Narrow-front loop migration in a population of the common cuckoo Cuculus canorus, as revealed by satellite telemetry

Seasonal migration patterns of Siberian Rubythroat (Calliope calliope) facing the Qinghai-Tibet Plateau

BACKGROUND

Small songbirds respond and adapt to various geographical barriers during their annual migration. Global flyways reveal the diverse migration strategies in response to different geographical barriers, among which are high-elevation plateaus. However, few studies have been focused on the largest and highest plateau in the world, the Qinghai-Tibet Plateau (QTP) which poses a significant barrier to migratory passerines. The present study explored the annual migration routes and strategies of a population of Siberian Rubythroats (Calliope calliope) that breed on the north-eastern edge of the QTP.

METHODS

Over the period from 2021 to 2023, we applied light-level geolocators (13 deployed, seven recollected), archival GPS tags (45 deployed, 17 recollected), and CAnMove multi-sensor loggers (with barometer, accelerometer, thermometer, and light sensor, 20 deployed, six recollected) to adult males from the breeding population of Siberian Rubythroat on the QTP. Here we describe the migratory routes and phenology extracted or inferred from the GPS and multi-sensor logger data, and used a combination of accelerometric and barometric data to describe the elevational migration pattern, flight altitude, and flight duration. All light-level geolocators failed to collect suitable data.

RESULTS

Both GPS locations and positions derived from pressure-based inference revealed that during autumn, the migration route detoured from the bee-line between breeding and wintering grounds, leading to a gradual elevational decrease. The spring route was more direct, with more flights over mountainous areas in western China. This different migration route during spring probably reflects a strategy for faster migration, which corresponds with more frequent long nocturnal migration flights and shorter stopovers during spring migration than in autumn. The average flight altitude (1856 ± 781 m above sea level) was correlated with ground elevation but did not differ between the seasons.

CONCLUSIONS

Our finding indicates strong, season-dependent impact of the Qinghai-Tibet Plateau on shaping passerine migration strategies. We hereby call for more attention to the unexplored central-China flyway to extend our knowledge on the environment-migration interaction among small passerines.

The effects of cities on quail (Coturnix coturnix) migration: a disturbing story of population connectivity, health, and ecography

The increasing impact of human activities on ecosystems is provoking a profound and dangerous effect, particularly in wildlife. Examining the historical migration patterns of quail (Coturnix coturnix) offers a compelling case study to demonstrate the repercussions of human actions on biodiversity. Urbanization trends, where people gravitate toward mega-urban areas, amplify this effect. The proliferation of artificial urban ecosystems extends its influence across every biome, as human reliance on infrastructure and food sources alters ecological dynamics extensively. We examine European quail migrations pre- and post-World War II and in the present day. Our study concentrates on the Italian peninsula, investigating the historical and contemporary recovery of ringed quail populations. To comprehend changes in quail migration, we utilize trajectory analysis, open statistical data, and linear generalized models. We found that while human population and economic growth have shown a linear increase, quail recovery rates exhibit a U-shaped trajectory, and cereal and legume production displays an inverse U-shaped pattern. Generalized linear models have unveiled the significant influence of several key factors-time periods, cereal and legume production, and human demographics-on quail recovery rates. These factors closely correlate with the levels of urbanization observed across these timeframes. These insights underscore the profound impact of expanding human populations and the rise of mega-urbanization on ecosystem dynamics and services. As our planet becomes more urbanized, the pressure on ecosystems intensifies, highlighting the urgent need for concerted efforts directed toward conserving and revitalizing ecosystem integrity. Simultaneously, manage the needs and demands of burgeoning mega-urban areas. Achieving this balance is pivotal to ensuring sustainable coexistence between urban improvement and the preservation of our natural environment.

Spring arrival of the common cuckoo at breeding grounds is strongly determined by environmental conditions in tropical Africa

Failure to adapt migration timing to changes in environmental conditions along migration routes and at breeding locations can result in mismatches across trophic levels, as occurs between the brood parasitic common cuckoo Cuculus canorus and its hosts. Using satellite tracking data from 87 male cuckoos across 11 years, we evaluate why the cuckoo has not advanced its arrival to the UK. Across years, breeding ground arrival was primarily determined by timing of departure from stopover in West Africa before northward crossing of the Sahara. Together with high population synchrony and low apparent endogenous control of this event, this suggests that a seasonal ecological constraint operating here limits overall variation in breeding grounds arrival, although this event was itself influenced by carry-over from timing of arrival into tropical Africa. Between-year variation within individuals was, in contrast, mostly determined by northward migration through Europe, probably due to weather conditions. We find evidence of increased mortality risk for (a) early birds following migration periods positively impacting breeding grounds arrival, and (b) late birds, possibly suffering energy limitation, after departure from the breeding grounds. These results help identify areas where demands of responding to global change can potentially be alleviated by improving stopover quality.

Satellite tracking resident songbirds in tropical forests

Advances in tracking technology have helped elucidate the movements of the planet's largest and most mobile species, but these animals do not represent faunal diversity as a whole. Tracking a more diverse array of animal species will enable testing of broad ecological and evolutionary hypotheses and aid conservation efforts. Small and sedentary species of the tropics make up a huge part of earth's animal diversity and are therefore key to this endeavor. Here, we investigated whether modern satellite tracking is a viable means for measuring the fine-scale movement patterns of such animals. We fitted five-gram solar-powered transmitters to resident songbirds in the rainforests of New Guinea, and analyzed transmission data collected over four years to evaluate movement detection and performance over time. Based upon the distribution of location fixes, and an observed home range shift by one individual, there is excellent potential to detect small movements of a few kilometers. The method also has clear limitations: total transmission periods were often short and punctuated by lapses; precision and accuracy of location fixes was limited and variable between study sites. However, impending reductions in transmitter size and price will alleviate many issues, further expanding options for tracking earth's faunal diversity.

Modeling Complex Seasonal Avian Migration: Predictions From the Thermal Environment and Resource Availability

Billions of birds undertake long-distance migration and the complexity of schedules has only recently become clear. Such movements occur as a response to seasonality but the ultimate drivers of these changing distributions remain difficult to study directly. Modeling seasonal distributions based fundamentally on climate and vegetation without parameterizing with empirical data, we focus on the potential role of ambient temperature and available resources in shaping the migratory program. We simulate the complete annual cycle over the Afro-Palearctic region in a round-trip migration model allowing full variation in the extent and timing of movement, and multiple stopovers. The resultant simulated tracks and associated environmental metrics are interrogated: we evaluate the thermal and resource consequences of staying in Europe versus crossing the Sahara, and secondly identify the movement patterns optimizing exposure to green vegetation and local surpluses. There is a distinct thermal gain from crossing the Sahara and the pattern emerging of optimal seasonal vegetation resembles contemporary migration routes regarding Sahara crossing, loop structure and itinerancy. Thus, our first-principle simulations suggest that variations in migration patterns among species are caused by a complex trade-off between risks and rewards of staying versus moving, including innate physiological constraints and the resultant gain of the high-risk Sahara crossing.

Differences in on-ground and aloft conditions explain seasonally different migration paths in Demoiselle crane

BACKGROUND

Although some migratory birds may take different routes during their outbound and inbound migration, the factors causing these differential migrations to and from the breeding grounds, have rarely been investigated. In Northeast Asia, Demoiselle crane (Anthropoides virgo) performs one of the most extreme "loop" migrations known to date. During outbound migration, they cross the Himalayas to non-breeding sites in northwest India. Contrastingly, during inbound migration to the breeding grounds, they fly around the western end of the Himalayas. We hypothesise that differences in prevailing environmental conditions aloft and/or on-ground during both seasonal migrations are at the core of this phenomenon.

METHODS

Based on the tracking data of 16 individuals of tagged Demoiselle crane, we compared conditions during actual migration with those of simulated "reverse" migration (i.e. by adding 180 degrees to the flight direction and adding and subtracting half a year to the timestamps of outbound and inbound migration, respectively).

RESULTS

The comparison of actual and simulated "reverse" migration indicated that cranes would have encountered poorer aloft (wind support and thermal uplift) and on-ground conditions (temperature) if they had migrated in a reverse outbound migration and poorer on-ground conditions (Normalised Difference Vegetation Indexes [NDVI]) if they had migrated in a reverse inbound direction.

CONCLUSIONS

Our analyses suggest that both on-ground and aloft conditions play a key role in explaining Demoiselle cranes' loop migration, during the periods that they chose to use these alternative routes. Knowledge on the determinants of (differential) migration routes allow predicting migration decisions and may be critical in mitigating global change effects on animal migrations.

Host-dependent dispersal demonstrates both-sex host specificity in cuckoos

In a parasite species, the dispersal of individuals should be dependent on the host species to which they are specialized; thus, any sexual/individual difference in host specificity may influence their dispersal patterns and, hence, population genetic structures. However, such predictions remain poorly verified in generalist avian brood parasites that are composed of multiple lineages of host-specific races. Here, we show the dispersal consequences inferred from spatial genetic structures and their association with host specificity in brood parasitic common cuckoos Cuculus canorus in which female-specific host race formation has been widely accepted. Genetic sampling from adult cuckoos confirmed restricted dispersal in both sexes and resultant genetic structures between populations where different host species breed allopatrically, whereas it was not the case between distant areas inhabited by the same host species. Contrary to the female host race hypothesis, our results demonstrate that male cuckoos may also have host specificity and disperse accordingly, conclusively allowing us to hypothesize the formation of a host race including both sexes.

Flying on their own wings: young and adult cuckoos respond similarly to long-distance displacement during migration

Common cuckoos Cuculus canorus are obligate nest parasites yet young birds reach their distant, species-specific wintering grounds without being able to rely on guidance from experienced conspecifics - in fact they never meet their parents. Naïve marine animals use an inherited navigational map during migration but in inexperienced terrestrial animal migrants unequivocal evidence of navigation is lacking. We present satellite tracking data on common cuckoos experimentally displaced 1,800 km eastward from Rybachy to Kazan. After displacement, both young and adult travelled similarly towards the route of non-displaced control birds. The tracking data demonstrate the potential for young common cuckoos to return to the species-specific migration route after displacement, a response so far reported exclusively in experienced birds. Our results indicate that an inherited map allows first-time migrating cuckoos to locate suitable wintering grounds. This is in contrast to previous studies of solitary terrestrial bird migrants but similar to that reported from the marine environment.

Releases of Asian houbara must respect genetic and geographic origin to preserve inherited migration behaviour: evidence from a translocation experiment

Maintaining appropriate migratory strategies is important in conservation; however, translocations of migratory animals may alter locally evolved migration behaviours of recipient populations if these are different and heritable. We used satellite telemetry and experimental translocation to quantify differences and assess heritability in migration behaviours between three migratory Asian houbara (Chlamydotis macqueenii) breeding populations (640 km range across eastern, central and western Uzbekistan). Adults from the eastern population migrated twice as far (mean = 1184 km ± 44 s.e.) as the western population (656 km ± 183 s.e.) and showed significantly less variation in migration distance than the central population (1030 km ± 127 s.e.). The western and central populations wintered significantly further north (mean: +8.32° N ± 1.70 s.e. and +4.19° N ± 1.16 s.e., respectively) and the central population further west (-3.47° E ± 1.46 s.e.) than individuals from the eastern population. These differences could arise from a differing innate drive, or through learnt facultative responses to topography, filtered by survival. Translocated birds from the eastern population (wild-laid and captive-reared, n = 5) migrated further than adults from either western or central recipient populations, particularly in their second migration year. Translocated birds continued migrating south past suitable wintering grounds used by the recipient populations despite having to negotiate mountain obstacles. Together, this suggests a considerable conserved heritable migratory component with local adaptation at a fine geographic scale. Surviving translocated individuals returned to their release site, suggesting that continued translocations would lead to introgression of the heritable component and risk altering recipient migration patterns. Conservation biologists considering translocation interventions for migratory populations should evaluate potential genetic components of migratory behaviour.

Terrestrial Bird Migration and West Nile Virus Circulation, United States

Host migration and emerging pathogens are strongly associated, especially with regard to zoonotic diseases. West Nile virus (WNV), a mosquitoborne pathogen capable of causing severe, sometimes fatal, neuroinvasive disease in humans, is maintained in highly mobile avian hosts. Using phylogeographic approaches, we investigated the relationship between WNV circulation in the United States and the flight paths of terrestrial birds. We demonstrated southward migration of WNV in the eastern flyway and northward migration in the central flyway, which is consistent with the looped flight paths of many terrestrial birds. We also identified 3 optimal locations for targeted WNV surveillance campaigns in the United States—Illinois, New York, and Texas. These results illustrate the value of multidisciplinary approaches to surveillance of infectious diseases, especially zoonotic diseases.

Territoriality and variation in home range size through the entire annual range of migratory great spotted cuckoos (Clamator glandarius)

Variation in home range size throughout the year and its causes are not well understood yet. Migratory brood parasites offer a unique opportunity to incorporate this spatio-temporal dimension into the study of the factors regulating home range dynamics. Using satellite transmitters, we tracked sixteen migratory great spotted cuckoos (Clamator glandarius) of both sexes for up to three years. We constructed home ranges in all major staging areas, from the Spanish breeding areas to the African wintering grounds, analyzed their temporal and geographical variation and investigated their main potential determinants (e.g. food and host availability). We found that home ranges were significantly larger in the breeding area compared to non-breeding areas. Using NDVI as a proxy for food availability, we showed that breeding area home ranges have significantly lower food availability per km² than home ranges elsewhere which could explain why cuckoos use alternative areas with higher food availability before initiating migration. We also found some evidence for sex differences. Additionally, we found no indications of territoriality in this species, providing novel information into the current debate on brood parasite territoriality. Overall, food availability seems to be an important factor regulating home range dynamics and influencing migratory patterns throughout the year in great spotted cuckoos.

Migration behavior and performance of the great spotted cuckoo (Clamator glandarius)

The study of brood parasitism has traditionally been focused on the breeding period, but recent evidence suggests that it urgently needs a new spatio-temporal perspective to explore novel avenues on brood parasite-host co-evolutionary interactions. Many brood parasites are migrants, but their ecology outside their short breeding season is poorly known. The great spotted cuckoo (Clamator glandarius) is one of the classical models in the study of brood parasitism, however, there is very little information on its migratory strategy, route and wintering grounds. Furthermore, there is no previous information on the geographical distribution of mortality and its causes in this species; information that is critical to understand the fluctuations in cuckoo populations and detect potential conservation risks. Using satellite tracking technology, we provide novel insight into the migratory behavior and performance of the great spotted cuckoo. We found individuals from southern Spain to be long-distance nocturnal migrants that use the East Atlantic Flyway for both post and pre-breeding migration, and that winter in the western Sahel. We found evidence of individual variation in their migration route, particularly regarding their post-breeding behavior in Spain. Our study also suggests that the south of Morocco is the most dangerous area due to a large number of deaths during the post-breeding migratory period. Furthermore, we found that natural predation seems to be the main cause of death, probably due to raptors, although human activities (i.e. hunting) could also played a role in the southern Mediterranean shore. Our study offers novel findings and challenges traditional ideas on the ecology of this species providing a good example of how the new spatio-temporal perspective can expand our knowledge on brood parasites.

The start of migration correlates with arrival timing, and the total speed of migration increases with migration distance in migratory songbirds: a cross-continental analysis

BACKGROUND

Anthropogenic changes in the climate and environment have globally affected ecological processes such that the spatiotemporal occurrence of the main annual cycle events (i.e., breeding, wintering, moulting, and migration) has shifted in migratory birds. Variation in arrival timing at migratory destinations can be proximately caused by an altered start of migration, total migration distance, and/or total speed of migration. Quantifying the relative contributions of these causes is important because this will indicate the mechanisms whereby birds could potentially adjust their annual cycle in response to global change. However, we have relatively little quantitative information about how each of these factors contributes to variation in arrival timing. My main aims are to estimate how arrival timing is correlated with variation in the start of migration and the total migration distance and how the total speed of migration may change with the total migration distance and body mass in a comprehensive analysis including multiple species.

METHODS

For this purpose, I considered individual tracks covering complete migrations from multiple species and distinguished between within- and between-species effects.

RESULTS

Assuming that the within- and between-species effects quantified under this approach agree with the effects acting at the individual level, starting migration one day later or increasing the total migration distance by 1000 km would result in later arrival timing by 0.4-0.8 days or 2-5 days, respectively. The generality with which the start of migration is correlated with arrival timing within species suggests that this is the general biological mechanism regulating arrival timing, rather than the total migration distance. The total speed of migration was positively correlated with the total migration distance but not with the bird's body mass.

CONCLUSIONS

As the start of migration is endogenously controlled and/or affected by hatching date, directional selection can probably act on existing within-species/within-population variation to alter arrival timing. This factor and the importance of variation in the start of migration for arrival timing suggest that migratory species/populations in which there is sufficient variation in the start of migration and transgenerational processes affect the corresponding timing may present an advantage over others in coping with anthropogenic-induced global changes.

Ten grams and 13,000 km on the wing - route choice in willow warblers Phylloscopus trochilus yakutensis migrating from Far East Russia to East Africa

BACKGROUND

High-latitude bird migration has evolved after the last glaciation, in less than 10,000-15,000 years. Migrating songbirds rely on an endogenous migratory program, encoding timing, fueling, and routes, but it is still unknown which compass mechanism they use on migration. We used geolocators to track the migration of willow warblers (Phylloscopus trochilus yakutensis) from their eastern part of the range in Russia to wintering areas in sub-Saharan Africa. Our aim was to investigate if the autumn migration route can be explained by a simple compass mechanism, based on celestial or geomagnetic information, or whether migration is undertaken as a sequence of differential migratory paths possibly involving a map sense. We compared the recorded migratory routes for our tracked birds with simulated routes obtained from different compass mechanisms.

RESULTS

The three tracked males were very similar in the routes they took to their final wintering sites in southern Tanzania or northern Mozambique, in their use of stopover sites and in the overall timing of migration. None of the tested compass mechanisms could explain the birds' routes to the first stopover area in southwest Asia or to the destination in Southeast Africa without modifications. Our compass mechanism simulations suggest that the simplest scenarios congruent with the observed routes are based on either an inclination or a sun compass, assuming two sequential steps.

CONCLUSIONS

The birds may follow a magnetoclinic route coinciding closely with the tracks by first moving west, i.e. closer to the goal, and thereafter follow a constant apparent angle of inclination to the stopover site. An alternative would be to use the sun compass, but with time-adjustments along the initial part of the migration to the first stopover, and thereafter depart along a new course to the winter destination. A combination of the two mechanisms cannot be ruled out, but needs to be confirmed in future studies.

From Svalbard to Siberia: Passerines breeding in the High Arctic also endure the extreme cold of the Western Steppe

Few species are adapted to high latitudes, and many over-winter in milder climates with migrations involving extensive barrier crossings. By escaping extreme conditions for the majority of the year, physiological and behavioural adaptations presumably need to be less pronounced. The snow bunting Plectrophenax nivalis is the most northerly breeding passerine. We tracked the Svalbard population using geolocators to reveal that these individuals not only breed in environmental extremes, but also spend the winters in the severe cold and highly stochastic weather conditions of the Siberian steppe. Migratory strategies appeared to be flexible between individuals and years. However, common wintering grounds in the Asian Western Steppe were identified, where birds could utilise vast crop- and grasslands while enduring low ambient temperatures. The timing of significant long distance movements was consistent among individuals, and the autumn routing of the >1000 km open water flight to Novaya Zemlya incurred favourable wind assistance and lower risk of precipitation, compared to the shorter route between Svalbard and Norway used in spring. Presumably, Svalbard snow buntings are physiologically well-adapted to extreme conditions and their migration, rather being a retreat from physiologically demanding conditions, allows utilisation of an abundance of resources in the Asian Steppe.

Consistent individual and sex-specific differences in behaviour of common cuckoo chicks: is there a potential impact on host-parasite coevolutionary dynamics?

Research on brood parasitism has focused primarily on specific host anti-parasite behaviours and parasite counter-adaptations, and little is known about other aspects of their behaviours such as consistent behavioural differences between individuals. Therefore, we examined consistency in behaviour of nestlings of common cuckoos (Cuculus canorus) raised by great reed warblers (Acrocephalus arundinaceus). Cuckoo chicks showed high repeatability of both aggressive behaviour and breath rate, and both traits were strongly correlated with each other. This represents the first evidence for consistent differences in behaviour among avian brood parasites. Males were consistently more aggressive and less stressed than females. Nestlings of both sexes that hatched later in the season exhibited higher levels of aggression and lower stress responses than nestlings hatched earlier. This suggests that rearing conditions (e.g., food availability and quality) may modulate stress and aggressive phenotypes of brood parasites. We discuss potential effects of the observed patterns on host-parasite dynamics.

Timing avian long-distance migration: from internal clock mechanisms to global flights

Migratory birds regularly perform impressive long-distance flights, which are timed relative to the anticipated environmental resources at destination areas that can be several thousand kilometres away. Timely migration requires diverse strategies and adaptations that involve an intricate interplay between internal clock mechanisms and environmental conditions across the annual cycle. Here we review what challenges birds face during long migrations to keep track of time as they exploit geographically distant resources that may vary in availability and predictability, and summarize the clock mechanisms that enable them to succeed. We examine the following challenges: departing in time for spring and autumn migration, in anticipation of future environmental conditions; using clocks on the move, for example for orientation, navigation and stopover; strategies of adhering to, or adjusting, the time programme while fitting their activities into an annual cycle; and keeping pace with a world of rapidly changing environments. We then elaborate these themes by case studies representing long-distance migrating birds with different annual movement patterns and associated adaptations of their circannual programmes. We discuss the current knowledge on how endogenous migration programmes interact with external information across the annual cycle, how components of annual cycle programmes encode topography and range expansions, and how fitness may be affected when mismatches between timing and environmental conditions occur. Lastly, we outline open questions and propose future research directions.This article is part of the themed issue 'Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals'.

Route simulations, compass mechanisms and long-distance migration flights in birds

Bird migration has fascinated humans for centuries and routes crossing the globe are now starting to be revealed by advanced tracking technology. A central question is what compass mechanism, celestial or geomagnetic, is activated during these long flights. Different approaches based on the geometry of flight routes across the globe and route simulations based on predictions from compass mechanisms with or without including the effect of winds have been used to try to answer this question with varying results. A major focus has been use of orthodromic (great circle) and loxodromic (rhumbline) routes using celestial information, while geomagnetic information has been proposed for both a magnetic loxodromic route and a magnetoclinic route. Here, we review previous results and evaluate if one or several alternative compass mechanisms can explain migration routes in birds. We found that most cases could be explained by magnetoclinic routes (up to 73% of the cases), while the sun compas s could explain only 50%. Both magnetic and geographic loxodromes could explain <25% of the routes. The magnetoclinic route functioned across latitudes (1°S-74°N), while the sun compass only worked in the high Arctic (61-69°N). We discuss the results with respect to orientation challenges and availability of orientation cues.

Actogram analysis of free-flying migratory birds: new perspectives based on acceleration logging

The use of accelerometers has become an important part of biologging techniques for large-sized birds with accelerometer data providing information about flight mode, wing-beat pattern, behaviour and energy expenditure. Such data show that birds using much energy-saving soaring/gliding flight like frigatebirds and swifts can stay airborne without landing for several months. Successful accelerometer studies have recently been conducted also for free-flying small songbirds during their entire annual cycle. Here we review the principles and possibilities for accelerometer studies in bird migration. We use the first annual actograms (for red-backed shrike Lanius collurio) to explore new analyses and insights that become possible with accelerometer data. Actogram data allow precise estimates of numbers of flights, flight durations as well as departure/landing times during the annual cycle. Annual and diurnal rhythms of migratory flights, as well as prolonged nocturnal flights across desert barriers are illustrated. The shifting balance between flight, rest and different intensities of activity throughout the year as revealed by actogram data can be used to analyse exertion levels during different phases of the life cycle. Accelerometer recording of the annual activity patterns of individual birds will open up a new dimension in bird migration research.

Simplification, Innateness, and the Absorption of Meaning from Context: How Novelty Arises from Gradual Network Evolution

How does new genetic information arise? Traditional thinking holds that mutation happens by accident and then spreads in the population by either natural selection or random genetic drift. There have been at least two fundamental conceptual problems with imagining an alternative. First, it seemed that the only alternative is a mutation that responds "smartly" to the immediate environment; but in complex multicellulars, it is hard to imagine how this could be implemented. Second, if there were mechanisms of mutation that "knew" what genetic changes would be favored in a given environment, this would have only begged the question of how they acquired that particular knowledge to begin with. This paper offers an alternative that avoids these problems. It holds that mutational mechanisms act on information that is in the genome, based on considerations of simplicity, parsimony, elegance, etc. (which are different than fitness considerations). This simplification process, under the performance pressure exerted by selection, not only leads to the improvement of adaptations but also creates elements that have the capacity to serve in new contexts they were not originally selected for. Novelty, then, arises at the system level from emergent interactions between such elements. Thus, mechanistically driven mutation neither requires Lamarckian transmission nor closes the door on novelty, because the changes it implements interact with one another globally in surprising and beneficial ways. Finally, I argue, for example, that genes used together are fused together; that simplification leads to complexity; and that evolution and learning are conceptually linked.

Honey buzzards don't always make a beeline

(a) European honey buzzards breeding in Western Europe primarily use soaring flight to make annual long-range migrations via the Strait of Gibraltar to winter in West Africa; this adult male was photographed on migration near Gibraltar. Photo: Javier Elloriaga. (b) Autumn migration routes of 12 satellite tagged adult European honey buzzards (colour-coded lines); compared with the shortest possible straight-line routes (dashed lines), most routes involved substantial westerly detours in Africa. Adapted from Vansteelant et al. (2016). (c) In contrast, Montagu's harriers predominantly use flapping flight during their migrations; this adult male is carrying a satellite transmitter. Photo: Theo van Kooten. (d) Autumn migration routes of 34 satellite tagged adult Montagu's harriers; migratory tracks more closely approached straight-line routes, and typically involved longer sea crossings, than seen in European honey buzzards. Adapted from Trierweiler et al. (). In Focus: Vansteelant, W.M.G., Shamoun-Baranes, J., van Manen, W., van Diermen, J. & Bouten, W. (2017) Seasonal detours by soaring migrants shaped by wind regimes along the East Atlantic Flyway. Journal of Animal Ecology, 86, 179-191. Migratory birds often make substantial detours from the shortest possible route during their annual migrations, which may potentially increase the duration and energetic cost of their journeys. Vansteelant et al. () investigate repeated migrations of adult European honey buzzards between the Netherlands and sub-Saharan Africa, and find that they make large westerly detours in Africa on both the spring and autumn routes. These detours allow migrants to capitalise on more favourable winds further along the route, thus reducing energy expenditure. Lifelong tracking studies will allow researchers to identify how migration routes have evolved to exploit predictable atmospheric and oceanic circulation patterns.

Resource tracking within and across continents in long-distance bird migrants

Migratory birds track seasonal resources across and between continents. We propose a general strategy of tracking the broad seasonal abundance of resources throughout the annual cycle in the longest-distance migrating land birds as an alternative to tracking a certain climatic niche or shorter-term resource surplus occurring, for example, during spring foliation. Whether and how this is possible for complex annual spatiotemporal schedules is not known. New tracking technology enables unprecedented spatial and temporal mapping of long-distance movement of birds. We show that three Palearctic-African species track vegetation greenness throughout their annual cycle, adjusting the timing and direction of migratory movements with seasonal changes in resource availability over Europe and Africa. Common cuckoos maximize the vegetation greenness, whereas red-backed shrikes and thrush nightingales track seasonal surplus in greenness. Our results demonstrate that the longest-distance migrants move between consecutive staging areas even within the wintering region in Africa to match seasonal variation in regional climate. End-of-century climate projections indicate that optimizing greenness would be possible but that vegetation surplus might be more difficult to track in the future.

First-Time Migration in Juvenile Common Cuckoos Documented by Satellite Tracking

Being an obligate parasite, juvenile common cuckoos Cuculus canorus are thought to reach their African wintering grounds from Palearctic breeding grounds without guidance from experienced conspecifics but this has not been documented. We used satellite tracking to study naïve migrating common cuckoos. Juvenile cuckoos left breeding sites in Finland moving slowly and less consistently directed than adult cuckoos. Migration of the juveniles (N = 5) was initiated later than adults (N = 20), was directed toward the southwest-significantly different from the initial southeast direction of adults-and included strikingly long Baltic Sea crossings (N = 3). After initial migration of juvenile cuckoos toward Poland, the migration direction changed and proceeded due south, directly toward the winter grounds, as revealed by a single tag transmitting until arrival in Northwest Angola where northern adult cuckoos regularly winter. Compared to adults, the juvenile travelled straighter and faster, potentially correcting for wind drift along the route. That both migration route and timing differed from adults indicates that juvenile cuckoos are able to reach proper wintering grounds independently, guided only by their innate migration programme.

Morphometrics and stable isotopes differentiate wintering populations of a migratory bird

BACKGROUND

Describing migratory connectivity in mobile animals is crucial for understanding the selective pressures acting on different populations throughout their life cycle. Tracking single individuals has provided valuable data, but for most species the data available are still spurious and usually limited to a few individuals. Since different populations of migratory birds can be distinguished by a combination of morphometric measurements and the isotopic composition of their feathers, it is possible to measure these parameters on a large sample to differentiate populations.

METHODS

We studied northern wheatears, Oenanthe oenanthe, captured in their African wintering range and applied discriminant analyses on morphometric measurements and stable isotope signatures to determine whether birds found in different areas were distinguishable from each other.

RESULTS

Morphometric and isotopic measurements alone were not sufficient to discriminate between the birds of ssp. oenanthe from different areas in Africa. When combining the two measurements, however, assignment to the different groups became substantially more accurate. Following the discriminant analysis of morphometrics and δ(2)H, δ(13)C, and δ(15)N isotopes signatures, 19 of 20 oenanthe from Kenya, 15 of 20 oenanthe from Mali/Mauritania, and 19 of 20 oenanthe from Niger were assigned correctly to their wintering area.

CONCLUSIONS

Our results show that birds at different wintering sites can be distinguished from each other when using a combination of markers. We discuss the possible breeding origins of these wintering birds.

Population decline is linked to migration route in the Common Cuckoo

Migratory species are in rapid decline globally. Although most mortality in long-distance migrant birds is thought to occur during migration, evidence of conditions on migration affecting breeding population sizes has been completely lacking. We addressed this by tracking 42 male Common Cuckoos from the rapidly declining UK population during 56 autumn migrations in 2011–14. Uniquely, the birds use two distinct routes to reach the same wintering grounds, allowing assessment of survival during migration independently of origin and destination. Mortality up to completion of the Sahara crossing (the major ecological barrier encountered in both routes) is higher for birds using the shorter route. The proportion of birds using this route strongly correlates with population decline across nine local breeding populations. Knowledge of variability in migratory behaviour and performance linked to robust population change data may therefore be necessary to understand population declines of migratory species and efficiently target conservation resources.

Whether conditions experienced on long-distance migrations affect breeding populations is not clear. Here, the authors tracked migrating Common Cuckoos from the UK to Africa and show that route choices affect mortality during migration, and population decline in this nocturnally migrating bird.

Flexible navigation response in common cuckoos Cuculus canorus displaced experimentally during migration

Migrating birds follow innate species-specific migration programs capable of guiding them along complex spatio-temporal routes, which may include several separate staging areas. Indeed, migration routes of common cuckoos Cuculus canorus show little variation between individuals; yet, satellite tracks of 11 experimentally displaced adults revealed an unexpected flexibility in individual navigation responses. The birds compensated for the translocation to unfamiliar areas by travelling toward population-specific staging areas, demonstrating true navigation capabilities. Individual responses varied from travelling toward the first stopover in northern Europe to flying toward the Central-African winter grounds, the latter including several stopovers in unfamiliar areas. Apparently, the cuckoos possess spatial knowledge far beyond their population-specific flyway scale, and make individual decisions likely based on an assessment of perceived gain and cost of alternative route options.

Global aerial flyways allow efficient travelling

Birds migrate over vast distances at substantial costs. The highly dynamic nature of the air makes the selection of the best travel route difficult. We investigated to what extent migratory birds may optimise migratory route choice with respect to wind, and if route choice can be subject to natural selection. Following the optimal route, calculated using 21 years of empirical global wind data, reduced median travel time by 26.5% compared to the spatially shortest route. When we used a time-dependent survival model to quantify the adaptive benefit of choosing a fixed wind-optimised route, 84.8% of pairs of locations yielded a route with a higher survival than the shortest route. This suggests that birds, even if incapable of predicting wind individually, could adjust their migratory routes at a population level. As a consequence, this may result in the emergence of low-cost flyways representing a global network of aerial migratory pathways.