Advanced autumn migration of sparrowhawk has increased the predation risk of long-distance migrants in Finland

Predation affects life history traits of nearly all organisms and the population consequences of predator avoidance are often larger than predation itself. Climate change has been shown to cause phenological changes. These changes are not necessarily similar between species and may cause mismatches between prey and predator. Eurasian sparrowhawk Accipiter nisus, the main predator of passerines, has advanced its autumn phenology by about ten days in 30 years due to climate change. However, we do not know if sparrowhawk migrate earlier in response to earlier migration by its prey or if earlier sparrowhawk migration results in changes to predation risk on its prey. By using the median departure date of 41 passerine species I was able to show that early migrating passerines tend to advance, and late migrating species delay their departure, but none of the species have advanced their departure times as much as the sparrowhawk. This has lead to a situation of increased predation risk on early migrating long-distance migrants (LDM) and decreased the overlap of migration season with later departing short-distance migrants (SDM). Findings highlight the growing list of problems of declining LDM populations caused by climate change. On the other hand it seems that the autumn migration may become safer for SDM whose populations are growing. Results demonstrate that passerines show very conservative response in autumn phenology to climate change, and thus phenological mismatches caused by global warming are not necessarily increasing towards the higher trophic levels.     

Avian population consequences of climate change are most severe for long-distance migrants in seasonal habitats

One consequence of climate change is an increasing mismatch between timing of food requirements and food availability. Such a mismatch is primarily expected in avian long-distance migrants because of their complex annual cycle, and in habitats with a seasonal food peak. Here we show that insectivorous long-distance migrant species in The Netherlands declined strongly (1984–2004) in forests, a habitat characterized by a short spring food peak, but that they did not decline in less seasonal marshes. Also, within generalist long-distance migrant species, populations declined more strongly in forests than in marshes. Forest-inhabiting migrant species arriving latest in spring declined most sharply, probably because their mismatch with the peak in food supply is greatest. Residents and short-distance migrants had non-declining populations in both habitats, suggesting that habitat quality did not deteriorate. Habitat-related differences in trends were most probably caused by climate change because at a European scale, long-distance migrants in forests declined more severely in western Europe, where springs have become considerably warmer, when compared with northern Europe, where temperatures during spring arrival and breeding have increased less. Our results suggest that trophic mismatches may have become a major cause for population declines in long-distance migrants in highly seasonal habitats.     

Nocturnal autumn bird migration at Falsterbo, South Sweden

We investigated the patterns of nocturnal bird migration in autumn 1998 at a coastal site on the Falsterbo peninsula in south-western Sweden, by means of a passive infrared device. In total 17 411 flight paths, including track direction and altitude, of migrating birds were recorded for 68 nights from August to October. Mean migratory traffic rate per night varied between 6 and 6618 birds km⁻¹ h⁻¹, with an average of 1319 birds km⁻¹ h⁻¹. Migration at Falsterbo showed a similar seasonal pattern to that reported for central Europe, with pronounced peaks of migration and intermittent periods with relatively low migratory intensities. Weather factors explained two thirds of the variance in the intensity of bird migration. During nights with intense migration, associated with weak winds, the mean track direction was close to that in central western Europe (225°). Birds usually maintained a constant heading independent of wind directions and, in consequence, were drifted by the wind. The mean orientation clearly differed from that of the nearest coastline, suggesting that the birds did not use the topography below to compensate for wind drift.     

气候变化对丹顶鹤秋季迁徙路线潜在生境适宜性的影响

目前全球变暖趋势的加剧对丹顶鹤等大型濒危水禽的栖息地造成了严重的威胁。由于监测方法和技术手段的限制,丹顶鹤在迁徙路线上潜在生境的分布范围尚不清楚,气候变化对丹顶鹤迁徙路线生境适宜性的影响机理有待进一步研究。基于138个丹顶鹤样本分布信息和19种环境变量数据,利用 BIOMOD2 软件包构建了丹顶鹤潜在生境评价的组合模型,对丹顶鹤在亚洲东部秋季迁徙路线上的生境适宜性进行数值模拟,并预测SSP1.2-6气候背景下2021-2040年、2041-2060年、2061-2080年、2081-2100年四个不同阶段的丹顶鹤潜在生境范围的变化趋势。研究结果表明:与单模型的模拟结果相比,集成9种单模型的BIOMOD2组合模型预测精度更高。集成模型的重要性分析表明,气温日较差是丹顶鹤生境适宜性变化的最重要的影响因子。受气候变化的影响2021-2040年、2041-2060年、2061-2080年、2081-2100年丹顶鹤潜在生境的面积将分别减少到2.60×10⁵km²、2.58×10⁵km²、2.75×10⁵km²、2.56×10⁵km²,迁徙路线上胶东半岛和环渤海地区适栖生境面积减少的最为显著。本研究对于迁徙路线上珍稀水禽潜在适宜生境的模拟及全球变化背景下珍稀水禽栖息地的保育和修复具有重要意义。     

Timing of autumn migration of young Corncrakes Crex crex

Knowledge of the timing of migration of young Corncrakes is required for effective implementation of conservation measures. Forty Corncrake chicks were radiotracked during the period between independence and departure from the natal area to determine the age and time at which fledged juveniles depart on migration. Departure occurred between early August and mid‐September at a mean age of 44 days, around the age at which growth of the primary feathers ceases. Chicks from later broods are exposed to greater risk of being killed during mowing, but provision of late mowing options in conservation schemes could enhance their survival.     

Migratory connectivity and population-specific migration routes in a long-distance migratory bird

Knowledge about migratory connectivity, the degree to which individuals from the same breeding site migrate to the same wintering site, is essential to understand processes affecting populations of migrants throughout the annual cycle. Here, we study the migration system of a long-distance migratory bird, the Montagu''s harrier Circus pygargus, by tracking individuals from different breeding populations throughout northern Europe. We identified three main migration routes towards wintering areas in sub-Saharan Africa. Wintering areas and migration routes of different breeding populations overlapped, a pattern best described by ‘weak (diffuse) connectivity’. Migratory performance, i.e. timing, duration, distance and speed of migration, was surprisingly similar for the three routes despite differences in habitat characteristics. This study provides, to our knowledge, a first comprehensive overview of the migration system of a Palaearctic-African long-distance migrant. We emphasize the importance of spatial scale (e.g. distances between breeding populations) in defining patterns of connectivity and suggest that knowledge about fundamental aspects determining distribution patterns, such as the among-individual variation in mean migration directions, is required to ultimately understand migratory connectivity. Furthermore, we stress that for conservation purposes it is pivotal to consider wintering areas as well as migration routes and in particular stopover sites.     

The daily pattern of autumn bird migration in the northern Sahara

The temporal pattern of migration by passerine birds during the night, and their arrival during the day at the Egyptian coast and in the northern Sahara Desert was investigated. The mean direction of nocturnal migration at the coast was south-southeast, while at all desert sites it was south-southwest.
Birds arrived at the Egyptian coast only during the second half of the night which is explained by the fact that no birds could have taken off from the Mediterranean Sea. At least some of the birds landed at the coast where they spent the day before taking off shortly after sunset. These birds passed the desert sites at the expected time of day assuming a ground speed of 18 m per second. However, the origin of the birds passing the desert sites early at night is unclear. They must either have spent the day in the desert north of the study sites or they had overflown the Egyptian coast in the afternoon without landing.
The landing of birds during the day at the desert sites was bimodal. This pattern of arrival is explained either by some birds having landed at the Egyptian coast in the early morning before continuing, or by deteriorating conditions later in the day during flight or when resting in the desert, that obliged them to seek shelter at the desert sites.
A correlation between the number of migrants observed during the night and the number of resting birds in the desert on the following day suggests that an unknown proportion of birds might regularly use an intermittent migratory strategy that includes rest periods by day when crossing the desert, whereas others might adapt a non-stop migratory strategy.     

Variable shifts in spring and autumn migration phenology in North American songbirds associated with climate change

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

Heritability of the timing of autumn migration in a natural bird population

In recent decades, global temperature has increased at an unprecedented rate. This has been causing rapid environmental shifts that have altered the selective regimes determining the annual organization of birds. In order to assess the potential for adaptive evolution in the timing of autumn migration, we estimated heritabilities of the onset of migratory activity in a southern German blackcap (Sylvia atricapilla) population. Heritabilities (h2=0.34-0.45) and coefficients of additive genetic variation (CV(A)=4.7-5.7) were significant and consistent when estimated by different methods, irrespective of whether they were derived from birds hatched in the wild or bred in captivity. In an artificial selection experiment, we selected for later onset of migratory activity, simulating expected natural selection on this trait. We obtained a significant delay in the mean onset of migratory activity by more than one week after two generations of selection. Realized heritability (h2=0.55) was in agreement with expected heritability in the cohort that the selection line was derived from. Our results suggest that evolutionary changes in the timing of autumn migration may take place over a very short time period and will most probably be unconstrained by the lack of additive genetic variation.     

Stable isotope analyses of feathers help identify autumn stopover sites of three long-distance migrants in northeastern Africa

The potential use of stable nitrogen (δ¹⁵N), carbon (δ¹³C) and hydrogen (δD) isotope ratios in feathers of marsh warblers Acrocephalus palustris , river warblers Locustella fluviatilis and whitethroats Sylvia communis was evaluated as a means to help identify the location and isotopic composition of autumn stopover sites in northeast Africa. Feather δD values were compared with regional precipitation δD maps averaged over autumn months. Compared with whitethroats, feather δ¹⁵N, δ¹³C, and δD values of marsh warblers and river warblers suggest the two warblers occupy and grow their feathers in geographic locations with relatively mesic environments, and with higher proportions of C3 (vs. C4) plants. However, δ¹³C values of marsh and river warblers were distinct enough to indicate use of different foodwebs. From previous studies, it is evident that during autumn stopover, river warblers moult their primaries in Ethiopia. It is likely that marsh warblers, like river warblers, stay in Ethiopia and/or in neighbouring regions. Based on feather δD values and regional δD precipitation maps, this region should lie between southeast Sudan and southwest Ethiopia. However, without additional regional isotopic maps in Africa, more precise locations of the stopover sites remain unclear. The relatively enriched δ¹⁵N and δ¹³C values of whitethroat feathers compared with the two other species, reflect the fact that whitethroats moult in relatively drier environments and/or with a lower proportion of C3 vs. C4 plants.     

Projected changes in wind assistance under climate change for nocturnally migrating bird populations

Current climate models and observations indicate that atmospheric circulation is being affected by global climate change. To assess how these changes may affect nocturnally migrating bird populations, we need to determine how current patterns of wind assistance at migration altitudes will be enhanced or reduced under future atmospheric conditions. Here, we use information compiled from 143 weather surveillance radars stations within the contiguous United States to estimate the daily altitude, density, and direction of nocturnal migration during the spring and autumn. We intersected this information with wind projections to estimate how wind assistance is expected to change during this century at current migration altitudes. The prevailing westerlies at midlatitudes are projected to increase in strength during spring migration and decrease in strength to a lesser degree during autumn migration. Southerly winds will increase in strength across the continent during both spring and autumn migration, with the strongest gains occurring in the center of the continent. Wind assistance is projected to increase across the central (0.44 m/s; 10.1%) and eastern portions of the continent (0.32 m/s; 9.6%) during spring migration, and wind assistance is projected to decrease within the central (0.32 m/s; 19.3%) and eastern portions of the continent (0.17 m/s; 6.6%) during autumn migration. Thus, across a broad portion of the continent where migration intensity is greatest, the efficiency of nocturnal migration is projected to increase in the spring and decrease in the autumn, potentially affecting time and energy expenditures for many migratory bird species. These findings highlight the importance of placing climate change projections within a relevant ecological context informed through empirical observations, and the need to consider the possibility that climate change may generate both positive and negative implications for natural systems.     

Favorable winds speed up bird migration in spring but not in autumn

Wind has a significant yet complex effect on bird migration speed. With prevailing south wind, overall migration is generally faster in spring than in autumn. However, studies on the difference in airspeed between seasons have shown contrasting results so far, in part due to their limited geographical or temporal coverage. Using the first full‐year weather radar data set of nocturnal bird migration across western Europe together with wind speed from reanalysis data, we investigate variation of airspeed across season. We additionally expand our analysis of ground speed, airspeed, wind speed, and wind profit variation across time (seasonal and daily) and space (geographical and altitudinal). Our result confirms that wind plays a major role in explaining both temporal and spatial variabilities in ground speed. The resulting airspeed remains relatively constant at all scales (daily, seasonal, geographically and altitudinally). We found that spring airspeed is overall 5% faster in Spring than autumn, but we argue that this number is not significant compared to the biases and limitation of weather radar data. The results of the analysis can be used to further investigate birds'' migratory strategies across space and time, as well as their energy use.     

Effects of high latitude protected areas on bird communities under rapid climate change

Anthropogenic climate change is rapidly becoming one of the main threats to biodiversity, along with other threats triggered by human‐driven land‐use change. Species are already responding to climate change by shifting their distributions polewards. This shift may create a spatial mismatch between dynamic species distributions and static protected areas (PAs). As protected areas represent one of the main pillars for preserving biodiversity today and in the future, it is important to assess their contribution in sheltering the biodiversity communities, they were designated to protect. A recent development to investigate climate‐driven impacts on biological communities is represented by the community temperature index (CTI). CTI provides a measure of the relative temperature average of a community in a specific assemblage. CTI value will be higher for assemblages dominated by warm species compared with those dominated by cold‐dwelling species. We here model changes in the CTI of Finnish bird assemblages, as well as changes in species densities, within and outside of PAs during the past four decades in a large boreal landscape under rapid change. We show that CTI has markedly increased over time across Finland, with this change being similar within and outside PAs and five to seven times slower than the temperature increase. Moreover, CTI has been constantly lower within than outside of PAs, and PAs still support communities, which show colder thermal index than those outside of PAs in the 1970s and 1980s. This result can be explained by the higher relative density of northern species within PAs than outside. Overall, our results provide some, albeit inconclusive, evidence that PAs may play a role in supporting the community of northern species. Results also suggest that communities are, however, shifting rapidly, both inside and outside of PAs, highlighting the need for adjusting conservation measures before it is too late.     

Pareto front analysis of flight time and energy use in long-distance bird migration

Optimality models are frequently used in studies of long distance bird migration to help understand and predict migration routes, stopover strategies and fuelling behaviour in a spatially varying environment. These models typically evaluate bird behaviour by focusing on a single optimization currency, such as total migration time or energy-use, without explicitly considering trade-offs between the involved objectives. In this paper, we demonstrate that this classic single-objective approach downplays the importance of variability in bird behaviour. In the light of these considerations, we therefore propose to use a full multi-criteria optimization method to isolate the set of non-dominated, efficient or Pareto optimal solutions. Unlike single-objective optimization where there is only one combination of bird behaviour maximizing fitness, the Pareto solution set represents a range of optimal solutions to conflicting objectives. Our results demonstrate that this multi-objective approach provides important new ways of analyzing how environmental factors and behavioural constraints have driven the evolution of migratory behaviour.     

Timing and body condition of dichromatic Black Redstarts during autumn migration

Individual variation in postjuvenile molt in male Black Redstart is pronounced with about 90% of young males retaining female‐like coloration (cairei plumage type) and about 10% acquiring adult male‐like feathers (paradoxus plumage type). We examined whether autumn migration timing and body condition differed between individuals of the two plumage types. We used the data of 10,977 Black Redstarts captured during autumn at a ringing site in northern Switzerland where a protocol to record plumage types of captures has been applied since 1980. As cairei individuals cannot be distinguished from young females while sexing is comparatively easy for paradoxus individuals, the proportion of missing data on sex was likely to be higher for cairei individuals than for paradoxus individuals. We formally accounted for captures with unidentified sex using a Bayesian approach and conducted a simulation study to show that our approach was able to provide unbiased results even if the proportion of unsexed captures was high. Applying the method to the Black Redstart data, we found that the proportion of individuals with paradoxus plumage type increased from 7.6% in 1980 to 18.1% in 2013. Individuals with the paradoxus plumage type were on average 0.25 g heavier and had 0.62 mm longer third primaries than individuals with the cairei plumage type. However, we found no support for our expectation of later migration of paradoxus males compared to cairei individuals based on the assumption that paradoxus individuals should occupy autumn territories like adult males. Our results shed new light on the understudied timing of autumn migration in birds and are in line with available studies on Black Redstarts, suggesting a molt‐constraint that allows only young males in good body condition to molt into adult‐like plumages.     

Body mass of six long-distance migrant passerine species along the autumn migration route

Summary We analysed body mass and moult data of six passerine species along their autumn migration route from northern Europe to North Africa and derived hypothetical models of the organisation of their migration in terms of fuel store accumulation. We analysed data of 46,541 first-year birds from 34 trapping sites, sampled in a network of collaborating European and African ringing stations. After accounting for effects of time of day and size, there were marked differences between the six species examined in the change of body mass along the migration route and in the timing of moult. Garden Warblers (Sylvia borin) and Pied Flycatchers (Ficedula hypoleuca) underwent their postjuvenile moult prior to migration and increased their average body mass along the migration route. Sedge Warblers (Acrocephalus schoenobaenus) also increased body mass towards the south, but started the migration bout without further refuelling well before the Sahara and moulted mainly in the wintering grounds. Reed Warblers (Acrocephalus scirpaceus) and Whitethroats (Sylvia communis) migrated while still moulting and did not increase average body mass towards south. They accumulated the energy needed to fly over the Sahara just before it. Spotted Flycatchers (Muscicapa striata) behaved in the same way, but contrary to Reed Warblers and Whitethroats they did not accumulate much fat stores in North Africa, which might urge them to stop and fuel up regularly in the Sahara. In the course of the season average body mass of all species increased slightly, which enabled them to migrate faster. In general, average body mass of first-year birds in northern and central Europe during the migration period was comparable to that of adults during breeding.

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Körpermassen von sechs Arten von Langstreckenziehern entlang des Herbstzugweges

Zusammenfassung Wir analysierten Körpermassen- und Mauserdaten von sechs Singvogelarten auf dem Herbstzug entlang ihres Zugweges von Nordeuropa bis Nordafrika. Die Körpermassen von 46 541 diesjährigen Vögeln, die auf 34 Beringungsstationen im Rahmen eines internationalen Projektes gesammelt wurden, sind die Grundlage dieser Untersuchung. Neben der Entwicklung der Körpermassen entlang des Zugweges untersuchten wir den Einfluss der Mauser, des Fangdatums, der Tageszeit und der Körpergröße auf die Körpermasse. Mausernde Vögel waren schwerer als nichtmausernde mit der gleichen Fettmenge. Die Körpermasse nahm im Laufe der Saison und im Verlauf des Tages zu. Große Vögel (mit einer längeren 8. Handschwinge) waren schwerer als kleine. Gartengrasmücken (Sylvia borin) und Trauerschnäpper (Ficedula hypoleuca) wurden entlang des Zugweges kontinuierlich schwerer. Beide Arten machten die Jugendteilmauser hauptsächlich noch im Brutgebiet durch. Schilfrohrsänger (Acrocephalus schoenobaenus) wurden nach Süden ebenfalls etwas schwerer, aber sie begannen mit einem Reiseabschnitt ohne weiteren Fettaufbau schon in Mitteleuropa und mauserten ihr Kleingefieder hauptsächlich erst in Afrika. Die Körpermassen von Teichrohrsängern (Acrocephalus scirpaceus) und Dorngrasmücken (Sylvia communis) blieben nach Süden weitgehend konstant. Beide Arten lagerten die nötigen Energiereserven zum Überfliegen der Sahara erst kurz davor an. Sie trennten Jugendteilmauser und Zug nicht vollständig. Ähnlich verhielten sich Grauschnäpper (Muscicapa striata), doch im Gegensatz zu den zwei vorherigen Arten, wurden sie auch im Mittelmeergebiet nicht wesentlich fetter. Sie scheinen daher regelmässig in der Wüste zu rasten und zu fressen.

     

The orientation system and migration pattern of long-distance migrants: conflict between model predictions and observed patterns

The requirements of the orientation system of naïve long-distance night migrants were analysed by comparing data on Barred Warbler Sylvia nisoria , Marsh Warbler Acrocephalus palustris and Spotted Flycatcher Muscicapa striata with data from a computer model of a clock-and-compass system. These species show, respectively, a rather restricted winter distribution in East Africa, migration through a very narrow corridor in East Africa, and rather widely distributed recoveries in the Mediterranean with more concentrated recoveries south of the Sahara. For all three species, to obtain the observed concentrations either a very high directional migratory concentration was needed in computer simulations to bring the birds successfully to their wintering areas or misorientating individuals would be subjected to a very high mortality. Neither the very high directional concentration nor the high mortality amongst misorientating individuals fit the empirical data sets. On the basis of the present study, the observed patterns seem difficult to explain by a simple clock-and-compass system only, and to account for the exceptionally precise migratory routes shown in this study it is proposed that first-time migrants might be able to use landscape topography on a regional scale in combination with corrections of directional mistakes/wind displacements.