Longer wings for faster springs – wing length relates to spring phenology in a long‐distance migrant across its range

In migratory birds, morphological adaptations for efficient migratory flight often oppose morphological adaptations for efficient behavior during resident periods. This includes adaptations in wing shape for either flying long distances or foraging in the vegetation and in climate‐driven variation of body size. In addition, the timing of migratory flights and particularly the timely arrival at local breeding sites is crucial because fitness prospects depend on site‐specific phenology. Thus, adaptations for efficient long‐distance flights might be also related to conditions at destination areas. For an obligatory long‐distance migrant, the common nightingale, we verified that wing length as the aerodynamically important trait, but not structural body size increased from the western to the eastern parts of the species range. In contrast with expectation from aerodynamic theory, however, wing length did not increase with increasing migration distances. Instead, wing length was associated with the phenology at breeding destinations, namely the speed of local spring green‐up. We argue that longer wings are beneficial for adjusting migration speed to local conditions for birds breeding in habitats with fast spring green‐up and thus short optimal arrival periods. We suggest that the speed of spring green‐up at breeding sites is a fundamental variable determining the timing of migration that fine tune phenotypes in migrants across their range.     

Clock gene polymorphism,migratory behaviour and geographic distribution: a comparative study of trans‐Saharan migratory birds

Migratory behaviour is controlled by endogenous circannual rhythms that are synchronized by external cues, such as photoperiod. Investigations on the genetic basis of circannual rhythmicity in vertebrates have highlighted that variation at candidate ‘circadian clock’ genes may play a major role in regulating photoperiodic responses and timing of life cycle events, such as reproduction and migration. In this comparative study of 23 trans‐Saharan migratory bird species, we investigated the relationships between species‐level genetic variation at two candidate genes, Clock and Adcyap1, and species’ traits related to migration and geographic distribution, including timing of spring migration across the Mediterranean Sea, migration distance and breeding latitude. Consistently with previous evidence showing latitudinal clines in ‘circadian clock’ genotype frequencies, Clock allele size increased with breeding latitude across species. However, early‐ and late‐migrating species had similar Clock allele size. Species migrating over longer distances, showing delayed spring migration and smaller phenotypic variance in spring migration timing, had significantly reduced Clock (but not Adcyap1) gene diversity. Phylogenetic confirmatory path analysis suggested that migration date and distance were the most important variables directly affecting Clock gene diversity. Hence, our study supports the hypothesis that Clock allele size increases poleward as a consequence of adaptation to the photoperiodic regime of the breeding areas. Moreover, we show that long‐distance migration is associated with lower Clock diversity, coherently with strong stabilizing selection acting on timing of life cycle events in long‐distance migratory species, likely resulting from the time constraints imposed by late spring migration.     

Length polymorphisms at two candidate genes explain variation of migratory behaviors in blackpoll warblers (Setophaga striata)

Migratory behaviors such as the timing and duration of migration are genetically inherited and can be under strong natural selection, yet we still know very little about the specific genes or molecular pathways that control these behaviors. Studies in candidate genes Clock and Adcyap1 have revealed that both of these loci can be significantly correlated with migratory behaviors in birds, though observed relationships appear to vary across species. We investigated geographic genetic structure of Clock and Adcyap1 in four populations of blackpoll warblers (Setophaga striata), a Neotropical–Nearctic migrant that exhibits geographic variation in migratory timing and duration across its boreal breeding distribution. Further, we used data on migratory timing and duration, obtained from light‐level geolocator trackers to investigate candidate genotype–phenotype relationships at the individual level. While we found no geographic structure in either candidate gene, we did find evidence that candidate gene lengths are correlated with five of the six migratory traits. Maximum Clock allele length was significantly and negatively associated with spring arrival date. Minimum Adcyap1 allele length was significantly and negatively associated with spring departure date and positively associated with fall arrival date at the wintering grounds. Additionally, we found a significant interaction between Clock and Adcyap1 allele lengths on both spring and fall migratory duration. Adcyap1 heterozygotes also had significantly shorter migration duration in both spring and fall compared to homozygotes. Our results support the growing body of evidence that Clock and Adcyap1 allele lengths are correlated with migratory behaviors in birds.     

Long‐term changes in the migration phenology of UK breeding birds detected by large‐scale citizen science recording schemes

The timing of migration is one of the key life‐history parameters of migratory birds. It is expected to be under strong selection, to be sensitive to changing environmental conditions and to have implications for population dynamics. However, most phenological studies do not describe arrival and departure phenologies for a species in a way that is robust to potential biases, or that can be clearly related to breeding populations. This hampers our ability to understand more fully how climate change may affect species’ migratory strategies, their life histories and ultimately their population dynamics. Using generalized additive models (GAMs) and extensive large‐scale data collected in the UK over a 40‐year period, we present standardized measures of migration phenology for common migratory birds, and examine how the phenology of bird migration has changed in the UK since the 1960s. Arrival dates for 11 of 14 common migrants became significantly earlier, with six species advancing their arrival by more than 10 days. These comprised two species, Blackcap Sylvia atricapilla and Chiffchaff Phylloscopus collybita, which winter closest to Britain in southern Europe and the arid northern zone of Africa, Common Redstart Phoenicurus phoenicurus, which winters in the arid zone, and three hirundines (Sand Martin Riparia riparia, House Martin Delichon urbicum and Barn Swallow Hirundo rustica), which winter in different parts of Africa. Concurrently, departure dates became significantly later for four of the 14 species and included species that winter in southern Europe (Blackcap and Chiffchaff) and in humid zones of Africa (Garden Warbler Sylvia borin and Whinchat Saxicola rubetra). Common Swift Apus apus was the exception in departing significantly earlier. The net result of earlier arrival and later departure for most species was that length of stay has become significantly longer for nine of the 14 species. Species that have advanced their timing of arrival showed the most positive trends in abundance, in accordance with previous studies. Related in part to earlier arrival and the relationship above, we also show that species extending their stay in Great Britain have shown the most positive trends. Further applications of our modelling approach will provide opportunities for more robust tests of relationships between phenological change and population dynamics than have been possible previously.     

Flyway‐scale analysis reveals that the timing of migration in wading birds is becoming later

1. Understanding the implications of climate change for migratory animals is paramount for establishing how best to conserve them. A large body of evidence suggests that birds are migrating earlier in response to rising temperatures, but many studies focus on single populations of model species.
2. Migratory patterns at large spatial scales may differ from those occurring in single populations, for example because of individuals dispersing outside of study areas. Furthermore, understanding phenological trends across species is vital because we need a holistic understanding of how climate change affects wildlife, especially as rates of temperature change vary globally.
3. The life cycles of migratory wading birds cover vast latitudinal gradients, making them particularly susceptible to climate change and, therefore, ideal model organisms for understanding its effects. Here, we implement a novel application of changepoint detection analysis to investigate changes in the timing of migration in waders at a flyway scale using a thirteen‐year citizen science dataset (eBird) and determine the influence of changes in weather conditions on large‐scale migratory patterns.
4. In contrast to most previous research, our results suggest that migration is getting later in both spring and autumn. We show that rates of change were faster in spring than autumn in both the Afro‐Palearctic and Nearctic flyways, but that weather conditions in autumn, not in spring, predicted temporal changes in the corresponding season. Birds migrated earlier in autumn when temperatures increased rapidly, and later with increasing headwinds.
5. One possible explanation for our results is that migration is becoming later due to northward range shifts, which means that a higher proportion of birds travel greater distances and therefore take longer to reach their destinations. Our findings underline the importance of considering spatial scale when investigating changes in the phenology of migratory bird species.

     

Increase in protandry over time in a long‐distance migratory bird

Protandry is a widespread life‐history phenomenon describing how males precede females at the site or state of reproduction. In migratory birds, protandry has an important influence on individual fitness, the migratory syndrome, and phenological response to climate change. Despite its significance, accurate analyses on the dynamics of protandry using data sets collected at the breeding site, are lacking. Basing our study on records collected during two time periods, 1979 to 1988 and 2006 to 2016, we aim to investigate protandry dynamics over 38 years in a breeding population of willow warblers (Phylloscopus trochilus). Change in the timing of arrival was analyzed in males and females, and protandry (number of days between male and female arrival) was investigated both at population level and within breeding pairs. Our results show advancement in the arrival time at the breeding site in both sexes, but male arrival has advanced to a greater extent, leading to an increase in protandry both at the population level and within breeding pairs. We did not observe any change in sex ratio that could explain the protandry increase, but pronounced temperature change has occurred and been reported in the breeding area and along the migratory route. Typically, natural selection opposes too early arrival in males, but given warmer springs, this counteracting force may be relaxing, enabling an increase in protandry. We discuss whether our results suggest that climate change has induced sex‐specific effects, if these could be evolutionary and whether the timing of important life‐history stages such as arrival at the breeding site may change at different rates in males and females following environmental shifts.     

Can long‐distance migratory birds adjust to the advancement of spring by shortening migration distance? The response of the pied flycatcher to latitudinal photoperiodic variation

Many organisms use day length as a cue for synchronizing their life cycles with seasonal changes in environmental productivity. Under rapid climate change, however, responses to day length may become maladaptive, and photo‐responsive organisms may only be able to evade increasingly unsuitable habitats if they can accommodate to a wide range of photoperiodic conditions. A previous experiment showed that the pied flycatcher, Ficedula hypoleuca, a Palaearctic‐Afrotropical migratory bird, would strongly advance the timing of spring migration and reproductive maturation if it shifted its wintering area from sub‐Saharan Africa to the Mediterranean region. However, it is unknown whether this marked response to latitudinal variation in photoperiodic conditions is continuous over the entire range of potential wintering areas, and if a shortening of migration distance would be an effective mechanism to adjust the timing of migration to rapidly changing climatic conditions. Here, we experimentally show that a moderate northward displacement of the pied flycatcher's current wintering grounds by 10° would result in a clear advancement of the termination of prenuptial moult and the initiation of spring migratory activity and gonadal growth. However, we found no further advancement under conditions simulating higher wintering latitudes, suggesting the existence of a critical photoperiodic threshold or a steep gradual response within a narrow geographical range between 10° and 20° northern latitude. Because habitat conditions in this area are deteriorating rapidly, the potential for pied flycatchers to adjust their life cycle to changing climatic conditions by shortening the migration distance may be limited in the future.     

Solar radiation and ENSO predict fruiting phenology patterns in a 15‐year record from Kibale National Park,Uganda

Fruiting, flowering, and leaf set patterns influence many aspects of tropical forest communities, but there are few long‐term studies examining potential drivers of these patterns, particularly in Africa. We evaluated a 15‐year dataset of tree phenology in Kibale National Park, Uganda, to identify abiotic predictors of fruit phenological patterns and discuss our findings in light of climate change. We quantified fruiting for 326 trees from 43 species and evaluated these patterns in relation to solar radiance, rainfall, and monthly temperature. We used time‐lagged variables based on seasonality in linear regression models to assess the effect of abiotic variables on the proportion of fruiting trees. Annual fruiting varied over 3.8‐fold, and inter‐annual variation in fruiting is associated with the extent of fruiting in the peak period, not variation in time of fruit set. While temperature and rainfall showed positive effects on fruiting, solar radiance in the two‐year period encompassing a given year and the previous year was the strongest predictor of fruiting. As solar irradiance was the strongest predictor of fruiting, the projected increase in rainfall associated with climate change, and coincident increase in cloud cover suggest that climate change will lead to a decrease in fruiting. ENSO in the prior 24‐month period was also significantly associated with annual ripe fruit production, and ENSO is also affected by climate change. Predicting changes in phenology demands understanding inter‐annual variation in fruit dynamics in light of potential abiotic drivers, patterns that will only emerge with long‐term data.     

Ongoing changes in migration phenology and winter residency at Bracken Bat Cave

Bats play an important role in agroecology and are effective bioindicators of environmental conditions, but little is known about their fundamental migration ecology, much less how these systems are responding to global change. Some of the world's largest bat populations occur during the summer in the south‐central United States, when millions of pregnant females migrate from lower latitudes to give birth in communal maternity colonies. Despite a relatively large volume of research into these colonies, many fundamental questions regarding their abundance—including their intra‐ and interseasonal variability—remain unanswered, and even estimating the size of individual populations has been a long‐running challenge. Overall, monitoring these bat populations at high temporal resolution (e.g., nightly) and across long time spans (e.g., decades) has been impossible. Here, we show 22 continuous years of nightly population counts at Bracken Cave, a large bat colony in south‐central Texas, enabling the first climate‐scale phenological analysis. Using quantitative radar monitoring, we found that spring migration and the summer reproductive cycle have advanced by approximately 2 weeks over the study period. Furthermore, we quantify the ongoing growth of a newly—established overwintering population that indicates a system‐wide response to changing environmental conditions. Our observations reveal behavioral plasticity in bats’ ability to adapt to changing resource availability, and provide the first long‐term quantification of their response to a changing climate. As aerial insectivores, these changes in bat phenology and propensity for overwintering indicate probable shifts in prey availability, with clear implications for pest management across wider regional agrisystems.     

Potential for an Arctic‐breeding migratory bird to adjust spring migration phenology to Arctic amplification

Arctic amplification, the accelerated climate warming in the polar regions, is causing a more rapid advancement of the onset of spring in the Arctic than in temperate regions. Consequently, the arrival of many migratory birds in the Arctic is thought to become increasingly mismatched with the onset of local spring, consequently reducing individual fitness and potentially even population levels. We used a dynamic state variable model to study whether Arctic long‐distance migrants can advance their migratory schedules under climate warming scenarios which include Arctic amplification, and whether such an advancement is constrained by fuel accumulation or the ability to anticipate climatic changes. Our model predicts that barnacle geese Branta leucopsis suffer from considerably reduced reproductive success with increasing Arctic amplification through mistimed arrival, when they cannot anticipate a more rapid progress of Arctic spring from their wintering grounds. When geese are able to anticipate a more rapid progress of Arctic spring, they are predicted to advance their spring arrival under Arctic amplification up to 44 days without any reproductive costs in terms of optimal condition or timing of breeding. Negative effects of mistimed arrival on reproduction are predicted to be somewhat mitigated by increasing summer length under warming in the Arctic, as late arriving geese can still breed successfully. We conclude that adaptation to Arctic amplification may rather be constrained by the (un)predictability of changes in the Arctic spring than by the time available for fuel accumulation. Social migrants like geese tend to have a high behavioural plasticity regarding stopover site choice and migration schedule, giving them the potential to adapt to future climate changes on their flyway.     

Large‐scale variations in the vegetation growing season and annual cycle of atmospheric CO2 at high northern latitudes from 1950 to 2011

We combine satellite and ground observations during 1950–2011 to study the long‐term links between multiple climate (air temperature and cryospheric dynamics) and vegetation (greenness and atmospheric CO₂ concentrations) indicators of the growing season of northern ecosystems (>45°N) and their connection with the carbon cycle. During the last three decades, the thermal potential growing season has lengthened by about 10.5 days (P < 0.01, 1982–2011), which is unprecedented in the context of the past 60 years. The overall lengthening has been stronger and more significant in Eurasia (12.6 days, P < 0.01) than North America (6.2 days, P > 0.05). The photosynthetic growing season has closely tracked the pace of warming and extension of the potential growing season in spring, but not in autumn when factors such as light and moisture limitation may constrain photosynthesis. The autumnal extension of the photosynthetic growing season since 1982 appears to be about half that of the thermal potential growing season, yielding a smaller lengthening of the photosynthetic growing season (6.7 days at the circumpolar scale, P < 0.01). Nevertheless, when integrated over the growing season, photosynthetic activity has closely followed the interannual variations and warming trend in cumulative growing season temperatures. This lengthening and intensification of the photosynthetic growing season, manifested principally over Eurasia rather than North America, is associated with a long‐term increase (22.2% since 1972, P < 0.01) in the amplitude of the CO₂ annual cycle at northern latitudes. The springtime extension of the photosynthetic and potential growing seasons has apparently stimulated earlier and stronger net CO₂ uptake by northern ecosystems, while the autumnal extension is associated with an earlier net release of CO₂ to the atmosphere. These contrasting responses may be critical in determining the impact of continued warming on northern terrestrial ecosystems and the carbon cycle.     

The effects of long‐distance migration on the evolution of moult strategies in Western‐Palearctic passerines

Although feathers are the unifying characteristic of all birds, our understanding of the causes, mechanisms, patterns and consequences of the feather moult process lags behind that of other major avian life‐history phenomena such as reproduction and long‐distance migration. Migration, which evolved in many species of the temperate and arctic zones, requires high energy expenditure to endure long‐distance journeys. About a third of Western‐Palearctic passerines perform long‐distance migrations of thousands of kilometres each year using various morphological, physiological, biomechanical, behavioural and life‐history adaptations. The need to include the largely non‐overlapping breeding, long‐distance migration and feather moult processes within the annual cycle imposes a substantial constraint on the time over which the moult process can take place. Here, we review four feather‐moult‐related adaptations which, likely due to time constraints, evolved among long‐distance Western‐Palearctic migrants: (i) increased moult speed; (ii) increased overlap between moult and breeding or migration; (iii) decreased extent of plumage moult; and (iv) moult of part or all of the plumage during the over‐wintering period in the tropics rather than in the breeding areas. We suggest that long‐distance migration shaped the evolution of moult strategies and increased the diversity of these strategies among migratory passerines. In contrast to this variation, all resident passerines in the Western Palearctic moult immediately after breeding by renewing the entire plumage of adults and in some species also juveniles, while in other species juvenile moult is partial. We identify important gaps in our current understanding of the moult process that should be addressed in the future. Notably, previous studies suggested that the ancestral moult strategy is a post‐breeding summer moult in the Western Palearctic breeding areas and that moult during the winter evolved due to the scheduling of long‐distance migration immediately after breeding. We offer an alternative hypothesis based on the notion of southern ancestry, proposing that the ancestral moult strategy was a complete moult during the ‘northern winter’ in the Afro‐tropical region in these species, for both adults and juveniles. An important aspect of the observed variation in moult strategies relates to their control mechanisms and we suggest that there is insufficient knowledge regarding the physiological mechanisms that are involved, and whether they are genetically fixed or shaped by environmental factors. Finally, research effort is needed on how global climate changes may influence avian annual routines by altering the scheduling of major processes such as long‐distance migration and feather moult.     

Patterns in departure phenology and mass gain on African non‐breeding territories prior to the Sahara crossing in a long‐distance migrant

Afro‐Palaearctic migrants are declining to a greater degree than other European species, suggesting that processes occurring in Africa or on migration may be driving these trends. Constraints on food availability on the wintering grounds may contribute to these declines but little is known about when and where these resource constraints may occur. Sufficient resources are particularly important prior to spring migration, when migrants must cross the Sahara Desert. We examined mass gain and departure phenology in a long‐distance Palaearctic passerine migrant to determine the degree to which pre‐migratory fattening occurs in their long‐term non‐breeding territories in the Guinea Savannah region of Africa. We monitored 75 Whinchats Saxicola rubetra for departure from their non‐breeding territories in one spring, and analysed mass data of 377 Whinchats collected over three non‐breeding seasons plus 141 migrating Whinchats caught in April over 8 years, all within the same few square kilometres of human‐modified Guinea Savannah in central Nigeria. Whinchats left their winter territories throughout April, with males departing on average 8 days earlier than females. However, there was no evidence that time of departure from territory was linked to age, body size or mass at capture. Whinchats departed their territories with a predicted mass of 16.8 ± 0.3 g, considerably less than the c. 24 g required for the average Whinchat to cross the Sahara directly. Comparing departure dates with arrival dates in southern Europe showed a discrepancy of at least 2 weeks, suggesting that many Whinchats spend considerable time on pre‐migratory fuelling outside their winter territory prior to crossing the Sahara. Overwintering birds gained mass slowly during February and March (0.03 g/day), and non‐territorial or migrating birds at a much higher rate in April (at least 0.23 g/day), with up to 20% of migrating Whinchats in April potentially having sufficient fuel loads to cross the Sahara directly from central Nigeria. Our results suggest that most Whinchats leave their winter territories to fatten up locally or, possibly, by staging further north, closer to the southern limit of the Sahara. Resource constraints are therefore likely to be particularly focused in West Africa during mid‐April and possibly at staging areas before the crossing of the Sahara Desert.     

Evidence of local short‐distance spawning migration of tropical freshwater eels,and implications for the evolution of freshwater eel migration

Freshwater eels have fascinated biologists for centuries due to the spectacular long‐distance migrations between the eels’ freshwater habitats and their spawning areas far out in the ocean and the mysteries of their ecology. The spawning areas of Atlantic eels and Japanese eel were located far offshore in the Atlantic Ocean and the Pacific Ocean, respectively, and their reproduction took place thousands of kilometers away from their growth habitats. Phylogenetic studies have revealed that freshwater eels originated in the Indonesian region. However, remarkably little is known about the life histories of tropical freshwater eels despite the fact that tropical eels are key to understanding the nature of primitive forms of catadromous migration. This study found spawning‐condition tropical freshwater eels in Lake Poso, central Sulawesi, Indonesia, with considerably high gonadosomatic index values and with histologically fully developed gonads. This study provides the first evidence that under certain conditions, freshwater eels have conditions that are immediately able to spawn even in river downstream. The results suggest that, in contrast to the migrations made by the Atlantic and Japanese eels, freshwater eels originally migrated only short distances of <100 kilometers to local spawning areas adjacent to their freshwater growth habitats. Ancestral eels most likely underwent a catadromous migration from local short‐distance movements in tropical coastal waters to the long‐distance migrations characteristic of present‐day temperate eels, which has been well established as occurring in subtropical gyres in both hemispheres.     

Genetic assignment of recruits reveals short‐ and long‐distance larval dispersal in Pocillopora damicornis on the Great Barrier Reef

Understanding connectivity of coral populations among and within reefs over ecologically significant timescales is essential for developing evidence‐based management strategies, including the design of marineprotected areas. Here, we present the first assessment of contemporary connectivity among populations of two Molecular Operational Taxonomic Units (MOTUs) of the brooding coral Pocillopora damicornis. We used individual‐based genetic assignment methods to identify the proportions of philopatric and migrant larval recruits, settling over 12 months at sites around Lizard Island (northern Great Barrier Reef [GBR]) and over 24 months at sites around the Palms Islands (central GBR). Overall, we found spatially and temporally variable rates of self‐recruitment and dispersal, demonstrating the importance of variation in local physical characteristics in driving dispersal processes. Recruitment patterns and inferred dispersal distances differed between the two P. damicornis MOTUs, with type α recruits exhibiting predominantly philopatric recruitment, while the majority of type β recruits were either migrants from identified putative source populations or assumed migrants based on genetic exclusion from all known populations. While P. damicornis invests much energy into brooding clonal larvae, we found that only 15% and 7% of type α and type β recruits, respectively, were clones of sampled adult colonies or other recruits, challenging the hypothesis that reproduction is predominantly asexual in this species on the GBR. We explain high rates of self‐recruitment and low rates of clonality in these MOTUs by suggesting that locally retained larvae originate predominantly from spawned gametes, while brooded larvae are mainly vagabonds.     

Long‐term continental changes in wing length,but not bill length,of a long‐distance migratory shorebird

We compiled a >50‐year record of morphometrics for semipalmated sandpipers (Calidris pusilla), a shorebird species with a Nearctic breeding distribution and intercontinental migration to South America. Our data included >57,000 individuals captured 1972–2015 at five breeding locations and three major stopover sites, plus 139 museum specimens collected in earlier decades. Wing length increased by ca. 1.5 mm (>1%) prior to 1980, followed by a decrease of 3.85 mm (nearly 4%) over the subsequent 35 years. This can account for previously reported changes in metrics at a migratory stopover site from 1985 to 2006. Wing length decreased at a rate of 1,098 darwins, or 0.176 haldanes, within the ranges of other field studies of phenotypic change. Bill length, in contrast, showed no consistent change over the full period of our study. Decreased body size as a universal response of animal populations to climate warming, and several other potential mechanisms, are unable to account for the increasing and decreasing wing length pattern observed. We propose that the post‐WWII near‐extirpation of falcon populations and their post‐1973 recovery driven by the widespread use and subsequent limitation on DDT in North America selected initially for greater flight efficiency and latterly for greater agility. This predation danger hypothesis accounts for many features of the morphometric data and deserves further investigation in this and other species.     

Novel methods reveal shifts in migration phenology of barn swallows in South Africa

Many migratory bird species, including the barn swallow (Hirundo rustica), have advanced their arrival date at Northern Hemisphere breeding grounds, showing a clear biotic response to recent climate change. Earlier arrival helps maintain their synchrony with earlier springs, but little is known about the associated changes in phenology at their non-breeding grounds. Here, we examine the phenology of barn swallows in South Africa, where a large proportion of the northern European breeding population spends its non-breeding season. Using novel analytical methods based on bird atlas data, we show that swallows first arrive in the northern parts of the country and gradually appear further south. On their north-bound journey, they leave South Africa rapidly, resulting in mean stopover durations of 140 days in the south and 180 days in the north. We found that swallows are now leaving northern parts of South Africa 8 days earlier than they did 20 years ago, and so shortened their stay in areas where they previously stayed the longest. By contrast, they did not shorten their stopover in other parts of South Africa, leading to a more synchronized departure across the country. Departure was related to environmental variability, measured through the Southern Oscillation Index. Our results suggest that these birds gain their extended breeding season in Europe partly by leaving South Africa earlier, and thus add to scarce evidence for phenology shifts in the Southern Hemisphere.     

Predicting bird phenology from space: satellite‐derived vegetation green‐up signal uncovers spatial variation in phenological synchrony between birds and their environment

Population‐level studies of how tit species (Parus spp.) track the changing phenology of their caterpillar food source have provided a model system allowing inference into how populations can adjust to changing climates, but are often limited because they implicitly assume all individuals experience similar environments. Ecologists are increasingly using satellite‐derived data to quantify aspects of animals' environments, but so far studies examining phenology have generally done so at large spatial scales. Considering the scale at which individuals experience their environment is likely to be key if we are to understand the ecological and evolutionary processes acting on reproductive phenology within populations. Here, we use time series of satellite images, with a resolution of 240 m, to quantify spatial variation in vegetation green‐up for a 385‐ha mixed‐deciduous woodland. Using data spanning 13 years, we demonstrate that annual population‐level measures of the timing of peak abundance of winter moth larvae (Operophtera brumata) and the timing of egg laying in great tits (Parus major) and blue tits (Cyanistes caeruleus) is related to satellite‐derived spring vegetation phenology. We go on to show that timing of local vegetation green‐up significantly explained individual differences in tit reproductive phenology within the population, and that the degree of synchrony between bird and vegetation phenology showed marked spatial variation across the woodland. Areas of high oak tree (Quercus robur) and hazel (Corylus avellana) density showed the strongest match between remote‐sensed vegetation phenology and reproductive phenology in both species. Marked within‐population variation in the extent to which phenology of different trophic levels match suggests that more attention should be given to small‐scale processes when exploring the causes and consequences of phenological matching. We discuss how use of remotely sensed data to study within‐population variation could broaden the scale and scope of studies exploring phenological synchrony between organisms and their environment.