Nocturnal passerine migration without tailwind assistance

Nocturnal passerine migrants could substantially reduce the amount of energy spent per distance covered if they fly with tailwind assistance and thus achieve ground speeds that exceed their airspeeds (the birds’ speed in relation to the surrounding air). We analysed tracking radar data from two study sites in southern and northern Scandinavia and show that nocturnally migrating passerines, during both spring and autumn migration, regularly travelled without tailwind assistance. Average ground and airspeeds of the birds were strikingly similar for all seasonal and site‐specific samples, demonstrating that winds had little overall influence on the birds’ resulting travel speeds. Distributions of wind effects, measured as (1) the difference between ground and airspeed and (2) the tail/headwind component along the birds’ direction of travel, showed peaks close to a zero wind effect, indicating that the migratory flights often occurred irrespective of wind direction. An assessment of prevailing wind speeds at the birds’ mean altitude indicated a preference for lower wind speeds, with flights often taking place in moderate winds of 3–10 m/s. The limited frequency of wind‐assisted flights among the nocturnal passerine migrants studied is surprising and in clear contrast to the strong selectivity of tailwinds exhibited by some other bird groups. Relatively high costs of waiting for favourable winds, rather low probabilities of occurrence of tailwind conditions and a need to use a large proportion of nights for flying are probably among the factors that explain the lack of a distinct preference for wind‐assisted flights among nocturnal passerine migrants.     

Wind estimation based on thermal soaring of birds

The flight performance of birds is strongly affected by the dynamic state of the atmosphere at the birds' locations. Studies of flight and its impact on the movement ecology of birds must consider the wind to help us understand aerodynamics and bird flight strategies. Here, we introduce a systematic approach to evaluate wind speed and direction from the high‐frequency GPS recordings from bird‐borne tags during thermalling flight. Our method assumes that a fixed horizontal mean wind speed during a short (18 seconds, 19 GPS fixes) flight segment with a constant turn angle along a closed loop, characteristic of thermalling flight, will generate a fixed drift for each consequent location. We use a maximum‐likelihood approach to estimate that drift and to determine the wind and airspeeds at the birds' flight locations. We also provide error estimates for these GPS‐derived wind speed estimates. We validate our approach by comparing its wind estimates with the mid‐resolution weather reanalysis data from ECMWF, and by examining independent wind estimates from pairs of birds in a large dataset of GPS‐tagged migrating storks that were flying in close proximity. Our approach provides accurate and unbiased observations of wind speed and additional detailed information on vertical winds and uplift structure. These precise measurements are otherwise rare and hard to obtain and will broaden our understanding of atmospheric conditions, flight aerodynamics, and bird flight strategies. With an increasing number of GPS‐tracked animals, we may soon be able to use birds to inform us about the atmosphere they are flying through and thus improve future ecological and environmental studies.     

Computer‐automated bird detection and counts in high‐resolution aerial images: a review

Bird surveys conducted using aerial images can be more accurate than those using airborne observers, but can also be more time‐consuming if images must be analyzed manually. Recent advances in digital cameras and image‐analysis software offer unprecedented potential for computer‐automated bird detection and counts in high‐resolution aerial images. We review the literature on this subject and provide an overview of the main image‐analysis techniques. Birds that contrast sharply with image backgrounds (e.g., bright birds on dark ground) are generally the most amenable to automated detection, in some cases requiring only basic image‐analysis software. However, the sophisticated analysis capabilities of modern object‐based image analysis software provide ways to detect birds in more challenging situations based on a variety of attributes including color, size, shape, texture, and spatial context. Some techniques developed to detect mammals may also be applicable to birds, although the prevalent use of aerial thermal‐infrared images for detecting large mammals is of limited applicability to birds because of the low pixel resolution of thermal cameras and the smaller size of birds. However, the increasingly high resolution of true‐color cameras and availability of small unmanned aircraft systems (drones) that can fly at very low altitude now make it feasible to detect even small shorebirds in aerial images. Continued advances in camera and drone technology, in combination with increasingly sophisticated image analysis software, now make it possible for investigators involved in monitoring bird populations to save time and resources by increasing their use of automated bird detection and counts in aerial images. We recommend close collaboration between wildlife‐monitoring practitioners and experts in the fields of remote sensing and computer science to help generate relevant, accessible, and readily applicable computer‐automated aerial photographic census techniques.     

Flight dynamics of Cory’s shearwater foraging in a coastal environment

Flight dynamics theories are influenced by two major topics: how birds adapt their flight to cope with heterogeneous habitats, and whether birds plan to use the wind field or simply experience it. The aim of this study was to understand the flight dynamics of free-flying Cory’s shearwaters in relation to the wind characteristics on the coastal upwelling region of continental Portugal. We deployed recently miniaturised devices—global positioning system loggers to collect precise and detailed information on birds’ positions and motions. Prevalent winds were blowing from the north-east and adults used those winds by adjusting their flight directions mainly towards north-west and south-west, flying with cross and tail winds, respectively, and avoiding head winds. This is confirmation that Cory’s shearwaters use a shear soaring flying strategy while exploiting the environment for food: adults foraged mainly with cross winds and their ground speed was not constant during all foraging trips as it changed dynamically as a result of the ocean surface shear winds. During travelling phases, ground speed was strongly influenced by the position of the bird with regard to the wind direction, as ground speed increased significantly with increasing tail wind component (TWC) values. Adults appear to choose foraging directions to exploit ambient wind, in order to improve shear soaring efficiency (cross winding) and exploit diurnal changes in tail wind strength to maximise commuting efficiency. We report, for the first time, precise ground speed values (GPS-derived data) and computed actual flight speed values (using TWC analysis) for Cory’s shearwater.     

Interactive effects of body mass changes and species‐specific morphology on flight behavior of chick‐rearing Antarctic fulmarine petrels under diurnal wind patterns

For procellariiform seabirds, wind and morphology are crucial determinants of flight costs and flight speeds. During chick‐rearing, parental seabirds commute frequently to provision their chicks, and their body mass typically changes between outbound and return legs. In Antarctica, the characteristic diurnal katabatic winds, which blow stronger in the mornings, form a natural experimental setup to investigate flight behaviors of commuting seabirds in response to wind conditions. We GPS‐tracked three closely related species of sympatrically breeding Antarctic fulmarine petrels, which differ in wing loading and aspect ratio, and investigated their flight behavior in response to wind and changes in body mass. Such information is critical for understanding how species may respond to climate change. All three species reached higher ground speeds (i.e., the speed over ground) under stronger tailwinds, especially on return legs from foraging. Ground speeds decreased under stronger headwinds. Antarctic petrels (Thalassoica antarctica; intermediate body mass, highest wing loading, and aspect ratio) responded stronger to changes in wind speed and direction than cape petrels (Daption capense; lowest body mass, wing loading, and aspect ratio) or southern fulmars (Fulmarus glacialoides; highest body mass, intermediate wing loading, and aspect ratio). Birds did not adjust their flight direction in relation to wind direction nor the maximum distance from their nests when encountering headwinds on outbound commutes. However, birds appeared to adjust the timing of commutes to benefit from strong katabatic winds as tailwinds on outbound legs and avoid strong katabatic winds as headwinds on return legs. Despite these adaptations to the predictable diurnal wind conditions, birds frequently encountered unfavorably strong headwinds, possibly as a result of weather systems disrupting the katabatics. How the predicted decrease in Antarctic near‐coastal wind speeds over the remainder of the century will affect flight costs and breeding success and ultimately population trajectories remains to be seen.     

Optimal flight behavior of soaring migrants: a case study of migrating steppe buzzards, Buteo buteo vulpinus

This article presents tests of the theoretical predictions onoptimal soaring and gliding flight of large, diurnal migrantsusing Pennycuick's program 2 for "bird flight performance."Predictions were compared with 141 observed flight paths ofmigrating steppe buzzards, Buteo buteo vulpinus. Calculationsof cross-country speed relative to the air included bird's airspeedsand sinking rates in interthermal gliding and climbing ratesin thermal circling. Steppe buzzards adjusted interthermal glidingairspeed . according to their actual climbing rate in thermalcircling. By optimizing their gliding airspeed, the birds maximizedtheir crosscountry performance relative to the air. Despitethis general agreement with the model, there was much scatterin the data, for the model neglects horizontal winds and updraftsduring the gliding phase. Lower sinking rates due to updraftsduring the gliding phases allowed many birds to achieve highercross-country speeds than predicted. In addition, birds reactedto different wind directions and speeds: in side and opposingwinds, the steppe buzzards compensated for wind displacementduring soaring and increased their gliding airspeed with decreasingtailwind component Nevenheless, cross-country speed relativeto the ground, which is the important measure for a migratorybird, was still higher under following winds. This study showsthat Pennycuick's program 2 provides reliable predictions onoptimal soaring and gliding behavior using realistic assumptionsand constants in the model, but a great deal of variation aroundthe mean is generated by factors not included in the model     

Soaring across continents: decision‐making of a soaring migrant under changing atmospheric conditions along an entire flyway

Thermal soaring birds reduce flight‐energy costs by alternatingly gaining altitude in thermals and gliding across the earth's surface. To find out how soaring migrants adjust their flight behaviour to dynamic atmospheric conditions across entire migration routes, we combined optimal soaring migration theory with high‐resolution GPS tracking data of migrating honey buzzards Pernis apivorus and wind data from a global numerical atmospheric model. We compared measurements of gliding air speeds to predictions based on two distinct behavioural benchmarks for thermal soaring flight. The first being a time‐optimal strategy whereby birds alter their gliding air speeds as a function of climb rates to maximize cross‐country air speed over a full climb– glide cycle (V_opt). The second a risk‐averse energy‐efficient strategy at which birds alter their gliding air speed in response to tailwinds/headwinds to maximize the distance travelled in the intended direction during each glide phase (V_bgw). Honey buzzards were gliding on average 2.05 ms^{– 1} slower than V_opt and 3.42 ms^{– 1} faster than V_bgw while they increased air speeds with climb rates and reduced air speeds in tailwinds. They adopted flexible flight strategies gliding mostly near V_bgw under poor soaring conditions and closer to V_opt in good soaring conditions. Honey buzzards most adopted a time‐optimal strategy when crossing the Sahara, and at the onset of spring migration, where and when they met with the best soaring conditions. The buzzards nevertheless glided slower than V_opt during most of their journeys, probably taking time to navigate, orientate and locate suitable thermals, especially in areas with poor thermal convection. Linking novel tracking techniques with optimal migration models clarifies the way birds balance different tradeoffs during migration.     

Avoidance of headwinds or exploitation of ground effect—why do birds fly low?

ABSTRACT Birds often fly close to the ground or water. Wind shear theory predicts that wind speeds decline with proximity to the substratum, so birds might be expected to fly lower when flying upwind than when flying downwind. We tested this prediction and found that the wind shear equation is valid at heights below 4 m, with wind speed over a smooth surface ～40% lower at a height of 0.08 m than at 4 m. Birds that fly close enough to smooth substrata can also benefit energetically from ground effect, where vortices generated by their flight interact with the ground or water. This suggests that birds should use ground effect more when flying upwind than when flying downwind. We determined the percent time spent flying in ground effect by 21 species of passerine and non‐passerine birds flying in sheltered coastal aquatic and nearby terrestrial areas of County Cork, Ireland. We found that use of ground effect was uncommon for passerines, but common for a variety of non‐passerine waterbirds. However, phylogenetic analysis indicates no linkage between phylogeny and incidence of ground effect use and it is probable that incidence of use is determined by ecology rather than phylogeny. Great Cormorants (Phalacrocorax carbo) used ground effect most frequently over water (59.4% of time in flight). Over land, Barn Swallows (Hirundo rustica) used ground effect most often (19.8% of time). Phylogenetic contrasts regression analysis showed no significant relationship between use of ground effect and either wing aspect ratio or wing loading for 18 of our focal species, though simple linear regression analysis indicated that birds with greater wing loading used ground effect slightly (but significantly) more often. We found that 95% of Great Cormorants flying upwind used ground effect whereas only 35% did so when flying downwind. Few Black‐headed Gulls (Chroicocephalus ridibundus) used ground effect (probably because they fly high to locate prey), but still showed greater use when flying upwind (25%) than downwind (2.5%). When flying upwind in ground effect at the wind speeds encountered in our study, the velocity for minimum power (V_mp) for Great Cormorants was exceeded, suggesting theoretical benefits of about 14.3%. Our study indicates that several species exploit both wind shear and ground effect to minimize energy expenditure during commuting and foraging, but that others do not, because of either complexity of habitat morphology or the demands of their foraging ecology.     

Local and regional weather patterns influencing post‐breeding migration counts of soaring birds at the Strait of Gibraltar,Spain

Migration is a significant event in the annual cycle of many avian species. During migration birds face many challenges, including unfamiliar foraging and refuge habitats, resulting in a much higher rate of mortality during migration than during other seasons of the year. Weather may significantly affect a bird's decision to initiate migration, the course and pace of migration, and its survival during migration. Each of these influences may impact the counts of migrating birds at geographical convergence zones or bottlenecks. It is important to quantify the effect of short‐term weather on these counts to appropriately interpret and use such counts in other analyses. To this end, we aim to assess the effects of local and regional weather conditions on the migration counts of soaring birds at the Strait of Gibraltar during post‐breeding migration. We used information‐theoretic approaches to analyse the influence of local weather and weather in northern Spain on the migration counts of five soaring bird species from two count sites near the Strait of Gibraltar. Migration counts were higher on days with local northerly and westerly winds, often following a day of easterly winds, on days with local high pressure systems, and often following a day of lower pressure. Weather conditions in northern Spain influenced migration counts at the Strait of Gibraltar, but the effects were much weaker than local weather conditions. We confirm that short‐term weather conditions, locally and regionally, can influence migration counts and should thus be considered when these counts are used in other analyses.     

Flying at no mechanical energy cost: disclosing the secret of wandering albatrosses

ALBATROSSES DO SOMETHING THAT NO OTHER BIRDS ARE ABLE TO DO: fly thousands of kilometres at no mechanical cost. This is possible because they use dynamic soaring, a flight mode that enables them to gain the energy required for flying from wind. Until now, the physical mechanisms of the energy gain in terms of the energy transfer from the wind to the bird were mostly unknown. Here we show that the energy gain is achieved by a dynamic flight manoeuvre consisting of a continually repeated up-down curve with optimal adjustment to the wind. We determined the energy obtained from the wind by analysing the measured trajectories of free flying birds using a new GPS-signal tracking method yielding a high precision. Our results reveal an evolutionary adaptation to an extreme environment, and may support recent biologically inspired research on robotic aircraft that might utilize albatrosses' flight technique for engineless propulsion.     

Is there a connection between weather at departure sites,onset of migration and timing of soaring‐bird autumn migration in Israel?

Aims Different aspects of soaring‐bird migration are influenced by weather. However, the relationship between weather and the onset of soaring‐bird migration, particularly in autumn, is not clear. Although long‐term migration counts are often unavailable near the breeding areas of many soaring birds in the western Palaearctic, soaring‐bird migration has been systematically monitored in Israel, a region where populations from large geographical areas converge. This study tests several fundamental hypotheses regarding the onset of migration and explores the connection between weather, migration onset and arrival at a distant site. Location Globally gridded meteorological data from the breeding areas in north‐eastern Europe were used as predictive variables in relation to the arrival of soaring migrants in Israel. Methods Inverse modelling was used to study the temporal and spatial influence of weather on initiation of migration based on autumn soaring‐bird migration counts in Israel. Numerous combinations of migration duration and temporal influence of meteorological variables (temperature, sea‐level pressure and precipitable water) were tested with different models for meteorological sensitivity. Results The day of arrival in Israel of white storks, honey buzzards, Levant sparrowhawks and lesser spotted eagles was significantly and strongly related to meteorological conditions in the breeding area days or even weeks before arrival in Israel. The cumulative number of days or cumulative value above or below a meteorological threshold performed significantly better than other models tested. Models provided reliable estimates of migration duration for each species. Main conclusions The meteorological triggers of migration at the breeding grounds differed between species and were related to deteriorating living conditions and deteriorating migratory flight conditions. Soaring birds are sensitive to meteorological triggers at the same period every year and their temporal response to weather appears to be constrained by their annual routine.     

Use of time‐lapse photography and digital image analysis to estimate breeding success of a cliff‐nesting seabird

Development of new methods for obtaining basic demographic data from difficult‐to‐access breeding colonies and easily disturbed species is an important challenge in studies of seabirds. We describe a method that can generate data concerning annual breeding success of cliff‐nesting seabirds or other colonial birds with open nests. Our method requires only a single visit to a colony every second or third year, and is based on the use of automated time‐lapse photography. To test our method, we used time‐lapse photos to examine the breeding success of Thick‐billed Murres (Uria lomvia) in two breeding colonies in Greenland during the years 2011, 2012, and 2014. Based on the analysis of time‐lapse photos, we found that hatching success during the 3 yr of our study ranged from 60% to 81%, fledging success from 89% to 95%, and breeding success from 53% to 74% (Table 1). Use of digital image analysis made it possible to differentiate between breeding and non‐breeding birds and determine if and when breeding attempts failed or succeeded. The key to making our method a realistic long‐term monitoring technique is the use of an automated, formal image analysis to process the thousands of photos from the time‐lapse cameras and, more specifically, to reduce a large number of photos to a manageable number. Using our method, we needed 12–22 h per study plot, depending on the number of breeding sites per plot (range = 47–127) and whether it was the first or the second time the plot was analyzed, to obtain our estimates of hatching, fledging, and breeding success. This included time for data preparation, image analyses, visual inspections, and summarizing data in a spreadsheet. We found that our estimates of breeding success were comparable to those obtained by direct observation in the field. An important aspect of using time‐lapse technology is to foresee potential reasons why time‐lapse cameras might stop taking pictures, for example, equipment failure (camera, timer, or battery) or interference by visitors (e.g., vandalism or theft). As such, thorough testing of time‐lapse systems and selecting camera locations less likely to be disturbed are most important. We believe that use of time‐lapse photography in combination with digital image analysis to estimate breeding success can be useful for determining the breeding success of other cliff‐nesting seabirds and, more generally, other birds that breed in colonies, especially those located in remote areas and where direct observation may disturb birds.     

On the importance of estimating detection probabilities from at‐sea surveys of flying seabirds

The primary and accepted method used to estimate seabird densities at sea from ships is the strip transect method, designed to correct for the effect of random directional bird movement relative to that of the ship. However, this method relies on the critical assumption that all of the birds within the survey strip are detected. We used the distance sampling method from line‐transects to estimate detection probability of a number of species of flying seabirds, and to test whether distance from the ship and bird body size affected detectability. Detection probability decreased from 0.987 (SE=0.029) to 0.269 (SE=0.035) with increasing strip half‐width from 100 to 1400 m. Detection probability also varied between size‐groups of species with strip half‐width. For all size‐groups, this probability was close to 1 for strip half‐width of 100 m, but was 0.869 (SE=0.115), 0.725 (SE=0.096) and 0.693 (SE=0.091) for strip half‐width of 300 m, a typical strip width used in seabird surveys, for respectively large, medium and small size flying seabirds. For larger strip half‐width, detection probability was higher for large sized species, intermediate for medium sized species and lower for smaller sized species. For strip half‐width larger than 100 m we suggest that more attention should be paid to testing the assumption of perfect detectability, because abundance estimates may be underestimated when this assumption is violated. Finally, the effect of the speed of travel of flying seabird on the detection probability was estimated in a simulation study, which suggests that detection probability was underestimated with increasing flying speed.     

Field studies of avian nocturnal migratory orientation II. Experimental manipulation of orientation in white-throated sparrows (Zonotrichia albicollis) released aloft

Migrant white-throated sparrows (Zonotrichia albicollis) were released from boxes carried aloft by balloon and tracked with radar. All birds were released on clear nights when winds were light and opposed to the normal migration direction for the season. Birds were treated in one of two ways: Lens birds were fitted with frosted lenses prior to release; No Lens birds were released without lenses. No Lens birds that engaged in straight and level flight generally headed in the predicted migratory direction and as a group were not oriented with respect to wind direction. Lens birds did not head in the predicted migratory direction, but instead oriented downwind. This orientation behaviour is consistent with the relationship of orientation cues inferred from the field observations described in part I of this paper. The data show that flying birds deprived of all detailed form vision can determine wind direction.     

Extracting bird migration information from C‐band Doppler weather radars

Although radar has been used in studies of bird migration for 60 years, there is still no network in Europe for comprehensive monitoring of bird migration. Europe has a dense network of military air surveillance radars but most systems are not directly suitable for reliable bird monitoring. Since the early 1990s, Doppler radars and wind profilers have been introduced in meteorology to measure wind. These wind measurements are known to be contaminated with insect and bird echoes. The aim of the present research is to assess how bird migration information can be deduced from meteorological Doppler radar output. We compare the observations on migrating birds using a dedicated X‐band bird radar with those using a C‐band Doppler weather radar. The observations were collected in the Netherlands, from 1 March to 22 May 2003. In this period, the bird radar showed that densities of more than one bird per km³ are present in 20% of all measurements. Among these measurements, the weather radar correctly recognized 86% of the cases when birds were present; in 38% of the cases with no birds detected by the bird radar, the weather radar claimed bird presence (false positive). The comparison showed that in this study reliable altitudinal density profiles of birds cannot be obtained from the weather radar. However, when integrated over altitude, weather radar reflectivity is correlated with bird radar density. Moreover, bird flight speeds from both radars show good agreement in 78% of cases, and flight direction in 73% of cases. The usefulness of the existing network of weather radars for deducing information on bird migration offers a great opportunity for a European‐wide monitoring network of bird migration.     

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.     

The flight behaviour of individual passerine nocturnal migrants: A tracking radar study

Nocturnal passerine migrants were tracked with a small automatic tracking radar during spring migration in eastern New York. Climbing, descending and markedly non-linear tracks were selected for analysis because they may reveal relationships not evident in normal straight and level tracks. Climbing individuals ascended at 1 to 2 vertical metres per second by heading into the wind and increasing their ascent angles while air speed tended to remain constant. Within individual tracks, birds flew slower when flying downwind than when flying into the wind and changes in air speed were performed over periods of a few seconds. A small amount of data suggested that this behaviour did not occur under overcast skies. Both the direction and speed of the wind force were important in predicting air speed. Multiple regression analysis indicated that faster flying birds were more likely to fly in winds of high speed and at large angles into the wind.     

The influence of weather on the flight altitude of nocturnal migrants in mid‐latitudes

By altering its flight altitude, a bird can change the atmospheric conditions it experiences during migration. Although many factors may influence a bird's choice of altitude, wind is generally accepted as being the most influential. However, the influence of wind is not clearly understood, particularly outside the trade‐wind zone, and other factors may play a role. We used operational weather radar to measure the flight altitudes of nocturnally migrating birds during spring and autumn in the Netherlands. We first assessed whether the nocturnal altitudinal distribution of proportional bird density could be explained by the vertical distribution of wind support using three different methods. We then used generalized additive models to assess which atmospheric variables, in addition to altitude, best explained variability in proportional bird density per altitudinal layer each night. Migrants generally remained at low altitudes, and flight altitude explained 52 and 73% of the observed variability in proportional bird density in spring and autumn, respectively. Overall, there were weak correlations between altitudinal distributions of wind support and proportional bird density. Improving tailwind support with height increased the probability of birds climbing to higher altitude, but when birds did fly higher than normal, they generally concentrated around the lowest altitude with acceptable wind conditions. The generalized additive model analysis also indicated an influence of temperature on flight altitudes, suggesting that birds avoided colder layers. These findings suggested that birds increased flight altitudes to seek out more supportive winds when wind conditions near the surface were prohibitive. Thus, birds did not select flight altitudes only to optimize wind support. Rather, they preferred to fly at low altitudes unless wind conditions there were unsupportive of migration. Overall, flight altitudes of birds in relation to environmental conditions appear to reflect a balance between different adaptive pressures.     

Testing an emerging paradigm in migration ecology shows surprising differences in efficiency between flight modes

To maximize fitness, flying animals should maximize flight speed while minimizing energetic expenditure. Soaring speeds of large-bodied birds are determined by flight routes and tradeoffs between minimizing time and energetic costs. Large raptors migrating in eastern North America predominantly glide between thermals that provide lift or soar along slopes or ridgelines using orographic lift (slope soaring). It is usually assumed that slope soaring is faster than thermal gliding because forward progress is constant compared to interrupted progress when birds pause to regain altitude in thermals. We tested this slope-soaring hypothesis using high-frequency GPS-GSM telemetry devices to track golden eagles during northbound migration. In contrast to expectations, flight speed was slower when slope soaring and eagles also were diverted from their migratory path, incurring possible energetic costs and reducing speed of progress towards a migratory endpoint. When gliding between thermals, eagles stayed on track and fast gliding speeds compensated for lack of progress during thermal soaring. When thermals were not available, eagles minimized migration time, not energy, by choosing energetically expensive slope soaring instead of waiting for thermals to develop. Sites suited to slope soaring include ridges preferred for wind-energy generation, thus avian risk of collision with wind turbines is associated with evolutionary trade-offs required to maximize fitness of time-minimizing migratory raptors.     

Confronting the winds: orientation and flight behaviour of roosting swifts, Apus apus

Swifts, Apus apus, spend the night aloft and this offers an opportunity to test the degree of adaptability of bird orientation and flight to different ecological situations. We predicted the swifts' behaviour by assuming that they are adapted to minimize energy expenditure during the nocturnal flight and during a compensatory homing flight if they become displaced by wind. We tested the predictions by recording the swifts' altitudes, speeds and directions under different wind conditions with tracking radar; we found an agreement between predictions and observations for orientation behaviour, but not for altitude and speed regulation. The swifts orientated consistently into the head wind, with angular concentration increasing with increasing wind speed. However, contrary to our predictions, they did not select altitudes with slow or moderate winds, nor did they increase their airspeed distinctly when flying into strong head winds. A possible explanation is that their head-wind orientation is sufficient to keep nocturnal displacement from their home area within tolerable limits, leaving flight altitude to be determined by other factors (correlated with temperature), and airspeed to show only a marginal increase in strong winds. The swifts were often moving "backwards", heading straight into the wind but being overpowered by wind speeds exceeding their airspeed. The regular occurrence of such flights is probably uniquely associated with the swifts' remarkable habit of roosting on the wing.