Felix Liechti

Birds: blowin’ by the wind?

Migration is a task that implies a route, a goal and a period of time. To achieve this task, it requires orientation abilities to find the goal and energy to cover the distance. Completing such a journey by flying through a moving airspace makes this relatively simple task rather complex. On the one hand birds have to avoid wind drift or have to compensate for displacements to reach the expected goal. On the other hand flight costs make up a large proportion of energy expenditure during migration and, consequently, have a decisive impact on the refuelling requirements and the time needed for migration. As wind speeds are of the same order of magnitude as birds’ air speeds, flight costs can easily be doubled or, conversely, halved by wind effects. Many studies have investigated how birds should or actually do react to winds aloft, how they avoid additional costs or how they profit from the winds for their journeys. This review brings together numerous theoretical and empirical studies investigating the flight behaviour of migratory birds in relation to the wind. The results of these studies corroborate that birds select for favourable wind conditions both at departure and aloft to save energy and that for some long-distance migrants a tail-wind is an indispensable support to cover large barriers. Compensation of lateral wind drift seems to vary between age classes, depending on their orientation capacities, and probably between species or populations, due to the variety of winds they face en route. In addition, it is discussed how birds might measure winds aloft, and how flight behaviour with respect to wind shall be tested with field data.

Year-Round Tracking of Small Trans-Saharan Migrants Using Light-Level Geolocators

Since 1899 ringing (or banding) remained the most important source of information about migration routes, stopover sites and wintering grounds for birds that are too small to carry satellite-based tracking systems. Despite the large quantity of migrating birds ringed in their breeding areas in Europe, the number of ring recoveries from sub-Saharan Africa is very low and therefore the whereabouts of most small bird species outside the breeding season remain a mystery. With new miniaturized light-level geolocators it is now possible to look beyond the limits of ring recovery data. Here we show for the first time year round tracks of a near passerine trans-Saharan migrant, the European Hoopoe (Upupa epops epops). Three birds wintered in the Sahel zone of Western Africa where they remained stationary for most of the time. One bird chose a south-easterly route following the Italian peninsula. Birds from the same breeding population used different migration routes and wintering sites, suggesting a low level of migratory connectivity between breeding and wintering areas. Our tracking of a near passerine bird, the European Hoopoe, with light-level geolocators opens a new chapter in the research of Palaearctic-African bird migration as this new tool revolutionizes our ability to discover migration routes, stopover sites and wintering grounds of small birds.

The relevance of wind for optimal migration theory

Tracking radar measurements of winds at the flight levels of bird migration during several migratory seasons in central Europe, southern Israel and the western Mediterranean are analysed with regard to optimal migratory behaviour in relation to wind. Birds migrating through central Europe and the Mediterranean area have to cope with windspeeds commonly ranging from 50 to 100% of their normal airspeed. Opposing winds prevail in central Europe, while at the western and eastern edge of the Mediterranean headand tailwinds occur in similar frequencies. Winds are highly variable in time and altitude. A bird migrating selectively during nights with favourable wind conditions speeds up its flight by 30% (on average) compared to an individual disregarding the wind situation. Selecting the most profitable flight altitude may result in an additional gain of 40% in flight speed. Therefore, taking into account the wind situation carefully, a bird can almost double its flight speed and save about half of the energy required for its migratory journey through central Europe and the Mediterranean. The time needed for refuelling decreases accordingly or the safety margins provided by fat reserves can be increased. For birds flying long distances in one step, the selection of favourable winds should be more important than adjusting departure to local fat accumulation rates.

Flight behaviour of nocturnally migrating birds in coastal areas - crossing or coasting

Numerous reports on homogeneous streams of nocturnal migrants over land and water, crossing coastlines without taking any notice of the habitat change underneath, seem to contradict recent infrared observations along the French and Spanish Mediterranean coasts in autumn, suggesting important deviations from the standard SSW or SW flow of migrants associated with the geographical situation and the course of the coastlines. In order to look for potential reactions of nocturnal migrants to the sea/land transition, the flight paths of night-migrating birds were recorded by tracking radar in autumn 1996 at the southern tip of Mallorca and at the E-W leading coast near Malaga. Within the tracking time of 40 s per individual there was no short-term adjustment of direction when crossing the coastline, neither were there differences in the average vertical speeds of birds approaching and leaving the coast. The mean altitudes of tracked birds, however, were about 9% higher above land than over water at both sites. Between-site comparison revealed higher average flight speeds of birds above the island than when leaving the mainland. At both sites important variation in flight behaviour was observed in the course of the night: near Malaga a continuous shift towards the E-W leading coast, simultaneously over water and land, suggests diminishing motivation to continue flights across the sea at times when decreasing fat reserves and/or an endogenous rhythm dissuade from crossing an obstacle of unknown dimensions. Increasing proportions of reverse migration in the course of the night at both sites, with birds flying at low levels and low speeds, are additional indications of motivational conflicts between continuing migration and landing. Optimal migratory behaviour therefore does not necessarily imply that birds should follow the shortest route, but appears to be an adjustable compromise between risk avoidance and straight flight depending on endogenous and environmental conditions.

Songbird migration across the Sahara: the non-stop hypothesis rejected!

Billions of songbirds breeding in the Western Palaearctic cross the largest desert of the world, the Sahara, twice a year. While crossing Europe, the vast majority use an intermittent flight strategy, i.e. fly at night and rest or feed during the day. However, it was long assumed that they overcome the Sahara in a 40 h non-stop flight. In this study, we observed bird migration with radar in the plain sand desert of the Western Sahara (Mauritania) during autumn and spring migration and revealed a clear prevalence of intermittent migration. Massive departures of songbirds just after sunset independent of site and season suggests strongly that songbirds spent the day in the plain desert. Thus, most songbirds cross the Sahara predominately by the intermittent flight strategy. Autumn migration took place mainly at low altitudes with high temperatures, its density decreased abruptly before sunrise, followed by very little daytime migration. Migration was highly restricted to night-time and matched perfectly the intermittent flight strategy. However, in spring, when migratory flights occurred at much higher altitudes than in autumn, in cool air, about 17% of the songbird migration occurred during the day. This suggests that flying in high temperatures and turbulent air, as is the case in autumn, may lead to an increase in water and/or energy loss and may prevent songbirds from prolonged flights into the day.

Bird migration flight altitudes studied by a network of operational weather radars

A fully automated method for the detection and quantification of bird migration was developed for operational C-band weather radar, measuring bird density, speed and direction as a function of altitude. These weather radar bird observations have been validated with data from a high-accuracy dedicated bird radar, which was stationed in the measurement volume of weather radar sites in The Netherlands, Belgium and France for a full migration season during autumn 2007 and spring 2008. We show that weather radar can extract near real-time bird density altitude profiles that closely correspond to the density profiles measured by dedicated bird radar. Doppler weather radar can thus be used as a reliable sensor for quantifying bird densities aloft in an operational setting, which—when extended to multiple radars—enables the mapping and continuous monitoring of bird migration flyways. By applying the automated method to a network of weather radars, we observed how mesoscale variability in weather conditions structured the timing and altitude profile of bird migration within single nights. Bird density altitude profiles were observed that consisted of multiple layers, which could be explained from the distinct wind conditions at different take-off sites. Consistently lower bird densities are recorded in The Netherlands compared with sites in France and eastern Belgium, which reveals some of the spatial extent of the dominant Scandinavian flyway over continental Europe.

First evidence of a 200-day non-stop flight in a bird

Being airborne is considered to be energetically more costly as compared with being on the ground or in water. Birds migrating or foraging while airborne are thought to spend some time resting on the ground or water to recover from these energetically demanding activities. However, for several decades ornithologists have claimed that some swifts may stay airborne for almost their whole lifetime. Here we present the first unequivocal evidence that an individual bird of the Alpine swift (Tachymarptis melba) can stay airborne for migration, foraging and roosting over a period of more than 6 months. To date, such long-lasting locomotive activities had been reported only for animals living in the sea. Even for an aerodynamically optimized bird, like the Alpine swift, flying requires a considerable amount of energy for continuous locomotive control. Our data imply that all vital physiological processes, including sleep, can be perpetuated during flight.

Nocturnal exploratory flights, departure time, and direction in a migratory songbird

Stopover studies have concentrated so far mostly on mechanisms regulating the temporal organisation on the day-to-day level. Taking advantage of the small and isolated island of Helgoland in the North Sea, we investigated the stopover and departure behaviour of a nocturnal migrant by using radio telemetry. Special attention was paid particularly to nocturnal behaviour, their departure times within the night, and departure directions. Here, we show that Northern Wheatears, Oenanthe oenanthe, performed regularly nocturnal exploratory flights on nights before and on departure night, which might be a common behaviour of nocturnal migrants to evaluate meteorological conditions aloft prior to departure. We proposed that migrants being prepared for an endurance flight would depart early in the night within a short time window, whereas individuals departing with low fuel load would be less prone to take off early. Our data, however, could not support this hypothesis. In respect of the migratory direction, there was a significant correlation between departure direction and departure fuel load. Northern Wheatears with high departure fuel loads headed more towards the north than lean migrants, which departed mostly towards the nearest coastline, i.e. east to south. Thus, birds with high fuel loads showed their seasonally appropriate migratory direction irrespective of the ecological barrier ahead, whereas lean birds avoided this direction. To our knowledge, this is the first study that investigates the relationship of fuel load and departure direction in a free-flying songbird.ZusammenfassungIn Studien zum Rastverhalten von Zugvögeln wurden bis jetzt hauptsächlich die Mechanismen untersucht, die die zeitliche Organisation des Rastverhaltens auf der Ebene von Tagen steuern. Wir haben das Rast- und Abzugsverhalten eines Nachtziehers mit Hilfe von Radiotelemetrie untersucht und uns dabei die isolierte Lage der kleinen Nordseeinsel Helgoland zu Nutze gemacht. Besondere Aufmerksamkeit galt dem nächtlichen Verhalten, der Abzugszeit in der Nacht, und der Abzugsrichtung. Hier zeigen wir, dass Steinschmätzer Oenanthe oenanthe regelmäßig nächtliche Erkundungsflüge in Nächten vor und während der Abzugsnacht durchführten, was ein typisches Verhalten von Nachtziehern sein könnte, um die Windbedingungen in verschiedenen Höhen zu testen. Wir nahmen an, dass Zugvögel, die ausreichende Reserven für einen Langstreckenflug hatten, früh in der Nacht während eines relativ engen Zeitfensters abziehen würden, wohingegen Individuen mit geringeren Energiereserven wahrscheinlich früh aber auch spät in der Nacht abziehen. Unsere Ergebnisse konnten diese Hypothese allerdings nicht bestätigen. Zwischen der Abzugsrichtung und den Energievorräten beim Abzug bestand ein signifikanter Zusammenhang. Steinschmätzer mit hohen Energiereserven flogen in nördlichere Richtungen als magere Vögel, die zum größten Teil in Richtung der nächstgelegenen Küste, d.h. nach Osten bis Süden, abzogen. Also zeigten Vögel mit großen Energiereserven ihre jahreszeitlich angemessene Abzugsrichtung unabhängig von der vor ihnen liegenden ökologischen Barriere, während magere Vögel diese Richtung vermieden. Unseres Wissens ist dies die erste Studie, die den Zusammenhang zwischen Energievorräten und Abzugsrichtung an Singvögeln im Freiland untersucht.

PREDICTING MIGRATORY FLIGHT ALTITUDES BY PHYSIOLOGICAL MIGRATION MODELS

Abstract Using the altitudinal profiles of wind, temperature, pressure, and humidity in three flight models, we tried to explain the altitudinal distributions of nocturnal migrants recorded by radar above a desert in southern Israel. In the simplest model, only the tailwind component was used as a predictor of the most preferred flight altitude (T model). The energy model (E model) predicted flight ranges according to mechanical power consumption in flapping flight depending on air density and wind conditions, assuming optimal adjustment of airspeed and compensation of crosswinds, and including the influence of mass loss during flight. The energy-water model (EW model) used the same assumptions and parameters as the E model but also included restrictions caused by dehydration. Because wind was by far the most important factor governing altitudinal distribution of nocturnal migrants, differences in predictions of the three models were small. In a first approach, the EW model performed slightly better than the E model, and both performed slightly better than the T model. Differences were most pronounced in spring, when migrants should fly high according to wind conditions, but when climbing and descending they must cross lower altitudes where conditions are better with respect to dehydration. A simplified energy model (Es model) that omits the effect of air density on flight costs explained the same amount of variance in flight altitude as the more complicated E and EW models. By omitting the effect of air density, the Es model predicted lower flight altitudes and thus compensated for factors that generally bias height distributions downward but are not considered in the models (i.e. climb and descent through lower air layers, cost of ascent, and decrease of oxygen partial pressure with altitude). Our results confirm that wind profiles, and thus energy rather than water limitations, govern the altitudinal distribution of nocturnal migrants, even under the extreme humidity and temperature conditions in the trade wind zone.

Automatic identification of bird targets with radar via patterns produced by wing flapping

Bird identification with radar is important for bird migration research, environmental impact assessments (e.g. wind farms), aircraft security and radar meteorology. In a study on bird migration, radar signals from birds, insects and ground clutter were recorded. Signals from birds show a typical pattern due to wing flapping. The data were labelled by experts into the four classes BIRD, INSECT, CLUTTER and UFO (unidentifiable signals). We present a classification algorithm aimed at automatic recognition of bird targets. Variables related to signal intensity and wing flapping pattern were extracted (via continuous wavelet transform). We used support vector classifiers to build predictive models. We estimated classification performance via cross validation on four datasets. When data from the same dataset were used for training and testing the classifier, the classification performance was extremely to moderately high. When data from one dataset were used for training and the three remaining datasets were used as test sets, the performance was lower but still extremely to moderately high. This shows that the method generalizes well across different locations or times. Our method provides a substantial gain of time when birds must be identified in large collections of radar signals and it represents the first substantial step in developing a real time bird identification radar system. We provide some guidelines and ideas for future research.