Thomas Alerstam

Optimal fat loads in migrating birds: a test of the time-minimization hypothesis

We tested the hypothesis that birds are selected to minimize the time spent on migration, that is, to migrate as fast as possible. Optimal fat loads in time-selected migration were predicted for different rates of fat accumulation at stopover sites. We analyzed departure fat loads of migrating bluethroats Luscinia svecica svecica, experimentally provided with extra food at a stopover site, and of migrating rufous hummingbirds Selasphorus rufus, which showed considerable individual variation in fat-deposition rate, in relation to these predictions. We found qualitative agreement with the time-minimization hypothesis. However, quantitative agreement requires that specific assumptions be fulfilled for both species: (1) consistent differences in expected speed of migration should exist between different individuals of the same species and/or (2) the expected speed of migration should increase along the route Both of these assumptions are probably valid, and ringing data suggest an increase in bluethroat autumn migration speed along the route Physiological and flight mechanical constraints will prevent birds from depositing excessively large amounts of fuel. These assumptions and constraints should be taken into account in future critical tests of the hypothesis that natural selection operates to maximize the speed of migration.

When and where does mortality occur in migratory birds? Direct evidence from long‐term satellite tracking of raptors

Information about when and where animals die is important to understand population regulation. In migratory animals, mortality might occur not only during the stationary periods (e.g. breeding and wintering) but also during the migration seasons. However, the relative importance of population limiting factors during different periods of the year remains poorly understood, and previous studies mainly relied on indirect evidence. Here, we provide direct evidence about when and where migrants die by identifying cases of confirmed and probable deaths in three species of long-distance migratory raptors tracked by satellite telemetry. We show that mortality rate was about six times higher during migration seasons than during stationary periods. However, total mortality was surprisingly similar between periods, which can be explained by the fact that risky migration periods are shorter than safer stationary periods. Nevertheless, more than half of the annual mortality occurred during migration. We also found spatiotemporal patterns in mortality: spring mortality occurred mainly in Africa in association with the crossing of the Sahara desert, while most mortality during autumn took place in Europe. Our results strongly suggest that events during the migration seasons have an important impact on the population dynamics of long-distance migrants. We speculate that mortality during spring migration may account for short-term annual variation in survival and population sizes, while mortality during autumn migration may be more important for long-term population regulation (through density-dependent effects).

The annual cycle of a trans-equatorial Eurasian-African passerine migrant: different spatio-temporal strategies for autumn and spring migration

The small size of the billions of migrating songbirds commuting between temperate breeding sites and the tropics has long prevented the study of the largest part of their annual cycle outside the breeding grounds. Using light-level loggers (geolocators), we recorded the entire annual migratory cycle of the red-backed shrike Lanius collurio, a trans-equatorial Eurasian-African passerine migrant. We tested differences between autumn and spring migration for nine individuals. Duration of migration between breeding and winter sites was significantly longer in autumn (average 96 days) when compared with spring (63 days). This difference was explained by much longer staging periods during autumn (71 days) than spring (9 days). Between staging periods, the birds travelled faster during autumn (356 km d–1) than during spring (233 km d–1). All birds made a protracted stop (53 days) in Sahelian sub-Sahara on southbound migration. The birds performed a distinct loop migration (22 000 km) where spring distance, including a detour across the Arabian Peninsula, exceeded the autumn distance by 22 per cent. Geographical scatter between routes was particularly narrow in spring, with navigational convergence towards the crossing point from Africa to the Arabian Peninsula. Temporal variation between individuals was relatively constant, while different individuals tended to be consistently early or late at different departure/arrival occasions during the annual cycle. These results demonstrate the existence of fundamentally different spatio-temporal migration strategies used by the birds during autumn and spring migration, and that songbirds may rely on distinct staging areas for completion of their annual cycle, suggesting more sophisticated endogenous control mechanisms than merely clock-and-compass guidance among terrestrial solitary migrants. After a century with metal-ringing, year-round tracking of long-distance migratory songbirds promises further insights into bird migration.

Optimal bird migration revisited

Using optimality perspectives is now regarded as an essential way of analysing and understanding adaptations and behavioural strategies in bird migration. Optimization analyses in bird migration research have diversified greatly during the two recent decades with respect to methods used as well as to topics addressed. Methods range from simple analytical and geometric models to more complex modeling by stochastic dynamic programming, annual routine models and multiobjective optimization. Also, game theory and simulation by selection algorithms have been used. A wide range of aspects of bird migration have been analyzed including flight, fuel deposition, predation risk, stopover site use, transition to breeding, routes and detours, daily timing, fly-and-forage migration, wind selectivity and wind drift, phenotypic flexibility, arrival time and annual molt and migration schedules. Optimization analyses have proven to be particularly important for defining problems and specifying questions and predictions about the consequences of minimization of energy, time and predation risk in bird migration. Optimization analyses will probably also be important in the future, when predictions about bird migration strategies can be tested by much new data obtained by modern tracking techniques and when the importance of new trade-offs, associated with, e.g., digestive physiology, metabolism, immunocompetence and disease, need to be assessed in bird migration research.ZusammenfassungIn der Vogelzugsforschung erwiesen sich Optimierungsperspektiven für die Analyse und das Verständnis von Adaptionen und Verhaltensstrategien als äusserst essentiell. Hierbei haben sich Optimierungsanalysen in den letzten zwei Jahrzehnten in der Vogelzugsforschung sowohl methodisch als auch thematisch stark diversifiziert. Dabei reichen die Methoden von einfachen, analytischen und geometrischen bis zu mehr komplexen Modellen mit stochastisch-dynamischer Programmierung, Jahresroutinemodellen und multiobjektiver Optimierung. Auch Spieltheorie und Simulierungen mit selektiven Algorithmen wurden angewandt. Analysiert wurde ein weites Spektrum von Vogelzugaspekten, darunter Vogelflug, Fettablagerung, Prädationsdruck, Rastverhalten, Übergang zum Brüten, Zugwege und Umwege, Flug- und Rastwanderungen, Tagesrhythmen, Windselektivität und Winddrift, phenotypische Plastizität, Ankunftszeit und jährliche Zug- und Mauser. Optimierungsanalysen haben sich für die Definition von Problemen und für das Spezifizieren von Fragestellungen und Voraussagen bezüglich Konsequenzen der Minimierung von Energie, Zeit und Prädationsrisiko als speziell wichtig erwiesen. Für die Zukunft werden Optimierungsanalysen wahrscheinlich an Bedeutung gewinnen, wenn es darum geht, Voraussagen über Vogelzugstrategien mit neuen Daten und moderner Technik zu testen und wenn abgeschätzt werden muss, wie wichtig neue Kompromisse in Verbindung mit zum Beispiel Verdauungsphysiologie, Metabolismus, Immunabwehr und Krankheiten sind.

Bird orientation: compensation for wind drift in migrating raptors is age dependent

Despite the potentially strong effect of wind on bird orientation, our understanding of how wind drift affects migrating birds is still very limited. Using data from satellite-based radio telemetry, we analysed the effect of changing winds on the variation of the track direction of individual birds. We studied adults and juveniles of two raptor species, osprey Pandion haliaetus and honey buzzard Pernis apivorus, on autumn migration between North Europe and Africa, and demonstrate an important difference between the age categories of both species in the extent of wind drift. For juveniles, side- and following-wind components affected the rates of movement perpendicular to and along the mean direction, respectively, to a similar degree, suggesting full wind drift. By contrast, for adults the rate of crosswind displacement was significantly smaller than the effect of wind on forward movement, showing much reduced wind drift (29%). This indicates that adults have acquired a more sophisticated orientation system, permitting detection of and compensation for wind drift, than juveniles. These drift effects are likely to reduce the ability of juveniles to locate species-specific wintering areas in case of rapid climatic wind change.

Differences in Speed and Duration of Bird Migration between Spring and Autumn

It has been suggested that birds migrate faster in spring than in autumn because of competition for arrival order at breeding grounds and environmental factors such as increased daylight. Investigating spring and autumn migration performances is important for understanding ecological and evolutionary constraints in the timing and speed of migration. We compiled measurements from tracking studies and found a consistent predominance of cases showing higher speeds and shorter durations during spring compared to autumn, in terms of flight speeds (airspeed, ground speed, daily travel speed), stopover duration, and total speed and duration of migration. Seasonal differences in flight speeds were generally smaller than those in stopover durations and total speed/duration of migration, indicating that rates of foraging and fuel deposition were more important than flight speed in accounting for differences in overall migration performance. Still, the seasonal differences in flight speeds provide important support for time selection in spring migration.

How fast can birds migrate

Bird migration is typically characterized by periods of flight, when fuel is consumed, and intervening stopover periods when fuel is deposited. The resulting overall migration speed can be calculated on the basis of flight speed, rate of fuel deposition and power consumption during flight. Energy deposition rate (Pdep) can be estimated as the difference between metabolic scope and field metabolic rate during stopover. Evaluating how migration speed scales with body mass yields a declining speed with increasing mass for flapping flight ( m 0m9), while migration speed increases with increasing mass for soaring flight. For flapping flight, migration speed and power can be calculated according to aerodynamic theory, and in soaring flight cross-country speed can be estimated from rate of climb (when circling in thermals) and the glide polar. Power in soaring/gliding flight is assumed to be a constant multiple of the basal metabolic rate (BMR). We used this approach to calculate expected speeds of migration for 15 species. For a small bird with a typical energy deposition rate (Pdep = 1.0-BMR) and using flapping flight, predicted migration speed is about 200 km/day, while a maximum energy

Individuality in bird migration: routes and timing

The exploration of animal migration has entered a new era with individual-based tracking during multiple years. Here, we investigated repeated migratory journeys of a long-distance migrating bird, the marsh harrier Circus aeruginosus, in order to analyse the variation within and between individuals with respect to routes and timing. We found that there was a stronger individual repeatability in time than in space. Thus, the annual timing of migration varied much less between repeated journeys of the same individual than between different individuals, while there was considerable variation in the routes of the same individual on repeated journeys. The overall contrast in repeatability between time and space was unexpected and may be owing to strong endogenous control of timing, while short-term variation in environmental conditions (weather and habitat) might promote route flexibility. The individual variation in migration routes indicates that the birds navigate mainly by other means than detailed route recapitulation based on landmark recognition.

Bimodal orientation and the occurrence of temporary reverse bird migration during autumn in south Scandinavia

Abstract Extensive ringing data from a coastal site (Falsterbo Bird Observatory) in southwesternmost Sweden were used to investigate the occurrence of reverse autumn migration among 20 passerine bird species of widely different migration categories. The data demonstrate that reverse migration is a widespread and regular phenomenon among nocturnal as well as diurnal migrants and among irruptive migrants, temperate zone migrants, and long-distance migrants destined for tropical winter quarters. The reoriented movements were directed approximately opposite to the normal migration direction, i.e. between NNW and ENE from the coast and towards inland. Median distances of reverse movements varied between 9 and 65 km. Some individuals of irruptive and partial migrants settled to winter in the reverse direction. Bird species with relatively small fat reserves at capture were more likely to perform reverse migratory movements than species with larger fat deposits. In two species birds performing forward migration were significantly heavier within 10 days after capture than individuals performing reverse movements. The reoriented movements probably are of adaptive significance for birds confronted with the sea and pre-disposed to refuelling during migration. A bimodal orientation mechanism will bring the birds from an area with high competition for food and high predation risk to more suitable resting and feeding grounds before resuming migration in the forward direction and crossing the barrier.

How hazardous is the Sahara Desert crossing for migratory birds? Indications from satellite tracking of raptors.

We investigated the risk associated with crossing the Sahara Desert for migrating birds by evaluating more than 90 journeys across this desert by four species of raptors (osprey Pandion haliaetus, honey buzzard Pernis apivorus, marsh harrier Circus aeruginosus and Eurasian hobby Falco subbuteo) recorded by satellite telemetry. Forty per cent of the crossings included events of aberrant behaviours, such as abrupt course changes, slow travel speeds, interruptions, aborted crossings followed by retreats from the desert and failed crossings due to death, indicating difficulties for the migrants. The mortality during the Sahara crossing was 31 per cent per crossing attempt for juveniles (first autumn migration), compared with only 2 per cent for adults (autumn and spring combined). Mortality associated with the Sahara passage made up a substantial fraction (up to about half for juveniles) of the total annual mortality, demonstrating that this passage has a profound influence on survival and fitness of migrants. Aberrant behaviours resulted in late arrival at the breeding grounds and an increased probability of breeding failure (carry-over effects). This study also demonstrates that satellite tracking can be a powerful method to reveal when and where birds are exposed to enhanced risk and mortality during their annual cycles.