Overwintering Grounds Research Articles

Temporal and spatial variation in reproductive benefits in a partial migrant.

In partial migrant systems, where residents and migrants coexist within a population, residents are commonly predicted to gain a reproductive advantage over migrants through priority access to high-quality territories and an earlier breeding start. Annual variation in reproductive benefits has been suggested to be important for the coexistence of both strategies in a population, as differences in wintering conditions experienced by the two strategies may result in a periodic reproductive advantage for migrants. However, the importance of spatial environmental variation for reproductive output in partially migrant populations remains largely unexplored. We investigated variation in the reproductive output of migrants and residents in a population of Swiss red kites (Milvus milvus) both temporally, across and within years, and spatially, along an elevational gradient. We gathered 4 years of reproductive data combined with 183 GPS-derived full annual cycles from individuals breeding in the Swiss Alpine foothills. At low, but not high, elevations, residents produced more fledglings than migrants. We also found evidence for annual variation in the reproductive advantage of the two strategies. Furthermore, while reproductive output did decline with a later breeding start, there was no difference in the start of breeding between the two migration strategies. The results of this study suggest that differences in reproductive output between migrants and residents in partial migrant populations can vary both due to the use of spatially distinct overwintering grounds and because the strategies are differently affected by spatial variables in the breeding area, such as elevation. The study emphasizes that spatial and temporal variation in reproductive benefits must be considered when predicting how migratory species will respond to future environmental change.

A Multi-Scale Species Distribution Model for Migrating and Overwintering Western Monarch Butterflies: Climate Is the Best Predictor

Western Monarch butterflies (Danaus plexippus) migrate from inland breeding ranges to coastal overwintering grounds in California. Given that migratory individuals may make multi-scale habitat selection decisions, we considered a multi-scale species distribution model (SDM) using range-wide climatic and local landscape-level predictors of migratory and overwintering habitat and community-science presence data. The range-wide model output was included as a predictor in the local-scale model, generating multi-scale habitat suitability. The top range-wide predictor was the minimum temperature in December, contributing 83.7% to the model, and was positively associated with presence. At the local scale, the strongest predictors of presence were the range-wide output and percent coverage of low and medium levels of development, contributing > 95%, with 61–63% from the range-wide output, with local-scale suitability coinciding with the California coastal zones. Development’s positive association with overwintering monarch presence was counterintuitive. It is likely that our local-scale model is overfit to these development zones, but it is unclear whether this overfitting resulted from modeler choices, monarchs overwintering close to human development, biased detection near human development, or a combination of these factors. Therefore, alternative approaches to collecting local-scale attribute data are suggested while recognizing the primacy of climate in restricting overwinter sites.

Temperature and body size affect movement of juvenile Atlantic cod (Gadus morhua) and saithe (Pollachius virens) at nearshore nurseries.

Seasonal migrations of marine fish between shallow summer feeding habitats and deep overwintering grounds are driven by fluctuations in the biotic and abiotic environment as well as by changes in the internal state. Ontogenetic shifts in physiology and metabolism affect the response to environmental drivers and may lead to changes in migration timing and propensity. In this study, we investigated the effect of temperature and body size on migration timing and depth distribution in acoustically tagged Atlantic cod, Gadus morhua, and saithe, Pollachius virens, during the period of seasonal migration from shallow summer habitats. The results from our study revealed a wide range of horizontal and vertical distribution of age 1 and 2 G. morhua within the fjord. Larger G. morhua inhabited deeper, cooler waters than smaller juveniles, likely reflecting size-dependent thermal preferences and predation pressure. Conversely, juvenile P. virens occupied primarily shallow waters close to land. The variation in depth distribution of G. morhua was mainly explained by body size and not, against our predictions, by water temperature. Conversely, the dispersal from the in-fjord habitats occurred when water temperatures were high, suggesting that seasonal temperature fluctuations can trigger the migration timing of P. virens and larger G. morhua from summer habitats. Partial migration of small juvenile G. morhua from in-fjord foraging grounds, likely influenced by individual body condition, suggested seasonal migration as a flexible strategy that individuals may use to reduce predation and energetic expenditure. Predation mortality rates of tagged juveniles were higher than previously suggested and are the first robust predation mortality rates for juvenile G. morhua and P. virens estimated based on acoustic transmitters with acidity sensors. The results have relevance for climate-informed marine spatial planning as under the scenario of increasing ocean temperatures, increasing summer temperatures may reduce the juveniles' resource utilization in the shallow summer nurseries, resulting in lower growth rates, increased predation pressure, and lower chances of juvenile winter survival.

Time-compensated sun compass in juvenile sprat (Sprattus sprattus) reveals the onset of migratory readiness.

Sprat (Sprattus sprattus) is one of the most commercially exploited fish species in the Baltic Sea and expresses a pronounced seasonal migration pattern. Spawning takes place, among other places, in the Kiel Bight and Kiel Fjord in early summer. Juvenile sprat leave the nursery areas in late summer/early autumn to move to their feeding and overwintering grounds. What kind of orientation mechanisms sprat use for migration is not known yet. This study shows that juvenile sprat can use a time-compensated sun compass, heading towards the northeast, in the direction of their proposed overwintering grounds in Bornholm Basin. The sprats tested at the end of August oriented themselves in the predicted direction, whereas the sprats tested at the beginning of August only showed a random orientation. For the first time, this demonstrates the onset of migratory readiness in juvenile sprat, indicating the preparation for starting their migration.

Got milkweed? Genetic assimilation as potential source for the evolution of nonmigratory monarch butterfly wing shape.

Monarch butterflies (Danaus plexippus) are well studied for their annual long-distance migration from as far north as Canada to their overwintering grounds in Central Mexico. At the end of the cold season, monarchs start to repopulate North America through short-distance migration over the course of multiple generations. Interestingly, some populations in various tropical and subtropical islands do not migrate and exhibit heritable differences in wing shape and size, most likely an adaptation to island life. Less is known about forewing differences between long- and short-distance migrants in relation to island populations. Given their different migratory behaviors, we hypothesized that these differences would be reflected in wing morphology. To test this, we analyzed forewing shape and size of three different groups: nonmigratory, lesser migratory (migrate short-distances), and migratory (migrate long-distances) individuals. Significant differences in shape appear in all groups using geometric morphometrics. As variation found between migratory and lesser migrants has been shown to be caused by phenotypic plasticity, and lesser migrants develop intermediate forewing shapes between migratory and nonmigratory individuals, we suggest that genetic assimilation might be an important mechanism to explain the heritable variation found between migratory and nonmigratory populations. Additionally, our research confirms previous studies which show that forewing size is significantly smaller in nonmigratory populations when compared to both migratory phenotypes. Finally, we found sexual dimorphism in forewing shape in all three groups, but for size in nonmigratory populations only. This might have been caused by reduced constraints on forewing size in nonmigratory populations.

Changes in Overwintering Ground of Small Yellow Croaker (Larimichthys polyactis) Based on MaxEnt and GARP Models: A Case Study of the Southern Yellow Sea Stock.

Small yellow croaker (Larimichthys polyactis), a commercially essential fish commonly caught in China and South Korea, is now facing a severe decline in resources. The recruitment and surplus of L. polyactis depend selecting a suitable marine environment for overwintering. However, the international overwintering migration habit of L. polyactis limits the investigation of its overwintering environment preferences and suitable grounds. In this study, based on the distribution data of L. polyactis in the southern Yellow Sea in winter from 2010 to 2019 and the ocean remote sensing data such as sea bottom temperature (SBT), sea bottom salinity, chlorophyll-a concentration and water depth (Depth), we used the maximum entropy (MaxEnt) and the genetic algorithm for rule-set production (GARP) models to investigate the overwintering grounds of the southern Yellow Sea stock (SYS). The jackknife test was used to assess the importance of various environmental factors. For modelling the overwintering ground distribution of SYS, the area under the curve values of both models ​​were higher than 0.9. The overwintering ground was at 32°10'N-33°48'N, 122°30'E-125°00'E. The direction of its distribution was consistent with the Yellow Sea Warm Current in the southern Yellow Sea during the winter. Compared with the suitable overwintering area during 2010-2014, the highly appropriate overwintering area for SYS to overwinter decreased significantly during 2015-2019, showing a trend of moving to the east and north, related to the increase in fishing pressure and strengthening of the Yellow Sea Warm Current in recent years. Depth was the most significant factor for SYS overwintering, followed by SBT. The overwintering ground was at a depth of 40-65 m during the two periods. Additionally, the suitability of overwintering grounds in the coastal waters of southwestern South Korea has gradually increased. This study provides a scientific basis for formulating effective strategies to manage L. polyactis resources under the China-South Korea Fisheries Agreement. This article is protected by copyright. All rights reserved.

Isotopic Evidence of Population Connectivity at Multiple Life Stages for Larimichthys polyactis in the Southern Yellow Sea and East China Sea

Larimichthys polyactis, a typically oceanodromous species found across the East China, Yellow, and Bohai Seas, is one of the most ecologically and commercially important species in East Asian countries. Moreover, over the past decades, L. polyactis has experienced significant changes in its biological characteristics under long-term intensive fishing pressure, suggesting a need for urgent protection. A robust understanding of connectivity at the spatiotemporal scale is critical for the effective and thorough management of L. polyactis. In the present study, the otolith stable carbon (δ13C) and oxygen (δ18O) isotope ratios of four adult L. polyactis groups (Y-1–4) sampled in the southern Yellow and East China Seas (SYS and ECS) were determined at multiple life stages, including the larval dispersal period (core), overwintering period (A1), and one whole life cycle except the post-spawning period (A2). Dunn’s multiple comparison test showed that there was no difference among the Y-1, Y-2, and Y-3 groups in the otolith core and A1 zones (p > 0.05); the otolith δ13C and δ18O of the Y-4 group were significantly higher than those of the Y-3 group (p < 0.05) in the three analysed zones. In the otolith A2 zone, δ13C and δ18O of the Y-1 group were significantly higher than those of the Y-3 group (p < 0.05). Combined with the biplot of otolith δ13C and δ18O, the quadratic discriminant analysis (QDA) of the otolith core, A1 and A2 zones showed that: (1) in the otolith core zone, two dominant larval aggregations occurred, likely distributed in the coastal waters of northern Lvsi and Haizhou Bay fishing grounds (Y-4 and part Y-1/2 groups) and Zhoushan and southern Yangtze Estuary fishing grounds (Y-3 and part Y-1/2); (2) in the otolith A1 zone, two clear aggregations were separated by Y-4 and Y-1/2/3, suggesting two dominant groups overwintering in the SYS and ECS, respectively; and (3) in the otolith A2 zone, two dominant aggregations were separated by Y-4/1 and Y-2/3. In addition to the long-term larval dispersal process, the migratory route between spawning and overwintering grounds may be diverse, explaining the non-significant genetic differentiation of L. polyactis from different groups. Combining these results with those of previous studies on the life history of L. polyactis, a schematic map of the migratory routes is depicted providing important references for effective resource management.

Contrasting stock status trends obtained from survey and fishery CPUE, taking Larimichthys polyactis in Yellow Sea Large Marine Ecosystem as an example

Biological conservation of exploited fish species involves characterizing key aspects of their population dynamics using models as tools to estimate their biomass. The Bayesian state-space surplus production model in the open-source stock assessment tool Just Another Bayesian Biomass Assessment (JABBA) was used to assess small yellow croaker stock (Larimichthys polyactis) in the Yellow Sea large marine ecosystem (YSLME). In this study, Catch and the scientific survey catch per unit effort (CPUE) data of the overwintering grounds of small yellow croaker from 1985 to 2020 and three fishery CPUE data series (one original and two reconstructed) from the Chinese Fishery Statistical Yearbook were used to fit JABBA, respectively. The results showed that the trends in biomass obtained from the survey CPUE and from the fishery CPUE contrasted sharply. The independent survey CPUE-based JABBA showed a 54.4 % probability that the current resource status is over exploited (0.65 and 1.06 for B2020/BMSY and F2020/FMSY, respectively), whereas the fishery CPUE-based JABBA showed that the resource is in a healthy or recovering state (0.88–1.32 and 0.40–0.70 for B2020/BMSY and F2020/FMSY, respectively), and the estimates of the resource are overly optimistic. This discrepancy in biomass assessment arises because the fishing effort time series does not adequately reflect the technological advances in fishing vessels and their equipment. Therefore, assessment based on survey CPUE (predicted total allowable catch, TAC = 150,000 tons) is the preferable, more precautionary approach for establishing management reference points and informing management decisions, and estimates generated from the fisheries CPUE-based JABBA model (TAC = 165,000–210,000 tons) should be treated with caution. We found that the two reconstructed fishery CPUEs are more likely to produce model results closer to that of the survey CPUE-based JABBA than the original fishery CPUE. Further review and research on the correction of fishing effort and fishery CPUE in the YSLME is recommended before this data series is used in assessments aimed at biological conservation.

Multi-Sensor Remote Sensing of Intertidal Flat Habitats for Migratory Shorebird Conservation

Many species of shorebirds migrate long distances from their overwintering grounds in the southern hemisphere to breeding grounds in the northern hemisphere. The coastal intertidal zone, consisting of sand and mud flats exposed at low tide and covered at high tide, is heavily used as a migratory stopover or overwintering habitat. Understanding the spatial distribution of sediment types at these stopover sites is a critical step for understanding habitat use by shorebird species. Due to their importance as overwintering and stopover habitat for the imperiled western Atlantic subpopulation of the shorebird, the red knot (Calidris canutus rufa), as well as other migratory shorebirds, the northern coast of Brazil between Pará and Maranhão, and Bahía Lomas in northern Tierra del Fuego, Chile, were selected for further investigation as to the applicability of remotely sensed characterization of the intertidal flat habitats. Examination of the Landsat 8 multispectral reflectance and Sentinel-1 SAR backscatter reveals that sand and mud represent endmembers at opposite ends of a continuous gradient in feature space. While remotely sensed data can be used to discriminate between mud and sand intertidal types, the spectral relationships varied between the two very different geographic locations. The inclusion of both multispectral and radar sensing imagery can lead to important insights about the physical properties of the sediment that would be omitted by using one data source alone. Spectral unmixing techniques in Google Earth Engine were used to map the intertidal zone into general sediment classes spanning the gradient (i.e., mud, sandy mud, muddy sand, and sand). Comparison of the mapped outputs with field reference data suggests that mapping of mud- vs. sand-dominated areas can be accomplished with reasonable accuracy (overall accuracy of 75%).

EDNA and Acoustic Tag Monitoring Reveal Congruent Overwintering Distributions of Striped Bass in a Hydrologically Complex Estuarine Environment

After collapsing in the late 1990s, the southern Gulf of St. Lawrence population of striped bass (Morone saxatilis) is recovering. Here, we evaluate the use of under-ice eDNA sampling to monitor the population and confirm overwintering locations. From 2018 to 2020, water samples were collected from transects spanning 35 km of the Miramichi River system, accounting for the effects of sampling site, month, sampling depth and tidal influence on eDNA concentration. We examined the distribution of eDNA in a complex tidal river system with a time series consisting of 12 h of continuous sampling throughout a tidal cycle, in conjunction with the use of artificial DNA tracers and acoustic Doppler current profiler flow measurements. The eDNA distribution correctly identified overwintering grounds based on acoustic tag data, including a perceptible upstream shift in 2020. Overall, there was no significant effect of year, sampling month (February or March), sampling depth or tidal phase on eDNA concentrations. The tidal time series revealed only weak patterns of eDNA recirculation. Monitoring eDNA concentration and distribution allows for a relative comparison of population size and location between years, and has the potential to be expanded to other river systems more easily than traditional acoustic fish tags and surveys.

Dynamic Cause of the Sudden Death Event of Small Yellow Croaker (Larimichthys polyactis) in the East China Sea in Winter 2017

Small yellow croaker (Larimichthys polyactis), a benthic fish species widely distributed in the Northwest Pacific Ocean, plays an important role in the fishery catch of coastal countries. A large amount of dead small yellow croaker floated on the sea surface near 30.2°N, 125.3°E in the East China Sea on 19 February 2017, and more than 9,000 kilograms were caught by fishermen, which had never been recorded before. This study aims to investigate the dynamic causes of this sudden death event based on available in-situ, satellite and modelling data. The event occurred near the overwintering ground of small yellow croaker, where a majority of these fish assembled in winter. There were persistent ocean fronts at target site with average intensity of 0.05°C/km at sea surface and bottom in February 2017, which was the strongest in the past 5 years from 2015 to 2019. The fronts provided nutrients and food to fish, and the intense gradients prevented fish from crossing, thus forming “water barriers” to restrict the horizontal movement of fish. Due to enhanced convection, the water was well-mixed, enabling the demersal croaker to float to sea surface. The sea temperature was particularly warm in winter 2017, but then experienced a significant drop by more than 4.5°C in 40 days according to satellite and modelling datasets due to the northerly wind and the southeastward movement of cold eddy. The drop of temperature led to ‘cold shock’ of fish, affecting their physiological regulation and survival. Under the combined influence of these factors, a large number of small yellow croaker died and aggregated near the target site. This paper has a great reference value for further study on the living habits of small yellow croaker and their responses to marine dynamic changes.

Versatile monarch butterflies can switch navigation strategy

While the logistics of international travel remain off-putting as the Covid-19 pandemic ebbs and flows, creatures that routinely circumnavigate oceans and continents have continued undaunted. Arctic terns are still smashing long distance records while wildebeest move en masse in colossal herds. But the sovereign of the multigenerational migration has to be the monarch butterfly. Each spring, the original pioneers depart their overwintering grounds in Mexico for their Canadian summer home, yet it is their great-great-great-grandchildren that return ∼8 months later. ‘To keep a constant direction over this enormous distance, these animals rely on the sun for orientation’, says Myriam Franzke from the University of Wuerzburg, Germany. However, when the insects near their final destination, they may switch to other forms of guidance to select their ultimate landing site. Yet, no one had tested whether monarch butterflies can use other forms of navigation, beyond being directed by the sun, so Franzke, Christian Kraus, Maria Gayler, Keram Pfeiffer and Basil el Jundi, from the University of Wuerzburg, with David Dreyer from Lund University, Sweden, teamed up to test out the insect's homing instinct.‘We ordered the butterfly pupae from Costa Rica where they were raised in captivity’ says el Jundi, recalling that the adults began emerging from their chrysalises 3 days after their arrival in Germany. ‘It is always a remarkable moment when the small chrysalises turn into colourful butterflies with large wings’, says Franzke. Next, the team gently tethered individual butterflies at the centre of a cylindrical flight arena, allowing the insects to swivel in any direction as they took to the wing. Then, the scientists tested which directions the insects selected when they illuminated LEDs lining the wall of the arena, providing the insects with different views.Switching off three LED columns to show a 4 cm wide vertical dark band against a bright background, the team filmed the butterflies’ manoeuvres for 8 min, switching the position of the bar to the opposite side of the arena from time to time to make sure that the insects were still paying attention. Although the butterflies took hardly any notice of the bar initially, swivelling continually in different directions, by the end they were occasionally holding a steady direction toward one edge of the bar, before veering off again.In contrast, the monarchs seemed far more fixated by a bright vertical bar against a dark background. Some flew directly toward the light for the full 8 min, only switching direction when the bar shifted spontaneously to the other side of the arena: they were definitely attracted to the bright object. But how would the insects fare when the team presented them with a single illuminated green LED, which the butterflies mistake for the sun?This time, the insects set a bearing relative to the fake sun and tended to keep to that path, rotating through 180 deg each time the simulated sun jumped to the other side of the arena. They were using the bright light as a compass guiding their choice of direction, as if they were dispersing toward Canada or returning to Mexico to overwinter. And when the researchers alternated the bright vertical bar with the simulated sun, the butterflies flew persistently toward the bar, but switched direction and set a bearing when the fake sun flashed into view.‘[Monarch] butterflies can switch between compass orientation and attraction’, says el Jundi, and the team suspects that the insects depend on specialised guidance circuits in the brain when navigating over long distances by the sun, but have the ability to switch with ease and simply gravitate toward an object when close to home.

An overwintering ground for fish in a fish-retreat ditch created in a paddy irrigation and drainage system near the sea.

水田生態系における生物多様性保全機能の重要性が広く認識されるようになり，水田地域に何かしらの水域を通年維持することの重要性が知られてきている．本研究では福井県小浜市国富地区に造成された水田退避溝における魚類の遡上および降下を報告する．海域に近い本地区の水田退避溝は，ドジョウおよびオオシマドジョウ（純淡水魚）が産卵場所としてだけでなく越冬場所として利用していた．ウグイ，ウキゴリ（回遊魚）およびマハゼ（汽水・海産魚）が成長場所としてだけでなく越冬場所として利用している可能性が示唆された．つまり，河川や排水路および海と比較して環境変化が小さい水田退避溝は，多くの魚類にとってゆりかごになっていると考えられる．

Shoaling behaviour in the European cuttlefishSepia officinalis

AbstractGroup living is a common behavioural feature in many different animal species. It offers a multitude of fitness benefits, such as increased opportunities to find resources, improved predator vigilance and potentially even social learning. In cephalopods, social grouping behaviour has mainly been reported for squid species, with up to several thousand individuals displaying different forms of shoaling and even schooling behaviour. Despite being held in groups in captivity, cuttlefish (Cephalopoda: Sepiidae) have long been considered rather asocial animals. However, reports of breeding aggregations and one recent schooling observation from the wild started to bring this characterisation into question. Following this, we here present 10 observations of the European cuttlefishSepia officinalis(Linnaeus 1758) forming groups of up to 30 individuals along the South Coast of the UK. The majority of the observed cuttlefish appeared to be juveniles or subadults and showed different shoaling orientations, such as linear or spherical‐shaped formations. This indicated the grouping behaviour did not derive from coincidental accumulations. No mating or courtship behaviour could be identified in these groups, and as all observations were made in August or September, and therefore outside their mating season (March to June), it is unlikely that reproductive behaviour motivates these aggregations. AsS. officinalisis known to migrate to deeper overwintering grounds in autumn, we propose that cuttlefish may temporarily form groups in late summer/early autumn as part of their migration pattern, and that their shoaling behaviour likely offers similar fitness benefits as in other migrating shoaling species.

Evolution of winter molting strategies in European and North American migratory passerines.

Molt is critical for birds as it replaces damaged feathers and worn plumage, enhancing flight performance, thermoregulation, and communication. In passerines, molt generally occurs on the breeding grounds during the postbreeding period once a year. However, some species of migrant passerines that breed in the Nearctic and Western Palearctic regions have evolved different molting strategies that involve molting on the overwintering grounds. Some species forego molt on the breeding grounds and instead complete their prebasic molt on the overwintering grounds. Other species molt some or all feathers a second time (prealternate molt) during the overwintering period. Using phylogenetic analyses, we explored the potential drivers of the evolution of winter molts in Nearctic and Western Palearctic breeding passerines. Our results indicate an association between longer photoperiods and the presence of prebasic and prealternate molts on the overwintering grounds for both Nearctic and Western Palearctic species. We also found a relationship between prealternate molt and generalist and water habitats for Western Palearctic species. Finally, the complete prealternate molt in Western Palearctic passerines was linked to longer days on the overwintering grounds and longer migration distance. Longer days may favor the evolution of winter prebasic molt by increasing the time window when birds can absorb essential nutrients for molt. Alternatively, for birds undertaking a prealternate molt at the end of the overwintering period, longer days may increase exposure to feather‐degrading ultra‐violet radiation, necessitating the replacement of feathers. Our study underlines the importance of the overwintering grounds in the critical process of molt for many passerines that breed in the Nearctic and Western Palearctic regions.

Magnetic Compass Orientation in a Palaearctic-Indian Night Migrant, the Red-Headed Bunting.

Simple SummaryThe earth’s magnetic field, celestial cues, and retention of geographical cues en route provide birds with compass knowledge during migration. The magnetic compass works on the direction of the magnetic field, specifically, the course of the field lines. We tested Red-headed Buntings in orientation cages in the evening during spring migration. Simulated overcast testing resulted in a northerly mean direction, while in clear skies, birds oriented in an NNW (north–northwest) direction. Buntings were exposed to 120° anticlockwise shifted magnetic fields under simulated overcast skies and responded by shifting their orientation accordingly. The results showed that this Palaearctic night migrant possesses a magnetic compass, as well as the fact that magnetic cues act as primary directional messengers. When birds were exposed to different environmental conditions at 22 °C and 38 °C temperatures under simulated overcast conditions, they showed a delay in Zugunruhe (migratory restlessness) at 22 °C, while an advance migratory restlessness was observed under 38 °C conditions. Hot and cold weather clearly influenced the timing of migrations in Red-headed Buntings, but not the direction.Red-headed Buntings (Emberiza bruniceps) perform long-distance migrations within their southerly overwintering grounds and breeding areas in the northern hemisphere. Long-distance migration demands essential orientation mechanisms. The earth’s magnetic field, celestial cues, and memorization of geographical cues en route provide birds with compass knowledge during migration. Birds were tested during spring migration for orientation under natural clear skies, simulated overcast skies at natural day length and temperature, simulated overcast at 22 °C and 38 °C temperatures, and in the deflected (−120°) magnetic field. Under clear skies, the Red-headed Buntings were oriented NNW (north–northwest); simulated overcast testing resulted in a northerly mean direction at local temperatures as well as at 22 °C and 38 °C. The Buntings reacted strongly in favor of the rotated magnetic field under the simulated overcast sky, demonstrating the use of a magnetic compass for migrating in a specific direction.

Season, anthocyanin supplementation, and flight training have mixed effects on the antioxidant system of migratory European Starlings

Abstract Migratory birds engage in 2 periods of endurance flight annually as they travel between summer breeding and overwintering grounds, and such endurance flights likely incur oxidative costs. These costs may differ between fall and spring migration, especially for females who must prepare for breeding and egg laying in spring. The objective of this study of a migratory bird was to test proposed hypotheses about how key components of the female’s antioxidant system differ in response to flight training in the fall and spring and to dietary antioxidant supplementation. We hand raised female European Starlings (Sturnus vulgaris) and fed them either a diet supplemented with dietary anthocyanins or a diet without added anthocyanins. We flew females in a wind tunnel for 15 days during fall and spring migration seasons and measured over time oxidative lipid damage (d-ROMs) and 3 components of the antioxidant system: nonenzymatic antioxidant capacity (OXY), uric acid, and glutathione peroxidase (GPx) activity. Prior to flight training, OXY and oxidative damage were lower in females during spring compared with fall, and females fed a low-antioxidant diet had consistently higher circulating uric acid. GPx activity decreased more in spring immediately after a long-duration flight. Females fed a high-antioxidant diet had a greater decrease in OXY after the 15-day flight training. Flight-trained females had higher circulating uric acid than untrained females immediately after the longest-duration flight and decreased GPx activity after the 15-day flight training. In sum, females upregulated enzymatic and nonenzymatic endogenous antioxidants in spring, and females fed a diet with less antioxidants appear to compensate by increasing circulating uric acid. Our findings emphasize the important role of dietary antioxidants for birds during migration, and similar flights in fall and spring likely represent distinct oxidative challenges in the life history of female birds.

Flexibility in migratory strategy contrasts with reliance on restricted staging and overwintering grounds for Sabine’s gulls from the Canadian High Arctic

Abstract Sabine’s gulls (Xema sabini) undertake the longest migration of any gull, a trans-equatorial journey between Arctic breeding and southern hemisphere wintering areas. For such long-distance migrants, quantifying within- and between-individual variation in migratory strategy is key towards understanding resilience to environmental variability encountered over migration. We tracked 22 birds on 32 migrations from the Canadian Arctic to evaluate strategies and quantify flexibility among individuals and years. All birds undertook extended stopovers in a geographically-restricted staging area halfway through migration in the California Current System in both directions. Individuals were otherwise flexible in most aspects of migration but were repeatable in arrival date and duration of the southbound staging phase. Routes taken during southbound migration and overlap in overwintering areas were significantly larger within the same year than among years. Overall, birds showed high individual flexibility in migratory strategies but made similar decisions to one another in the same years. Every year, all birds showed repeatable, consistent reliance on the staging grounds as a key stopover site in both directions. This suggests Sabine’s gulls adjust to environmental change in many aspects of their migration but may be vulnerable to climate change and other anthropogenic influences during critical stages of the journey.

Migration gives sleepless nights to the birds: a study on a Palaearctic–Indian migrant, Emberiza bruniceps

Migration is a seasonal phenomenon, wherein the birds swap between their breeding and overwintering grounds to avoid harsh climatic conditions and maximally utilize the geographical resources available. The annual cycle of a migratory bird broadly comprises non-breeding and breeding phases. The non-breeding phase corresponds to non-migratory life history state (LHS), whereas the breeding phase can be bifurcated into migratory (spring/autumn migration) and reproductive stages. Since most of the migratory birds are nocturnal migrants, this results either in shift or compromised sleep in these organisms for the sake of their reproduction and survival. Thus, an obvious question arises regarding the prevalence and distribution of sleep in these night migrants. Therefore, the present study focuses on the life history state-dependent as well as daily (with reference to night) differences in sleep behavior with special mention of different sleep postures in a migratory bunting, Emberiza bruniceps. For this purpose, male redheaded buntings were procured from the wild, and after acclimation in laboratory condition, were subjected to short (8L:16D) and long (13L:11D) photoperiodic schedules. Short photoperiod corresponds to non-migratory, whereas long photoperiod leads to pre-migratory and migratory LHSs. The results reveal that sleep behavior varied with different life history states of the bird. Non-migratory and pre-migratory states were dominated by front and back sleep postures, whereas migratory phase was dominated by active wakefulness. Not only life history state, even the night itself showed gradation in occurrence of different sleep behaviors. As the study was conducted in different phases of night, phasic distribution of different sleep behaviors was also observed. Thus, the bird under study, redheaded bunting, clearly demonstrated life history state, as well as daily differences in distribution of different sleep behaviors.

Earlier and slower or later and faster: Spring migration pace linked to departure time in a Neotropical migrant songbird.

Migratory birds travel vast distances and the timing of migratory flights can affect survival and the ability to reproduce. For Neotropical migrant songbirds, early spring departure from wintering sites, early arrival to the breeding grounds and higher reproductive success have been related to the use of suitable habitats and environmental conditions during the non-breeding season. However, how migratory strategies are shaped by winter habitat choice is largely unknown due to the general inability to track birds from specific wintering habitats to stopovers or breeding destinations. We assessed how winter habitat (native forest vs. shade-grown coffee plantations) relates to spring departure date and migration pace in Swainson's Thrush Catharus ustulatus. We also determined the effect of departure date and total migration duration on the arrival date of birds detected near or within their breeding range. We used a novel application of Motus radiotelemetry arrays to track individuals from their wintering grounds in the Andes of South America along their migratory journey to North America. We found variation in migratory strategies between habitats, with birds wintering in native forest departing later than birds in coffee. We present isotopic evidence for native forest being of higher quality than shade-coffee for Swainson's Thrush and hypothesize that moister conditions in forest, as shown by stable isotope (δ13 C) analysis of thrush whole blood, provides favourable pre-migratory conditions allowing birds to delay departure from wintering grounds. Habitat, between-site and -year variation in departure date, suggests that birds made facultative adjustments to winter habitat quality and environmental conditions. Independent of habitat, birds that departed later migrated faster and this pattern was maintained along the migration route (n=44). Migrating earlier and slower or later and faster was unlikely to result in significant differences in arrival time to breeding destinations. Our findings reveal underappreciated complexity in migratory decisions by long-distance migrants that contrast with the current paradigm of earlier departures and arrival from optimal habitats. The next step is to understand the relative fitness benefits of early versus late schedules or whether each strategy is an equally good response to experienced conditions.