Bruno Bruderer

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.

The Study of Bird Migration by Radar Part 1: The Technical Basis*

Since the 1960s radar has been an established research tool in bird migration studies. Radar informs us about the actual course of migration under various environmental conditions: it covers wide distances, is independent of light and reasonably independent of weather, provides data on migratory intensity, flight paths and with special equipment the wing-beat pattern of birds. It thus fills an important gap left by other methods such as visual and auditory observations, laboratory research, trapping, and ringing studies. For an appropriate use of the sophisticated tool, however, it is important to know its capabilities and limitations.

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.

The Study of Bird Migration by Radar Part 2: Major Achievements

This is a brief review of the main insights into bird migration provided by radar. Radar is the main tool to study the flight behavior of migratory birds under the influence of environmental factors, i.e., the ecology of migratory flights, ranging from the large-scale pattern of migration in relation to the distribution of land masses, geomorphology, and weather systems down to the variation of flight behavior of single birds in response to leading lines, obstacles, particular atmospheric conditions, and flight phases. Radar has revolutionized research on bird migration. It provides data on the patterns of migration in various geographical areas and is the main tool for studying the reactions of migrants to topographical features and weather, and the variation of flight behavior in time and space under the influence of environmental conditions. It will continue to play an important role, particularly for the analysis of flight and orientation strategies. This review briefly summarizes the major contributions of radar ornithology to our knowledge of bird migration. To limit the reference list I refer to review papers whenever possible. The most comprehensive synopses are [1] for migration as a whole (with an update on migratory strategies in [2]), [3] for radar ornithology, and [4, 5] for weather-dependence of migration. A methodological update is provided in [6].

The evolution of bird migration—a synthesis

We approach the problem of the evolution of bird migration by asking whether migration evolves towards new breeding areas or towards survival areas in the non-breeding season. Thus, we avoid the ambiguity of the usually discussed “southern-home-theory” or “northern-home-theory”. We argue that migration evolved in birds that spread to seasonal habitats through gradual dispersal to enhance survival during the non-breeding season; this in contrast to the alternative idea suggesting that migration evolved towards new breeding areas to increase reproductive success. Our synthesis is based on the threshold model explaining how migratory traits can change rapidly through microevolutionary processes. Our model brings former theories together and explains how bird migration, with the appropriate direction and time program, evolves through selection after genetically non-directed events such as dispersal and colonization. The model does not need the former untested assumptions such as competition as a reason for migration and for the disappearance of sedentary populations or higher reproductive success in temperate breeding areas. Our theory offers answers to questions such as how birds with a southern origin may gradually reach northern latitudes, why migration routes may follow historical expansion routes and why birds leave an area for the non-breeding season and move back instead of breeding on their wintering grounds. The theory proposes gradual change through selection and not sudden changes such as long distance dispersal or mutations and can be applied to migration at all latitudes and in all directions. The scenario provides a reasonable concept to understand most of the existing migratory phenomena on the basis of the ecology and genetics of migratory behaviour.

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.

TO CROSS THE SEA OR TO FOLLOW THE COAST? FLIGHT DIRECTIONS AND BEHAVIOUR OF MIGRATING RAPTORS APPROACHING THE MEDITERRANEAN SEA IN AUTUMN

Summary We studied the flight behaviour of migrating raptors confronted with the Mediterranean sea at an average coast site near Malaga (Spain) in autumn by means of a tracking radar. Behavioural reactions to the water barrier were species-specific, but modified by environmental conditions. Honey buzzards Pernis apivorus and black kites Milvus migrans were reluctant to crossing the water and followed the east-west leading coast; the few honey buzzards crossing the sea at the end of the migratory period were probably juveniles. Considerable numbers of harriers Circus sp., falcons Falco sp. and ospreys Pandion haliaetus crossed the Mediterranean sea. Falcons crossing the sea had higher flight altitudes than those following the coast or crossing the bay. Sea crossings occurred preferably in following winds and also in sidewinds, whereas no birds were observed to cross the sea in strong opposing winds. However, tailwind-support only partly explained for different migratory routes. Raptors crossing the sea in flapping-gliding flight increased airspeeds with sidewinds to reduce drift, but, different from theory, they did not decrease airspeed with increasing tailwind-support indicating that they minimised flight time above sea. Time and energy related consequences of different flight routes are discussed.

Migration by Flapping or Soaring: Flight Strategies of Marsh, Montagu's and Pallid Harriers in Southern Israel

Migratory flights of Marsh Harriers (Circus aeruginosus), Montagus Harriers (Circus pygargus) and Pallid Harriers (Circus macrourus) in southern Israel were used to test flight theory predictions. The body sizes of these closely related species are between those of the typical large soaring migrants, such as eagles and storks, and the typical flapping migrants, such as small falcons and sparrowhawks. In soaring-gliding flight, Marsh Harriers reacted to different thermal conditions by adjusting their gliding airspeed to the actual climbing rate in thermal circling; consequently, cross-country speed was related to climbing rate. In contrast, the smaller Montagus and Pallid Harriers did not adopt gliding airspeeds according to thermal conditions. All harrier species regularly used flapping-gliding flight, predominately soon after sunrise and before sunset, and more often in opposing winds than in following winds. Montagus/Pallid Harriers used flapping-gliding more frequently than Marsh Harriers. Because they alternate between different flight styles, harriers are more independent of environmental factors, such as thermal activity and wind, compared to pure soaring migrants. This allows harriers to migrate under unfavorable thermal and wind conditions. Marsh Harriers are similar to typical soaring migrants in maximizing cross-country speed in soaring-gliding flight, whereas Montagus and Pallid Harriers are less adapted to soaring-gliding flight and thus are similar to smaller flapping migrants. Optimal soaring-gliding flight seems to be less relevant for these smaller harriers; they maximize cross-country performance by efficiently combining different flight styles.

The air speed of migrating birds and its relationship to the wind

SummaryThis paper deals with the flight speeds of migrating birds observed by radar over a Swiss Alpine pass. The distributions of air speeds for different classes of birds and the influence of the wind on air speeds were investigated. Our findings differ greatly from observations of bird migration over the North Atlantic Ocean and the North American continent. Our data reveal: (1) that the air speeds of the bulk of migrating birds were in the range of 8–18 m/s and that the amount of ‘slow’ flyers (air speed below 5 m/s) was less than 5%; (2) that there was an obvious influence of the wind on air speeds and, moreover, the data showed a distinctly variable compensatory behavior among different bird classes; (3) that the wind component in the direction of the birds heading was a better predictor for air speed than the wind direction. Although we do not speculate about possibilities and mechanisms of wind detection, a simple argument shows how birds could estimate wind directions if they do use the moving surface as a reference system.