Potential use of weather radar to study movements of wintering waterfowl

To protect and restore wintering waterfowl habitat, managers require knowledge of routine wintering waterfowl movements and habitat use. During preliminary screening of Doppler weather radar data we observed biological movements consistent with routine foraging flights of wintering waterfowl known to occur near Lacassine National Wildlife Refuge (NWR), Louisiana. During the winters of 2004–2005 and 2005–2006, we conducted field surveys to identify the source of the radar echoes emanating from Lacassine NWR. We compared field data to weather radar reflectivity data. Spatial and temporal patterns consistent with foraging flight movements appeared in weather radar data on all dates of field surveys. Dabbling ducks were the dominant taxa flying within the radar beam during the foraging flight period. Using linear regression, we found a positive log-linear relationship between average radar reflectivity (Z) and number of birds detected over the study area (P < 0.001, r² = 0.62, n = 40). Ground observations and the statistically significant relationship between radar data and field data confirm that Doppler weather radar recorded the foraging flights of dabbling ducks. Weather radars may be effective tools for wintering waterfowl management because they provide broad-scale views of both diurnal and nocturnal movements. In addition, an extensive data archive enables the study of wintering waterfowl response to habitat loss, agricultural practices, wetland restoration, and other research questions that require multiple years of data. © 2011 The Wildlife Society.     

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.     

Spring nocturnal migration of Reed Warblers Acrocephalus scirpaceus: departure, landing and body condition

Nocturnal migration of Reed Warblers Acrocephalus scirpaceus was studied by trapping with 'high nets' on the Courish Spit (Eastern Baltic) during spring 1998–2000. In spring, Reed Warblers left the stopover site between 45 and 240 min after sunset (median 84 min), although 85% of birds took off between 45 and 120 min after sunset. Birds did not arrive until the fifth hour after sunset; 67% of birds ended their nocturnal flights in the penultimate hour before sunrise, i.e. at dawn. At the moment of migratory departure, the average Reed Warbler body mass was 12.79 ± 0.66 g ( n  = 60). Average body mass of birds ending migratory flight was 11.69 ± 0.67 g ( n  = 18). The difference was highly significant. However, more than half of the birds completed migratory flights with a considerable fuel load, and some even had energy stores sufficient for a migratory flight on the next night. The spring migratory strategy of Reed Warblers over Central and Northern Europe probably includes a succession of short migratory flights (4–6 h) during several subsequent nights with 1-day stopovers.     

Detection and discrimination of fauna in the aerosphere using Doppler weather surveillance radar

Organisms in the aerosphere have been detected by radar sinceits development in the 1940s. The national network of Dopplerweather radars (WSR-88D) in the United States can readily detectbirds, bats, and insects aloft. Level-II data from the radarcontain information on the reflectivity and radial velocityof targets and on width of the spectrum (SD of radial velocitiesin a radar pulse volume). Information on reflectivity can beused to quantify density of organisms aloft and radial velocitycan be used to discriminate different types of targets basedon their air speeds. Spectral width can also provide some usefulinformation when organisms with very different air speeds arealoft. Recent work with dual-polarization radar suggests thatit may be useful for discriminating birds from insects in theaerosphere, but more development and biological validation arerequired.     

Circadian flight schedules in night-migrating birds caught on migration

Many species of migratory birds migrate in a series of solitary nocturnal flights. Between flights, they stop to rest and refuel for the next segment of their journey. The mechanism controlling this behaviour has long remained elusive. Here, we show that wild-caught migratory redstarts (Phoenicurus phoenicurus) are consistent in their flight scheduling. An advanced videographic system enabled us to determine the precise timing of flight activity in redstarts caught at a northern European stopover site during their return trip from Africa. Birds were held captive for three days in the absence of photoperiodic cues (constant dim light) and under permanent food availability. Despite the absence of external temporal cues, birds showed clear bimodal activity patterns: intense nocturnal activity alternating with diurnal foraging and resting periods. The onset of their migratory activity coincided with the time of local sunset and was individually consistent on consecutive nights. The data demonstrate that night-migrating birds are driven by autonomous circadian clocks entrained by sunset cues. This timekeeping system is probably the key factor in the overall control of nocturnal songbird migration.     

Within night correlations between radar and ground counts of migrating songbirds

ABSTRACT.   Studies comparing numbers of nocturnal migrants in flight with numbers of migrants at stopover sites have produced equivocal results. In 2003, we compared numbers of nocturnal migrants detected by radar to numbers of passerines observed at the Atlantic Bird Observatory in southwestern Nova Scotia, Canada. Numbers of nocturnal migrants detected by radar were positively correlated with numbers of migrants as determined by mist-netting, censuses, and daily estimated totals (daily estimates of birds present based netting and census results and casual observations) the following day. On nights with winds favorable for migration (tailwinds), the peak correlation between ground counts and radar counts the night before occurred just after sunset. On nights with unfavorable winds (headwinds), the correlation increased through the night, with a peak just before sunrise. The patterns of correlation are consistent with a scenario where birds accumulate at the coastline during periods of unfavorable wind, likely because they are not willing to cross a major ecological barrier, the Gulf of Maine. On nights with favorable winds, many birds departed, but some, possibly after testing wind conditions, apparently decided not to cross the Gulf of Maine and returned. Our results suggest that combining data collected using different methods to generate a daily estimated total provides the best estimate of the number of migrants present at a stopover site. Simultaneous studies at multiple locations where different census methods are used, making more effective use of temporal data (both from radar and diurnal counts), will more clearly elucidate patterns of flight behavior by migratory songbirds and the relationship between ground counts and counts of birds aloft.     

Environmental effects on flying migrants revealed by radar

Migratory animals are affected by various factors during their journeys, and the study of animal movement by radars has been instrumental in revealing key influences of the environment on flying migrants. Radars enable the simultaneous tracking of many individuals of almost all sizes within the radar range during day and night, and under low visibility conditions. We review how atmospheric conditions, geographic features and human development affect the behavior of migrating insects and birds as recorded by radars. We focus on flight initiation and termination, as well as in‐flight behavior that includes changes in animal flight direction, speed and altitude. We have identified several similarities and differences in the behavioral responses of aerial migrants including an overlooked similarity in the use of thermal updrafts by very small (e.g. aphids) and very large (e.g. vultures) migrants. We propose that many aerial migrants modulate their migratory flights in relation to the interaction between atmospheric conditions and geographic features. For example, aerial migrants that encounter crosswind may terminate their flight or continue their migration and may also drift or compensate for lateral displacement depending on their position (over land, near the coast or over sea). We propose several promising directions for future research, including the development and application of algorithms for tracking insects, bats and large aggregations of animals using weather radars. Additionally, an important contribution will be the spatial expansion of aeroecological radar studies to Africa, most of Asia and South America where no such studies have been undertaken. Quantifying the role of migrants in ecosystems and specifically estimating the number of departing birds from stopover sites using low‐elevation radar scans is important for quantifying migrant–habitat relationships. This information, together with estimates of population demographics and migrant abundance, can help resolve the long‐term dynamics of migrant populations facing large‐scale environmental changes.     

Waterfowl on weather radar: applying ground-truth to classify and quantify bird movements

ABSTRACT Local and migratory movements aloft have important implications for the ecology and conservation of birds, but are difficult to quantify. Weather surveillance radar (WSR) offers a unique tool for observing movements of birds, but until now has been used primarily to address broad taxonomic questions. Herein, we demonstrate how natural history information and ground‐truthing can be used to answer quantitative and taxon‐specific questions regarding bird movements on WSR. We found that super‐resolution Level II data from the National Oceanic and Atmospheric Administration's mass storage system was the most effective format and source of WSR data, and that several software packages were needed for thorough analysis of WSR data. Using WSR, we identified potential movements of birds emigrating from a waterfowl stopover area in Illinois in fall (1 September–31 December) 2006 and 2007. We compared spatial and temporal patterns of these movements to the natural history of taxa occupying the source habitat and classified these radar targets as dabbling ducks (tribe Anatini). A portable X‐band radar measured the cruising heights of ducks at 400–600 m. During fall 2008, we conducted ground‐truthing with a thermal infrared camera to enumerate birds passing over our field site during nocturnal migration events. This estimate of bird density, paired with an associated sample of WSR echo strength, provided a mean radar cross section the same as dabbling ducks (112.5 cm²) and supported our natural‐history‐based classification. Thermal infrared‐estimated duck densities explained most of the variation (R²= 0.91) in WSR echo strength across seven migration events of varying intensities, suggesting that radar cross sections of dabbling ducks and WSR reflectivity can be used to estimate duck numbers in other comparable contexts. Our results suggest that careful investigation of the spatial and temporal patterns of movements on radar, along with field‐based ground‐truthing, can be used to study and quantify the movements of specific bird taxa.     

Nocturnal migratory songbirds adjust their travelling direction aloft: evidence from a radiotelemetry and radar study

In order to fully understand the orientation behaviour of migrating birds, it is important to understand when birds set their travel direction. Departure directions of migratory passerines leaving stopover sites are often assumed to reflect the birds'' intended travel directions, but this assumption has not been critically tested. We used data from an automated radiotelemetry system and a tracking radar at Falsterbo peninsula, Sweden, to compare the initial orientation of departing songbirds (recorded by radiotelemetry) with the orientation of songbird migrants in climbing and level flight (recorded by radar). We found that the track directions of birds at high altitudes and in level flight were more concentrated than the directions of departing birds and birds in climbing flight, which indicates that the birds adjust their travelling direction once aloft. This was further supported by a wide scatter of vanishing bearings in a subsample of radio-tracked birds that later passed an offshore radio receiver station 50 km southeast of Falsterbo. Track directions seemed to be more affected by winds in climbing compared with level flights, which may be explained by birds not starting to partially compensate for wind drift until they have reached cruising altitudes.     

The Flight Behavior of Migrating Birds in Changing Wind Fields: Radar and Visual Analyses

This paper examines the influence of atmospheric structure andmotion (principally winds aloft) on the flight behavior andaltitudinal distribution of migrating songbirds. Bird migrationdata that I gathered using surveillance radars operated by theUnited States National Weather Service and the Federal AviationAdministration and a vertically directed fixed-beam marine radarmounted on a mobile laboratory are analyzed in relation to windsaloft. Migrating birds appear to fly at altitudes where windswill minimize the cost of transport and assist movements inseasonally appropriate directions. When migratory flights occurat altitudes that are higher than usual, a significant correlationexists between the altitude of densest migration and the altitudeof most favorable wind. Lower altitudes may be favored overslightly more favorable winds at much higher altitudes. Radardata on the flight behavior of migrating birds in the vicinityof frontal systems is also examined. The flight strategies ofmigrants (fly over the front, change the direction of flight,or land and terminate the flight) differ depending on seasonand the "thickness" of the front. Recent migration studies thatare related to atmospheric structure and motion are summarizedand related to atmospheric processes operating simultaneouslyat vastly different spatial and temporal scales.     

Analyzing NEXRAD doppler radar images to assess nightly dispersal patterns and population trends in Brazilian free-tailed bats (Tadarida brasiliensis)

Operators of early weather-surveillance radars often observedechoes on their displays that did not behave like weather pattern,including expanding ring-like shapes they called angels. Theseechoes were caused by high-flying insects, migrating birds,and large colonies of bats emerging from roosts to feed. Modernweather-surveillance radar stations in the United States (NEXt-generationRADar or NEXRAD) provide detailed images that clearly show eveningbat emergences from large colonies. These images can be usedto investigate the flight behavior of groups of bats and populationtrends in large colonies of Brazilian free-tailed bats (Tadaridabrasiliensis) in south-central Texas which are clearly imagedby local NEXRAD radar stations. In this study, we used radarreflectivity data from the New Braunfels, Texas NEXRAD stationto examine relative colony size, direction of movement, speedof dispersion, and altitude gradients of bats from these coloniesfollowing evening emergence. Base reflectivity clear-air-modeLevel-II images were geo-referenced and compiled in a GIS alongwith locations of colonies and features on the landscape. Temporalsequences of images were filtered for the activity of bats,and from this, the relative size of bat colonies, and the speedand heading of bat emergences were calculated. Our results indicatecyclical changes in colony size from year to year and that initialheadings taken by bats during emergence flights are highly directional.We found that NEXRAD data can be an effective tool for monitoringthe nightly behavior and seasonal changes in these large colonies.Understanding the distribution of a large regional bat populationon a landscape scale has important implications for agriculturalpest management and conservation efforts.     

Strategies of passerine migration across the Mediterranean Sea and the Sahara Desert: a radar study

Radar observations of the diurnal timing of bird migration in the Sahara Desert are presented for autumn migration. Study sites were on a transect along the north-south migratory direction. Three groups of birds migrating either during day, evening or night in the northern part of the Western desert in Egypt were identified. The maximum of day and night groups occurred later the further south the study sites were. Based on the distance between sites and the timing of peak migration, birds were flying at an estimated ground speed of about 20 m/s. The maximum of the evening group was at about 21:00 h at all sites. The three groups were classified according to three different strategies of migration across the Mediterranean Sea and the Sahara Desert: (1) the day group of birds performed a non-stop flight across the sea and at least the northern part of the desert; [2] the night group performed an intermittent migratory strategy with stopover at the coast of Egypt to continue migration the next evening; (3) the evening group birds were also intermittent migratory fliers, but they stopped somewhere in the desert after a continuous flight across the sea and part of the desert. About 20% of all migrants are involved in non-stop migration and 80% in intermittent migration with stopover at the coast (70%) or with stopover in the desert (10%). It is argued that any species of small passerine has the option to use any of the three strategies.     

Evidence for widely dispersed birds migrating together at night

Lore and indirect evidence from previous studies suggest thatnocturnally migrating vertebrates (perhaps bats but mostly birds)sometimes fly widely dispersed from each other, but in flocks.The observations include stationary and scanning radars, recordingsof flight calls, and watching the moon with telescopes. Directobservations of such flocks have been lacking. This articlepresents data from novel tracking of nocturnal aggregationsof radar targets. Statistical analysis of straight, detailedflight paths supported the hypothesis that vertebrates, almostcertainly birds, flying within about 200–300 m of eachother fly parallel (in the same direction at the same speed)more often than do vertebrates flying farther apart. This inferencewas strengthened by comparisons with a partial control for windand for small-scale atmospheric structure: namely, small nocturnalarthropods tracked by the identical method did not fly parallel.Radar data also indicated that birds flying together may havesimilar wing beats, suggesting taxonomic similarity betweenbirds flying parallel. Possible functions include not only mutualbenefits on the ground during migratory stopover (habitat use,avoidance of predators, and social feeding) but also in-flightsharing of information about orientation.     

Barometer logging reveals new dimensions of individual songbird migration

Recent advances in tracking technology are based on the use of miniature sensors for recording new aspects of individual migratory behaviour. In this study, we have used activity data loggers with barometric and temperature sensors to record the flight altitudes as well as ground elevations during stationary periods of migratory songbirds. We tracked one individual of red‐backed shrike and one great reed warbler along their autumn migration from Europe to Africa. Both individuals performed their migration stepwise in travel segments and climbed most metres during the passage across the Mediterranean Sea and the Sahara Desert and least metres during the first flight segment in Europe. The great reed warbler reached its highest flight altitude of 3950 m a.s.l. during the travel segment from Europe to west Africa, while the red‐backed shrike reached 3650 m a.s.l as maximum flight altitude during its travel segment from Sahel to southern Africa. Both individuals used both lowlands and highlands for resting periods along their migrations. Furthermore, temperature decreased with increasing altitude during migratory flights for both individuals, highlighting the potential to determine flight duration from temperature measurements. Finally, we discuss how barometric data could be used to investigate birds’ responses to changes in air pressure as a cue for departures on migratory flights. This new technique, i.e. using a miniature data logger with barometric pressure sensor to estimate flight altitudes and ground elevations, will open up new avenues for research and importantly advance our understanding on how small birds behave during migratory flights.     

Do bird captures reflect migration intensity? – Trapping numbers on an Alpine pass compared with radar counts

A limitation of standardized mist netting for monitoring migration is caused by the lack of knowledge about the relationship between trapped birds and birds flying aloft. Earlier studies related nocturnal radar counts with trapping data of the following day. In this study, we compared for the first time data gathered simultaneously by radar and mist netting, separately for diurnal and nocturnal migration. Trapping numbers were strongly correlated with migratory intensities measured by radar (r>0.6). A multiple regression analysis, including wind speed and wind direction explained 61% of variation in the number of captures. During the night, and particularly with favourable winds, birds flew at higher altitudes and hence escaped the nets to a higher proportion. The number of nocturnal migrants trapped during daytime was well correlated with migratory intensities observed by radar in the preceding night. The diurnal time patterns, however, revealed fundamental differences between trapping counts and radar observations. This was mainly due to increasing and decreasing flight altitudes in the course of the night, and by the limitations of the radar technique that underestimates migratory intensities during the day when birds aggregate in flocks. In relation to the migratory intensity recorded by radar, diurnal migrants are trapped in a much higher proportion than nocturnal migrants. Finally, our results confirm that trapping data from a site hardly used for stopover are well suited to represent the ongoing migration during the day and night.     

Wind patterns as a potential driver in the evolution and maintenance of a North American migratory suture zone

Suture zones are areas where range contact zones and hybrid zones of multiple taxa are clustered. Migratory divides, contact zones between divergent populations that breed adjacent to one another but use different migratory routes, are a particular case of suture zones. Although multiple hypotheses for both the formation and maintenance of migratory divides have been suggested, quantitative tests are scarce. Here, we tested whether a novel factor, prevailing winds, was sufficient to explain both the evolution and maintenance of the Cordilleran migratory divide using individual‐based models. Empirical observations of eastern birds suggest a circuitous migratory route across Canada before heading south. Western breeders, however, travel south along the Pacific coast to their wintering grounds. We modeled the effect of wind on bird migratory flights by allowing them to float at elevation using spatially explicit modeled wind data. Modeled eastern birds had easterly mean trajectories, whereas western breeders showed significantly more southern trajectories. We also determined that a mean airspeed of 18.5 m s^–1 would be necessary to eliminate this difference in trajectory, a speed that is achieved by waterfowl and shorebirds, but is faster than songbird flight speeds. These results lend support for the potential importance of wind in shaping the phylogeographic history of North American songbirds.     

10. PROCEEDINGS OF THE SEVENTEENTH ANNUAL GENERAL MEETING OF THE S.A. ORNITHOLOGICAL SOCIETY HELD IN THE COMMITTEE ROOM OF THE PUBLIC LIBRARY. JOHANNESBURG,AT 3 P.M., SATURDAY, 8TH MARCH. 1947

Bruderer, B. 1994. Nocturnal bud migration in the Negev (Israel) a tracking radar study. Ostrich 65: 204–212.

The present publication summarizes the methodological possibilities of tracking radar and describes some features of nocturnal migration at two sites in the Negev, which include anwers to basic questions of bird migration. The directions of spring and autumn migration were practically opposite; only the headings in spring indicated some more compensation for stronger westerly winds. The volume of nocturnal spring migration was only about 65% of autumn migration, which may be an indication of mortality outside tie breeding area. Highest densities of migration at the two radar sites in the Negev Highlands (450 m above sea level) and in the Arava Valley (150 m below sea level) indicated flightlevels adjusted to atmospheric conditions aloft, and not to round level. Due to the trade-wind system, the birds heading southward in autumn flew mainly below flew mainly above 1500 m above sea level, while in spring they tended to make use of the anti-trades at higher altitudes. The decisive factor for altitude choice was the speed of tailwind in spring and autumn; other factors, such as temperature, humidity and pressure had no significant influence on the altitude distributions. With respect to the question of non-stop or intermittent flight across large desert areas, the data show that between the eastern deserts of Egypt and the Sinai/Negev complex the nocturnal migrants maintained their schedule of nocturnal flight and diurnal rest. A few exceptions of nocturnal migrants continuing migration at high altitudes into the day were identified mainly as heron- and gull-type birds. The proportion of waders and waterfowl identified by wing-beat pattern in nocturnal migration is nearly the same at both sites, indicating broad-front migration across the desert. The numbers of birds with continuous wingbeats is, however, so large compared to available estimates of waders and waterfowl wintering in Africa that careful reconsideration of the underlying assumptions in the radar and field estimates is necessary.     

Pointed wings, low wingloading and calm air reduce migratory flight costs in songbirds

Migratory bird, bat and insect species tend to have more pointed wings than non-migrants. Pointed wings and low wingloading, or body mass divided by wing area, are thought to reduce energy consumption during long-distance flight, but these hypotheses have never been directly tested. Furthermore, it is not clear how the atmospheric conditions migrants encounter while aloft affect their energy use; without such information, we cannot accurately predict migratory species' response(s) to climate change. Here, we measured the heart rates of 15 free-flying Swainson's Thrushes (Catharus ustulatus) during migratory flight. Heart rate, and therefore rate of energy expenditure, was positively associated with individual variation in wingtip roundedness and wingloading throughout the flights. During the cruise phase of the flights, heart rate was also positively associated with wind speed but not wind direction, and negatively but not significantly associated with large-scale atmospheric stability. High winds and low atmospheric stability are both indicative of the presence of turbulent eddies, suggesting that birds may be using more energy when atmospheric turbulence is high. We therefore suggest that pointed wingtips, low wingloading and avoidance of high winds and turbulence reduce flight costs for small birds during migration, and that climate change may have the strongest effects on migrants' in-flight energy use if it affects the frequency and/or severity of high winds and atmospheric instability.     

Cyprus wheatears Oenanthe cypriaca likely reach sub‐Saharan African wintering grounds in a single migratory flight

Long‐distance migratory flights with multiple stop‐overs, multiple wintering sites, and small‐scale connectivity in Afro‐Palearctic migrants are likely to increase their vulnerability to environmental change and lead to declining populations. Here we present the migration tracks and wintering locations of the first six Cyprus wheatears to be tracked with geolocators: a species with high survival and a stable population. We therefore predicted a non‐stop flight from Cyprus to sub‐Saharan wintering grounds, a single wintering area for each individual and a wide spread of wintering locations representing low migratory connectivity at the population level. The sub‐Saharan wintering grounds in south Sudan, Sudan and Ethiopia were likely reached by a single flight of an average straight‐line distance of 2538 km in ca 60 h, with an average minimum speed of 43.1 km h^–1. The high speed of migration probably ruled out stop‐overs greater than a few hours. Cyprus wheatears migrated from Cyprus in mid‐late October and most probably remained at a single location throughout winter; three out of five birds with available data may have used a second site < 100 km away during February; all returned between 7–22 March when accurate geolocation data are not possible due to the equinox. Wintering locations were spread over at least 950 km. There were no tag effects on survival. Cyprus wheatears showed a migratory strategy in accordance with their observed high survival rate and demonstrated a routine flight range that allows much of the Mediterranean and the Sahara to be crossed in a rapid two and a half‐day flight.     

Effects of a sea barrier on large-scale migration patterns studied by a network of weather radars

Capsule: Nocturnal passerine migration patterns were studied by a network of weather radars within the East-Atlantic flyway providing large-scale information on the effect of a geographical barrier.

Aims: The aim of this study was to obtain a large-scale spatial overview of the effects of a sea barrier on migratory flyways in northern Spain/western France.

Methods: Weather radar data were used from five sites at the Bay of Biscay during nights in spring 2015 to calculate flight directions and migration traffic rates (MTRs).

Results: The highest MTRs were registered by the radars at the southeastern edge of the bay, with a gradual decrease northwards. Spring migration direction was generally NNE/NE. Continuous nocturnal migration pattern indicated migration over land in the south. The radar half way up the French coast exhibited bimodal migration intensities at night, indicating sea crossing, and the northernmost corner of the bay showed little migration.

Conclusions: Radar patterns indicated migration over land and sea during spring migration. Sea crossing occurred with flight distances of up to about 500?km. Most migration activity was observed in the radars along the southeastern section of the bay, indicating that the general migration flyway from Spain funnels through the eastern side of the north Iberian Peninsula.