that classes with the farthest to travel passed through (transients) or settled (residents) earliest in autumn. We conclude that comparison of autumn migration schedules of transient and resident passerine birds at a single location can yield considerable information about the dynamics of settlement of the entire winter range, including possible information about differences in destination of subsets of tran- sients.

title: Climate and the complexity of migratory phenology: sexes, migratory distance, and arrival distributions
abstract: The intra- and inter-season complexity of bird migration has received limited attention in climatic change research. Our phenological analysis of 22 species collected in Chicago, USA, (1979–2002) evaluates the relationship between multi-scalar climate variables and differences (1) in arrival timing between sexes, (2) in arrival distributions among species, and (3) between spring and fall migration. The early migratory period for earliest arriving species (i.e., short-distance migrants) and earliest arriving individuals of a species (i.e., males) most frequently correlate with climate variables. Compared to long-distance migrant species, four times as many short-distance migrants correlate with spring temperature, while 8 of 11 (73%) of long-distance migrant species’ arrival is correlated with the North Atlantic Oscillation (NAO). While migratory phenology has been correlated with NAO in Europe, we believe that this is the first documentation of a significant association in North America. Geographically proximate conditions apparently influence migratory timing for short-distance migrants while continental-scale climate (e.g., NAO) seemingly influences the phenology of Neotropical migrants. The preponderance of climate correlations is with the early migratory period, not the median of arrival, suggesting that early spring conditions constrain the onset or rate of migration for some species. The seasonal arrival distribution provides considerable information about migratory passage beyond what is apparent from statistical analyses of phenology. A relationship between climate and fall phenology is not detected at this location. Analysis of the within-season complexity of migration, including multiple metrics of arrival, is essential to detect species’ responses to changing climate as well as evaluate the underlying biological mechanisms.