Quo vadis amphibia? Global warming and breeding phenology in frogs,toads and salamanders

As the earth is getting warmer, many animals and plants have shifted their timing of breeding towards earlier dates. However, there is substantial variation between populations in phenological shifts that typically goes unexplained. Identification of the different location and species characteristics that drive such variable responses to global warming is crucial if we are to make predictions for how projected climate change scenarios will play out on local and global scales. Here we conducted a phylogenetically controlled meta‐analysis of breeding phenology across frogs, toads and salamanders to examine the extent of variation in amphibian breeding phenology in response to global climate change. We show that there is strong geographic variation in response to global climate change, with species at higher latitudes exhibiting a more pronounced shift to earlier breeding than those at lower latitudes. Our analyses suggest that this latitude effect is a result of both the increased temperature (but not precipitation) at higher latitudes as well as a greater responsiveness by northern populations of amphibians to this change in temperature. We suggest that these effects should reinforce any direct effect of increasing warming at higher latitudes on breeding phenology. In contrast, we found very little contribution from other location factors or species traits. There was no evidence for a phylogenetic signal on advancing breeding phenology or responsiveness to temperature, suggesting that the amphibians that have been studied to date respond similarly to global warming.     

Spring migration phenology of birds in the Northern Prairie region is correlated with local climate change

ABSTRACT In apparent response to recent periods of global warming, some migratory birds now arrive earlier at stopover sites and breeding grounds. However, the effects of this warming on arrival times vary among locations and species. Migration timing is generally correlated with temperature, with earlier arrival during warm years than during cold years, so local variation in climate change might produce different effects on migration phenology in different geographic regions. We examined trends in first spring arrival dates (FADs) for 44 species of common migrant birds in South Dakota (1971–2006) and Minnesota (1964–2005) using observations compiled by South Dakota and Minnesota Ornithologists’ Unions. We found significant trends in FAD over time for 20 species (18 arriving earlier and two later) in South Dakota and 16 species (all earlier) in Minnesota. Of these species, 10 showed similar significant trends for both states. All 10 of these species exhibited significantly earlier arrival, and all were early spring migrants, with median FADs before 10 April in both states. Eighteen of the 44 species showed significant negative correlations of FADs with either winter (December–February) or spring (arrival month plus previous month) temperatures in one or both states. Interestingly, spring temperatures in both South Dakota and Minnesota did not warm significantly from 1971–2006, but winter temperatures in both states warmed significantly over the same time period. This suggests that the warmer winters disproportionately affected early spring migrants, especially those associated with aquatic habitats (seven of the 10 species showing significantly earlier spring arrival in both states). The stronger response to climate change by early spring migrants in our study is consistent with the results of several other studies, and suggests that migrants, especially early migrants, are capable of responding to local temperature conditions experienced on wintering grounds or along the migration route.     

A critical thermal transition driving spring phenology of Northern Hemisphere conifers

Despite growing interest in predicting plant phenological shifts, advanced spring phenology by global climate change remains debated. Evidence documenting either small or large advancement of spring phenology to rising temperature over the spatio-temporal scales implies a potential existence of a thermal threshold in the responses of forests to global warming. We collected a unique data set of xylem cell-wall-thickening onset dates in 20 coniferous species covering a broad mean annual temperature (MAT) gradient (−3.05 to 22.9°C) across the Northern Hemisphere (latitudes 23°–66° N). Along the MAT gradient, we identified a threshold temperature (using segmented regression) of 4.9 ± 1.1°C, above which the response of xylem phenology to rising temperatures significantly decline. This threshold separates the Northern Hemisphere conifers into cold and warm thermal niches, with MAT and spring forcing being the primary drivers for the onset dates (estimated by linear and Bayesian mixed-effect models), respectively. The identified thermal threshold should be integrated into the Earth-System-Models for a better understanding of spring phenology in response to global warming and an improved prediction of global climate-carbon feedbacks.     

Effect of open-field experimental warming on the leaf phenology of oriental oak (Quercus variabilis) seedlings

Aims An open-field warming experiment enables us to test the effects of projected temperature increase on change in plant phenology with fewer confounding factors and to study phenological response to temperature ranges beyond natural variability. This study aims to (i) examine the effect of temperature increase on leaf unfolding and senescence of oriental oak (Quercus variabilis Blume) under experimental warming and (ii) measure temperature-related parameters used in estimating phenological response to temperature elevation.Methods Using an open-field warming system with infrared heaters, we increased the air temperature by ~3°C in the warmed plots compared with that of the control plots consistently for 2 years. Leaf unfolding and senescence dates of Q. variabilis seedlings were recorded and temperature-related phenological parameters were analysed.Important findings The timing of leaf unfolding was advanced by 3–8 days (1.1–3.0 days/°C) and the date of leaf senescence was delayed by 14–19 days (5.0–7.3 days/°C) under elevated air temperatures. However, the cumulative degree days (CDD) of leaf unfolding were not significantly changed by experimental warming, which suggest the applicability of a constant CDD value to estimate the change in spring leaf phenology under 3°C warming. Consistent ranges of advancement and temperature sensitivity in spring phenology and delayed autumn phenology and proposed temperature parameters from this study might be applied to predict future phenological change.     

Effects of climatic change on the phenology of butterflies in the northwest Mediterranean Basin

Phenological changes in response to climatic warming have been detected across a wide range of organisms. Butterflies stand out as one of the most popular groups of indicators of climatic change, given that, firstly, they are poikilothermic and, secondly, have been the subject of thorough monitoring programmes in several countries for a number of decades. Here we provide for the first time strong evidence of phenological change as a consequence of recent climatic warming in butterflies at a Spanish site in the northwest Mediterranean Basin. By means of the widely used Butterfly Monitoring Scheme methodology, three different phenological parameters were analysed for the most common species to test for trends over time and relationships with temperature and precipitation. Between 1988 and 2002, there was a tendency for earlier first appearance dates in all 17 butterfly species tested, and significant advances in mean flight dates in 8 out of 19 species. On the other hand, the shape of the curve of adult emergence did not show any regular pattern. These changes paralleled an increase of 1–1.5°C in mean February, March and June temperatures. Likewise, a correlation analysis indicated the strong negative effect of spring temperature on phenological parameters (i.e. higher temperatures tended to produce phenological advances), and the opposite effect of precipitation in certain months. In addition, there was some evidence to indicate that phenological responses may differ between taxonomic lineages or species with similar diets. We discuss the consequences that these changes may have on species' population abundances, especially given the expected increase in aridity in the Mediterranean Basin caused by current climatic warming. We predict that varying degrees of phenological flexibility may account for differences in species' responses and, for multivoltine species, predict strong selection favouring local seasonal adaptations such as diapause phenomena or migratory behaviour.     

Phenology in a warming world: differences between native and non‐native plant species

Phenology is a harbinger of climate change, with many species advancing flowering in response to rising temperatures. However, there is tremendous variation among species in phenological response to warming, and any phenological differences between native and non‐native species may influence invasion outcomes under global warming. We simulated global warming in the field and found that non‐native species flowered earlier and were more phenologically plastic to temperature than natives, which did not accelerate flowering in response to warming. Non‐native species' flowering also became more synchronous with other community members under warming. Earlier flowering was associated with greater geographic spread of non‐native species, implicating phenology as a potential trait associated with the successful establishment of non‐native species across large geographic regions. Such phenological differences in both timing and plasticity between native and non‐natives are hypothesised to promote invasion success and population persistence, potentially benefiting non‐native over native species under climate change.     

Phenological response to climate change in China: a meta‐analysis

The change in the phenology of plants or animals reflects the response of living systems to climate change. Numerous studies have reported a consistent earlier spring phenophases in many parts of middle and high latitudes reflecting increasing temperatures with the exception of China. A systematic analysis of Chinese phenological response could complement the assessment of climate change impact for the whole Northern Hemisphere. Here, we analyze 1263 phenological time series (1960–2011, with 20+ years data) of 112 species extracted from 48 studies across 145 sites in China. Taxonomic groups include trees, shrubs, herbs, birds, amphibians and insects. Results demonstrate that 90.8% of the spring/summer phenophases time series show earlier trends and 69.0% of the autumn phenophases records show later trends. For spring/summer phenophases, the mean advance across all the taxonomic groups was 2.75 days decade^?1 ranging between 2.11 and 6.11 days decade^?1 for insects and amphibians, respectively. Herbs and amphibians show significantly stronger advancement than trees, shrubs and insect. The response of phenophases of different taxonomic groups in autumn is more complex: trees, shrubs, herbs and insects show a delay between 1.93 and 4.84 days decade^?1, while other groups reveal an advancement ranging from 1.10 to 2.11 days decade^?1. For woody plants (including trees and shrubs), the stronger shifts toward earlier spring/summer were detected from the data series starting from more recent decades (1980s–2000s). The geographic factors (latitude, longitude and altitude) could only explain 9% and 3% of the overall variance in spring/summer and autumn phenological trends, respectively. The rate of change in spring/summer phenophase of woody plants (1960s–2000s) generally matches measured local warming across 49 sites in China (R = ?0.33, P < 0.05).     

Advances in first bloom dates and increased occurrences of yearly second blooms in eastern China since the 1960s: further phenological evidence of climate warming

Confirming the results of previous regional studies on changes in first bloom dates (FBD) in China, this study provides evidence that complements conclusions drawn from studies of phenological changes in other dynamic climate systems in the Northern Hemisphere. Furthermore, increased occurrences of yearly second blooms (YSB) further reinforce results derived from other studies indicating a recent trend of generalized climate warming across China. Additionally, ascertaining changes in FBD and YSB against a recent background not only provides a hitherto poorly formulated autumnal equivalent to the well-studied shifts in FBD, but also formulates both spring and autumn flowering changes in recent years. Data in this study are derived from observations made from 1963 to 2006 by the Chinese Phenological Observation Network (CPON)—a nationwide system of monitoring stations that has made observations of over 173 species from across China since 1963. At each site, the mean value of each species’ annual deviation and spring mean surface temperatures were calculated. For each species, years and locations were also recorded for species in which second blooms (YSB) occurred. Of the 46 FBD samples, 31 showed advances from the mean, blooming earlier over the course of the study period. Notably, although only 8 of the 46 FBD samples showed significance levels of 0.1 or better, the average FBD did advance by 5.3 days. After the 1980s, the frequency of YSB occurrence remained steady, declining a little from the peak in the 1980s, but still exhibiting occurrences far more than were observed earlier. The data from this study clearly indicate that both the phenological advance of FBD in spring and the increased occurrence of YSB are consistent with climate warming.     

Relationship between laying dates of black kites Milvus migrans and spring temperatures in Italy: rapid response to climate change?

Many Holarctic bird species have been shown to be laying progressively earlier in response to global warming. Most studies have been conducted on small-bodied species at northern latitudes. In Italy, black kite Milvus migrans populations are concentrated near large pre-Alpine lakes, whose water temperatures have been increasing steadily in recent decades. Annual black kite laying dates have become 10–11 days earlier during the last nine years. This shift was related to temporal variations in spring air temperatures, with earlier laying in warmer springs. The progressive advancement in laying dates was not associated with an increase in population level productivity, despite the fact that productivity declined with laying date at the individual territory level within years. Avian response to climate change may be occurring more rapidly than previously thought, even in relatively long-lived species, and not only at the most northern latitudes or highest elevations. Because this change has occurred so rapidly, and because laying dates match annual variations in spring temperatures, the response must be facultatively driven by prevailing conditions, rather than genetic in response to natural selection.     

Impact of climate warming on phenological asynchrony of plankton dynamics across Europe

Climate warming alters the seasonal timing of biological events. This raises concerns that species-specific responses to warming may de-synchronize co-evolved consumer-resource phenologies, resulting in trophic mismatch and altered ecosystem dynamics. We explored the effects of warming on the synchrony of two events: the onset of the phytoplankton spring bloom and the spring/summer maximum of the grazer Daphnia. Simulation of 16 lake types over 31 years at 1907 North African and European locations under 5 climate scenarios revealed that the current median phenological delay between the two events varies greatly (20–190 days) across lake types and geographic locations. Warming moves both events forward in time and can lengthen or shorten the delay between them by up to ±60 days. Our simulations suggest large geographic and lake-specific variations in phenological synchrony, provide quantitative predictions of its dependence on physical lake properties and geographic location and highlight research needs concerning its ecological consequences.     

Altered geographic and temporal variability in phenology in response to climate change

Aim In response to recent climate warming, numerous studies have reported an earlier onset of spring and, to a lesser degree, a later onset of autumn, both determined from phenological observations. Here, we examine whether these reported changes have affected the synchronization of events on a regional level by examining temporal and spatial variability in phenology. In particular, we study whether years with earlier springs are associated with an altered spatial variability in phenology. Location Germany and the United Kingdom. Methods Plant phenological observations of 35 different phases (events such as flowering and leafing) collected by the German Weather Service (1951–2002) and butterfly phenological records of 29 species collected by the UK Butterfly Monitoring Scheme (1976–2003) are used. In these long‐term records, we examine the temporal (year‐to‐year) variability and the spatial (geographic or between site) variability with particular emphasis on how they vary with time of the year and with earliness or lateness of the phase. Results Early phenological events (i.e. spring) are more variable than later events, both in time and in space, although the pattern is clearer for plants than for butterflies. Confirming previous results, we find a clear relationship between the mean date of spring and summer phases and the degree to which they have become earlier. The spatial variability of spring events is greater in warmer years that have faster plant development. However, late spring and summer events do not show a consistent relationship. Autumn events are somewhat more spatially variable in years characterized by later seasons. Main conclusions This is the first examination of spatial variability of plant and animal phenological events at a multinational scale. Earlier spring events are likely to be associated with increased spatial variability in plants, although this is unlikely to also be true for summer events. If species experience differential changes in geographic variation this may disrupt interactions among them, e.g. in food webs. On the other hand, these may offer advantages for mobile species. Further research on linked species is recommended.     

Similarities in butterfly emergence dates among populations suggest local adaptation to climate

Phenology shifts are the most widely cited examples of the biological impact of climate change, yet there are few assessments of potential effects on the fitness of individual organisms or the persistence of populations. Despite extensive evidence of climate‐driven advances in phenological events over recent decades, comparable patterns across species' geographic ranges have seldom been described. Even fewer studies have quantified concurrent spatial gradients and temporal trends between phenology and climate. Here we analyse a large data set (~129 000 phenology measures) over 37 years across the UK to provide the first phylogenetic comparative analysis of the relative roles of plasticity and local adaptation in generating spatial and temporal patterns in butterfly mean flight dates. Although populations of all species exhibit a plastic response to temperature, with adult emergence dates earlier in warmer years by an average of 6.4 days per °C, among‐population differences are significantly lower on average, at 4.3 days per °C. Emergence dates of most species are more synchronised over their geographic range than is predicted by their relationship between mean flight date and temperature over time, suggesting local adaptation. Biological traits of species only weakly explained the variation in differences between space‐temperature and time‐temperature phenological responses, suggesting that multiple mechanisms may operate to maintain local adaptation. As niche models assume constant relationships between occurrence and environmental conditions across a species' entire range, an important implication of the temperature‐mediated local adaptation detected here is that populations of insects are much more sensitive to future climate changes than current projections suggest.     

Morphological constraints on changing avian migration phenology

Many organisms at northern latitudes have responded to climate warming by advancing their spring phenology. Birds are known to show earlier timing of spring migration and reproduction in response to warmer springs. However, species show heterogeneous phenological responses to climate warming, with those that have not advanced or have delayed migration phenology experiencing population declines. Although some traits (such as migration distance) partly explain heterogeneity in phenological responses, the factors affecting interspecies differences in the responsiveness to climate warming have yet to be fully explored. In this comparative study, we investigate whether variation in wing aspect ratio (reflecting relative wing narrowness), an ecomorphological trait that is strongly associated with flight efficiency and migratory behaviour, affects the ability to advance timing of spring migration during 1960–2006 in a set of 80 European migratory bird species. Species with larger aspect ratio (longer and narrower wings) showed smaller advancement of timing of spring migration compared to species with smaller aspect ratio (shorter and wider wings) while controlling for phylogeny, migration distance and other life‐history traits. In turn, migration distance positively predicted aspect ratio across species. Hence, species that are better adapted to migration appear to be more constrained in responding phenologically to rapid climate warming by advancing timing of spring migration. Our findings corroborate the idea that aspect ratio is a major evolutionary correlate of migration, and suggest that selection for energetically efficient flights, as reflected by high aspect ratio, may hinder phenotypically plastic/microevolutionary adjustments of migration phenology to ongoing climatic changes.     

Breeding phenological response to spring weather conditions in common Finnish birds: resident species respond stronger than migratory species

National bird‐nest record schemes provide a valuable data source to study large‐scale changes in basic breeding biology and effects of climate change on birds. Using nest‐record scheme data from 26 common Finnish breeding bird species from whole Finland, we estimated the laydate of the first egg for 129 063 nesting attempts. We then investigated the relationship of mean spring temperature and spring precipitation sum to changes in the onset of laying over the period 1961–2012. In addition, we examine differences in response to these climatic variables for species grouped for different life history strategies; migration, diet and habitat. Finally, we test whether body size is related to the strength of phenological response. We show that 26 common Finnish breeding bird species have advanced their laying dates over time and to an increase in the mean spring temperature over the study period. When species are grouped according life history strategies, we find that breeding phenological change is negatively associated with changes in the mean spring temperature where residents respond strongest to changes in mean spring temperature, but also short‐ and long‐distance migrants advance laydates with increasing spring temperatures. Breeding phenological change is also associated with spring precipitation, where resident species delay and short‐distance migrants advance the onset of breeding. In addition we find that omnivorous species respond stronger than insectivorous species to changes in spring temperature. In contrast to results from an earlier study, we do not find evidence that small‐sized species respond stronger to spring temperature than large‐sized species. As climate warming is predicted to continue in the future, long‐term citizen science schemes, such as the Finnish nest‐card scheme, prove to be a valuable cost‐effective way to monitor the environment and allow investigation into how species are responding to changes in their environment.     

Extreme spring conditions in the Arctic delay spring phenology of long-distance migratory songbirds

Arctic regions are warming rapidly, with extreme weather events increasing in frequency, duration, and intensity just as in other regions. Many studies have focused on how shifting seasonality in environmental conditions affects vegetation phenology, while far fewer have examined how the breeding phenology of arctic fauna responds. We studied two species of long-distance migratory songbirds, Lapland longspurs, Calcarius lapponicus, and white-crowned sparrows, Zonotrichia leucophrys gambelii, across five consecutive breeding seasons in northern Alaskan tundra. We aimed to understand how spring environmental conditions affected breeding cycle phenology, including the timing of arrival on breeding grounds, territory establishment, and clutch initiation. Spring temperatures, precipitation, and snow-free dates differed significantly among years, with 2013 characterized by unusually late snow cover. In response, we found a significant delay in breeding-cycle phenology for both study species in 2013 relative to other study years: the first bird observed was delayed by 6–10 days, with mean arrival by 3–6 days, territory establishment by 6–13 days, and clutch initiation by 4–10 days. Further, snow cover, temperature, and precipitation during the territory establishment period were important predictors of clutch initiation dates for both species. These findings suggest that Arctic-breeding passerine communities may have the flexibility required to adjust breeding phenology in response to the increasingly extreme and unpredictable environmental conditions—although future generations may encounter conditions that exceed their current range of phenological flexibility.     

Climate warming: a loss of variation in populations can accompany reproductive shifts

The most documented response of organisms to climate warming is a change in the average timing of seasonal activities (phenology). Although we know that these average changes can differ among species and populations, we do not know whether climate warming impacts within‐population variation in phenology. Using data from five study sites collected during a 13‐year survey, we found that the increase in spring temperatures is associated with a reproductive advance of 10 days in natural populations of common lizards (Zootoca vivipara). Interestingly, we show a correlated loss of variation in reproductive dates within populations. As illustrated by a model, this shortening of the reproductive period can have significant negative effects on population dynamics. Consequently, we encourage tests in other species to assess the generality of decreased variation in phenological responses to climate change.     

Sensitivity of Spring Phenology to Warming Across Temporal and Spatial Climate Gradients in Two Independent Databases

Disparate ecological datasets are often organized into databases post hoc and then analyzed and interpreted in ways that may diverge from the purposes of the original data collections. Few studies, however, have attempted to quantify how biases inherent in these data (for example, species richness, replication, climate) affect their suitability for addressing broad scientific questions, especially in under-represented systems (for example, deserts, tropical forests) and wild communities. Here, we quantitatively compare the sensitivity of species first flowering and leafing dates to spring warmth in two phenological databases from the Northern Hemisphere. One??PEP725??has high replication within and across sites, but has low species diversity and spans a limited climate gradient. The other??NECTAR??includes many more species and a wider range of climates, but has fewer sites and low replication of species across sites. PEP725, despite low species diversity and relatively low seasonality, accurately captures the magnitude and seasonality of warming responses at climatically similar NECTAR sites, with most species showing earlier phenological events in response to warming. In NECTAR, the prevalence of temperature responders significantly declines with increasing mean annual temperature, a pattern that cannot be detected across the limited climate gradient spanned by the PEP725 flowering and leafing data. Our results showcase broad areas of agreement between the two databases, despite significant differences in species richness and geographic coverage, while also noting areas where including data across broader climate gradients may provide added value. Such comparisons help to identify gaps in our observations and knowledge base that can be addressed by ongoing monitoring and research efforts. Resolving these issues will be critical for improving predictions in understudied and under-sampled systems outside of the temperature seasonal mid-latitudes.     

Influences of species, latitudes and methodologies on estimates of phenological response to global warming

New analyses are presented addressing the global impacts of recent climate change on phenology of plant and animal species. A meta‐analysis spanning 203 species was conducted on published datasets from the northern hemisphere. Phenological response was examined with respect to two factors: distribution of species across latitudes and taxonomic affiliation or functional grouping of target species. Amphibians had a significantly stronger shift toward earlier breeding than all other taxonomic/functional groups, advancing more than twice as fast as trees, birds and butterflies. In turn, butterfly emergence or migratory arrival showed three times stronger advancement than the first flowering of herbs, perhaps portending increasing asynchrony in insect–plant interactions. Response was significantly stronger at higher latitudes where warming has been stronger, but latitude explained < 4% of the variation. Despite expectation, latitude was not yet an important predictor of climate change impacts on phenology. The only two previously published estimates of the magnitude of global response are quite different: 2.3 and 5.1 days decade⁻¹ advancement. The scientific community has assumed this difference to be real and has attempted to explain it in terms of biologically relevant phenomena: specifically, differences in distribution of data across latitudes, taxa or time periods. Here, these and other possibilities are explored. All analyses indicate that the difference in estimated response is primarily due to differences between the studies in criteria for incorporating data. It is a clear and automatic consequence of the exclusion by one study of data on ‘stable’ (nonresponsive) species. Once this is accounted for, the two studies support each other, generating similar conclusions despite analyzing substantially nonoverlapping datasets. Analyses here on a new expanded dataset estimate an overall spring advancement across the northern hemisphere of 2.8 days decade⁻¹. This is the first quantitative analysis showing that data‐sampling methodologies significantly impact global (synthetic) estimates of magnitude of global warming response.     

European phenological response to climate change matches the warming pattern

Global climate change impacts can already be tracked in many physical and biological systems; in particular, terrestrial ecosystems provide a consistent picture of observed changes. One of the preferred indicators is phenology, the science of natural recurring events, as their recorded dates provide a high-temporal resolution of ongoing changes. Thus, numerous analyses have demonstrated an earlier onset of spring events for mid and higher latitudes and a lengthening of the growing season. However, published single-site or single-species studies are particularly open to suspicion of being biased towards predominantly reporting climate change-induced impacts. No comprehensive study or meta-analysis has so far examined the possible lack of evidence for changes or shifts at sites where no temperature change is observed. We used an enormous systematic phenological network data set of more than 125 000 observational series of 542 plant and 19 animal species in 21 European countries (1971–2000). Our results showed that 78% of all leafing, flowering and fruiting records advanced (30% significantly) and only 3% were significantly delayed, whereas the signal of leaf colouring/fall is ambiguous. We conclude that previously published results of phenological changes were not biased by reporting or publication predisposition: the average advance of spring/summer was 2.5 days decade⁻¹ in Europe. Our analysis of 254 mean national time series undoubtedly demonstrates that species' phenology is responsive to temperature of the preceding months (mean advance of spring/summer by 2.5 days°C⁻¹, delay of leaf colouring and fall by 1.0 day°C⁻¹). The pattern of observed change in spring efficiently matches measured national warming across 19 European countries (correlation coefficient r =−0.69, P <0.001).     

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.