Nocturnal Passerine Migrants Research Articles

An Analysis of Orientation Cage Field Data – A Case Study of Headings of the Blackcap, Sylvia atricapilla, at the Bukowo/Kopań Ringing Site in Poland

Abstract One of the most effective methods of studying the migratory patterns of nocturnal passerine migrants is the use of orientation cages as a supplementary procedure at ringing sites. The most comprehensive studies using orientation cages (Busse’s Flat Orientation Cage) were conducted within the SEEN network (SE European Bird Migration Network), with more than 43,000 tests performed at more than 40 ringing sites in autumn. A number of papers were published based on these data, presenting an overall pattern of passerine migration over SE Europe-Middle East-NE Africa. For more detailed analyses, it was first necessary to solve some methodical problems within case studies. The current work presents details for discussion based on data from 1338 tests of Blackcaps performed during the years 1995–2010 at a single ringing station, Bukowo/Kopań, located on the Polish Baltic coast. The birds were tested according to the standard methodology of the SEEN network (Busse 2000). The problems investigated were (1) the repeatability of heading patterns obtained in different years, (2) the linearity of the estimated arrival and departure headings, and (3) quantitative aspects of the results in the description of the heading pattern when migratory groups are found. It was determined that (1) yearly heading patterns were generally coherent and could be analysed as uniform case data; (2-1) the hypothesis that the arrival/departure heading axes are generally linear is accurate to within about one 10° sector, at least in the case of the Blackcap. This is coherent with impressions from other known data sets, but does not mean that the rule always applies to all species at every location on the migration route. In the future, this problem should be studied on a more detailed scale. It was further established that (2-2) estimating arrival and departure headings makes it possible to define migratory groups (populations) passing the study site. The direct heading estimation procedure seems to be more sensitive in identifying migratory groups than the calculation procedure. Moreover, (3-1) the calculation procedure makes it possible to estimate some quantitative properties of headings of migratory groups and define some interesting, though preliminary, number patterns of local migratory patterns; (3-2) the number relations between birds demonstrating the arrival and departure headings of the migratory group seem to be an interesting parameter for study on the distance of migration of groups within a species and, possibly, between species. This is another interesting problem that cage tests could be used to solve in the future. Finally, (3-3) knowledge about trends of individual populations passing the defined site becomes accessible only using analysis of data from orientation cages. The estimated quantitative indices discussed above could be helpful in presenting the general migratory pattern of the species on a geographical scale.

Effect of geomagnetic field on migratory activity in a diurnal passerine migrant, the dunnock, Prunella modularis

Migratory songbirds are guided by an endogenous programme during their first migration, encoding timing of migration, distance and direction. To successfully perform migration, birds have evolved phenotypic adaptations for flight, fuelling and navigation. Migratory distance in different species of birds is encoded as a period of expressed migratory restlessness, for which the length is correlated with migratory distance. Most of the work so far has been on nocturnal passerine migrants, while much less is known about the phenotypic adaptations for migration in diurnal migrants. Here we studied autumn migration fuelling and expression of migratory activity in caged diurnally migrating juvenile dunnocks in response to magnetic displacements. We kept one group (control) indoors at the location of capture in south Sweden, while the two experimental groups were gradually (over 5 days) displaced magnetically to locations to the northeast or to the wintering destination in southwest France. We found that all birds showed two peaks of activity during the day, for which the onset of activity was tightly timed to sunrise and sunset. The longest activity (2–3 h) occurred in the morning, coinciding with the natural period of migration for this species. For the control group, the migratory (flight) activity increased with the season (15 days), while it was strongly reduced for the dunnocks displaced to the wintering areas. Birds displaced to the north showed a stable, but slightly reduced migratory activity over time. The results support the finding that geomagnetic information expected to be met en route is important for triggering level of migratory activity in juveniles of a diurnal songbird migrant.

Further evidence of a time-independent stellar compass in a night-migrating songbird

We tested whether European robins Erithacus rubecula have a time-independent stellar compass, i.e. whether they can use the stellar pattern for finding the seasonally appropriate migratory direction without reference to their internal clock. It has been claimed earlier on the basis of experiments with other nocturnal migrants, indigo buntings Passerina cyanea, pied flycatchers Ficedula hypoleuca and blackcaps Sylvia atricapilla, but none of the previous experiments was performed in the vertical magnetic field, which rules out the use of the magnetic compass, and under the natural nocturnal sky. We argue that pervious experiments allow alternative interpretations, albeit unlikely. Our data independently confirm that European robins, like other nocturnal passerine migrants hitherto studied, use their stellar compass independently of their internal clock. It is generally assumed since the classic experiments of Stephen Emlen performed in the 1960s that star compass of migrating songbirds is time-independent, i.e. that the birds use the pattern of constellations and not the angle of rotation of the starry sky. However, it has not been shown unequivocally, because early experiments did not consider the possibility that the birds relied on magnetic compass rather than on time-independent stellar compass system. Here, we show that stellar compass of European robins tested in the vertical magnetic field that did not provide compass information is indeed independent of the birds’ internal clock and cannot be manipulated by resetting this clock. Cognitive challenges of learning the pattern of constellations, which is necessary to use time-independent star compass, are most fascinating and deserve detailed behavioural and neurobiological study.

Evolution of the Western Palaearctic Passerine Migration Pattern Presentation Style

Abstract The scientific knowledge available in many detailed studies needs, from time to time, some generalization that allows to provide a synthesis or at least presentation of certain problem to both, scientific community and wide public interested in the topic. This article presents evolution of the presentation style of spatial course of the passerine migration in the Western Palaearcic. According to developing knowledge in the topic the style of presentation of general migration pattern evolved from a “line–arrow ” style suggesting that the birds use narrow “corridors” to more adequate to the phenomenon “carpet” style with called as “bottle-necks” concentrations being local and temporal effects of existing migratory barriers and guiding lines as maritime coasts, mountains and deserts. These details of migratory flyways are less visible in nocturnal passerine migrants than diurnal movements of both of passerines and gliding big birds. Generally, according to spatial relations between breeding and wintering areas of the bird populations living in Western Palaearctic four main flyways are defined and presented on maps: Western (Atlantic), Central (Apennine), South- Eastern (Balkan) and Eastern (Indian). Their background lies in the post-ice age history of distribution changes of the bird species, but details still evolve.

Testing avian compass calibration: comparative experiments with diurnal and nocturnal passerine migrants in South Sweden.

ABSTRACTCue-conflict experiments were performed to study the compass calibration of one predominantly diurnal migrant, the dunnock (Prunella modularis), and two species of nocturnal passerine migrants, the sedge warbler (Acrocephalus schoenobaenus), and the European robin (Erithacus rubecula) during autumn migration in South Sweden. The birds' orientation was recorded in circular cages under natural clear and simulated overcast skies in the local geomagnetic field, and thereafter the birds were exposed to a cue-conflict situation where the horizontal component of the magnetic field (mN) was shifted +90° or −90° at two occasions, one session starting shortly after sunrise and the other ca. 90 min before sunset and lasting for 60 min. The patterns of the degree and angle of skylight polarization were measured by full-sky imaging polarimetry during the cue-conflict exposures and orientation tests. All species showed orientation both under clear and overcast skies that correlated with the expected migratory orientation towards southwest to south. For the European robin the orientation under clear skies was significantly different from that recorded under overcast skies, showing a tendency that the orientation under clear skies was influenced by the position of the Sun at sunset resulting in more westerly orientation. This sun attraction was not observed for the sedge warbler and the dunnock, both orientating south. All species showed similar orientation after the cue-conflict as compared to the preferred orientation recorded before the cue-conflict, with the clearest results in the European robin and thus, the results did not support recalibration of the celestial nor the magnetic compasses as a result of the cue-conflict exposure.

Seasonal modulation of flight speed among nocturnal passerine migrants: differences between short- and long-distance migrants

Migrating birds are expected to fly at higher airspeeds when minimizing time rather than energy costs of their migratory journeys. Spring migration has often been suggested to be more time selected than autumn migration, because of the advantage of early arrival at breeding sites. We have earlier demonstrated that nocturnal passerine migrants fly at higher airspeeds during spring compared to autumn, supporting time-selected spring migration. In this study, we test the hypothesis that seasonal airspeeds are modulated differently between short- and long-distance migrants, because of a stronger element of time selection for autumn migration over long distances. In support of this hypothesis, we demonstrate that the seasonal difference in airspeed is significantly larger (spring airspeed exceeding autumn airspeed by a factor of 1.16 after correcting for the influence of altitude, wind and climb/descent on airspeed) among short-distance compared to long-distance (factor 1.12) migrants. This result is based on a large sample of tracking radar data from 3 years at Falsterbo, South Sweden. Short-distance migrants also tend to fly with more favourable winds during autumn, indicating relaxed time constraints (being able to afford to wait for favourable winds) compared to long-distance migrants. These results indicate surprisingly fine-tuned seasonal modulation of airspeed and responses to wind, associated with behavioural strategies adapted to different levels of time selection pressures during spring and autumn migration.

Evidence that rates of advancement of long-distance passerine migrant arrival in the UK have been higher among earlier-arriving species

This study investigated the change with year in the arrival timing in the UK of 22 long-distance summer migrants, using median arrival dates estimated from BTO/RSPB/BirdWatch Ireland/SOC BirdTrack survey data. An apparent advancement in arrival dates was found, on average, for the data set as a whole. A significant correlation was demonstrated between the rate of advancement of individual species and their average median arrival date over the study period, with earlier-arriving species generally showing greater rates of advancement. This correlation was particularly strong among a subset of nocturnal passerine migrants. It is suggested that this observation may reflect a phenotypically plastic response to temperatures encountered en route.

Nocturnal passerine migrants fly faster in spring than in autumn: a test of the time minimization hypothesis

It has been suggested that time selection and precedence in arrival order are more important during spring than autumn migration. Migrating birds are expected to fly at faster airspeeds if they minimize duration rather than energy costs of migration, and they are furthermore expected to complete their journeys by final sprint flights if it is particularly important to arrive at the destination before competitors. We tested these hypotheses by tracking-radar studies of nocturnal passerine migrants during several spring and autumn seasons at Lund (56°N) and Abisko (68°N) at the southern and northern ends of the Scandinavian Peninsula, respectively. The samples from these two sites represent migrants that are mostly rather far from (Lund) or close to (Abisko) their breeding destinations. We found that the birds were flying at clearly faster airspeeds in spring than in autumn at both study sites, with spring speeds exceeding autumn speeds by, on average, 16%, after taking effects of wind conditions and vertical flight speeds into account. This difference in speeds could not be explained by seasonal differences in body mass or wing morphology and thus supports the hypothesis of time-selected spring migration. There was also a significantly larger seasonal difference in airspeed at Abisko than at Lund, suggesting that the birds may have shown an inclination to sprint on their final spring flights to the breeding destinations, although this possible extra sprint effort was modest.

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.

Autumnal patterns of nocturnal passerine migration in the St. Lawrence estuary region, Quebec, Canada: a weather radar study

We documented the pattern of nocturnal passerine migration on each side of the St. Lawrence estuary (Côte-Nord north and Gaspésie south), using the Doppler Canadian weather surveillance radar of Val d’Irène (XAM). We examined whether autumnal migrants flew across the St. Lawrence, resulting in a uniform broad-front migration, or avoided crossing it, resulting in a bird concentration along the north coast. We found that a proportion of migrants crossed the estuary but that most followed the north coast. Ranges at which birds were detected were, on average, greater on Côte-Nord, thereby rejecting the uniform broad-front migration hypothesis, inasmuch as reflectivity measurements suggested that bird concentrated along Côte-Nord. The mean flight direction on Côte-Nord was southwest but shifted westward as the night progressed, avoiding crossing the estuary by late night. In Gaspésie, the mean flight direction over land was south and no directional shift was observed throughout the night. Flight altitude reach up to 1000 m above sea level (a.s.l.), but migratory activity was highest in the first 500 m a.s.l. It appears that the St. Lawrence estuary acts as a leading line and a barrier for nocturnal passerine migrants, and likely shapes migration farther south in Canada and in the United States.

Compensation for wind drift by migrating swifts

We investigated the orientation in relation to wind of common swifts, Apus apus, during nocturnal spring and autumn migration. Swifts are highly adapted to a life in the air, showing wind-dependent orientation during nocturnal roosting flights, and may be expected to be more efficient in their wind drift/compensation behaviour than nocturnal passerine migrants, which are usually subjected to full or partial wind drift. A tracking radar at Lund in southern Sweden was used to record the orientation of common swifts (identified by their characteristic radar echo signature) on nocturnal migration flights and to measure wind conditions at the altitudes at which the birds were flying. Comparing track and heading directions under easterly and westerly winds revealed that the swifts shifted their heading distinctly into the wind, with the result that track directions were similar under the different wind conditions. As this pattern of complete compensation for drift from cross-winds occurred during both spring and autumn migration, there were no indications of differences between age classes. In addition, we found an effect of side-winds on equivalent airspeeds, with swifts increasing their airspeed with increasing wind speed. Such a response has been theoretically predicted as part of an optimal behaviour for counteracting wind drift but has hereto not been empirically demonstrated. There was also a positive correlation between overall wind speed and equivalent airspeed, making it difficult to interpret whether the swifts respond to the total wind speed rather than specifically to the side-wind effect. Our results suggest that important differences may exist between species in their capacities to orient in relation to the wind and that the swift may be particularly efficient in adjusting heading direction and airspeed to obtain complete compensation for wind drift during high-altitude nocturnal migratory flights. (C) 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. (Less)

Daytime passerine migrants over the Sahara — are these diurnal migrants or prolonged flights of nocturnal migrants?

Passerine migrants are usually divided into diurnal and nocturnal migrants. When crossing the Sahara it was expected that nocturnal migrants would continue their flight into the day. We investigated whether migration taking place in the Sahara during daytime comprises the normal diurnal migrant species or the nocturnal ones prolonging their flight into the day. Birds departing from the Sahel in spring had to cross an ecological barrier of 300km before reaching our study site, an oasis in central Mauritania. The intensity of passerine migration measured by radar varied from night to night and decreased towards sunrise. Under good wind conditions some passerine migration continued into the day. The landing tendency (sink rate) correlated negatively with the tail wind component. Transect counts on the ground revealed very low proportions of diurnal migrants, not matching the relatively high densities of passerine migration during the day, and a high correlation between transect density of nocturnal migrants and nocturnal passage of passerine migrants. Therefore we conclude that nocturnal passerine migrants are responsible for most of the recorded daytime passage (swifts, swallows and soaring birds being excluded). Flight and landing behaviour varied with environmental conditions and nocturnal passerine migrants adjusted their flight schedules opportunistically, continuing into the day in particularly favourable winds.

Migratory stopovers of passerines in an oasis at the crossroads of the African and Indian flyways

We studied migratory stopovers of nocturnal passerine migrants migrating between the Urals and Siberia and the Near East and Africa (five species) and between European Russia and the Indian subcontinent (two species) in an oasis in the arid belt of the northwestern edge of western Central Asia. In autumn, fuel loads of Blyth's Reed Warblers Acrocephalus dumetorum before their desert crossing were much greater than in Palaearctic‐African migrants, which face a much narrower barrier, and also greater than in conspecifics captured during and after the desert crossing. However, another Indian migrant, the Redbreasted Flycatcher Ficedula parva, had low average fuel stores. In spring, Blyth's Reed Warblers carried less fuel than in autumn, and less than African migrants such as Garden Warblers Sylvia borin in spring. This suggests that the arid belt of western Central Asia is a serious ecological barrier for Palaearctic‐Indian passerine migrants in autumn, but much less of one in spring. Palaearctic‐African migrants which make a detour around this barrier to the northwest and migrate north of the Caspian Sea do not need large fuel stores like those they deposit before crossing the Sahara.

Effects of magnetic manipulations on orientation: comparing diurnal and nocturnal passerine migrants on Capri, Italy in autumn

Orientation cage experiments were performed on Capri in Italy, with a diurnal passerine migrant (Tree Pipit Anthus trivialis), and a nocturnal passerine migrant (Garden Warbler Sylvia borin), to study the use of magnetic compass information during autumn passage migration. The experiments were performed outdoors at sunset in: (1) the local geomagnetic field under natural clear skies, (2) a shifted magnetic field (mN −90°) under clear skies, and (3) a shifted magnetic field (mN −90°) under simulated overcast skies. Day migrating Tree Pipits showed a clear shift in orientation compared to controls (i.e. local geomagnetic field and clear sky conditions) corresponding roughly with the magnetic shift (mN −90°) under clear as well as overcast skies, while the Garden Warbler migrating at night, did not respond to the same magnetic manipulations by shifting their preferred directions in the cages. The mean orientation of Tree Pipits did not differ from the sun’s position during experiments, while it was clearly different in Garden Warblers. Species-specific orientation responses to experimental manipulations in caged compared to freeflying migrants is discussed.

Habitat selection by nocturnal passerine migrants en route: mechanisms and results

Habitat use and habitat selection are essential for successful stopovers. Passerine migrants use habitats in a clearly non-random manner, even if many species utilise a broader range of habitats during passage than during breeding or wintering. Habitat selection proceeds as a sequence of events: landfall; search/settling, including redistribution across habitats if necessary; and habitat exploitation, with all stages probably condition-dependent. This review is aimed at studying this sequence and also the factors which govern decision-making in nocturnal passerine migrants at all levels. In most cases, habitats are (pre)selected by migrants already at landfall using both visual and acoustic cues. After landfall, migrants start to perform direct sampling of habitats during which they may move constantly and perform fine-tuning of their habitat choice. Some species subsequently occupy a small home range in a particular (micro)habitats, whereas others continue to move broadly during the whole stopover period. An interaction of several factors shapes the use of habitats after landing, among which are migrants’ innate preferences and functional morphology, foraging strategies and food resource distribution, habitat carrying capacity and exposure to predators. The large-scale spatial context probably also plays a role which might be currently underestimated.

Orientation and migratory activity of Song Thrushes (Turdus philomelos) in northern Italy: cage and release experiments under overcast conditions

Orientation and migratory activity of Song Thrushes (Turdus philomelos) in northern Italy: cage and release experiments under overcast conditions The autumn orientation and migratory activity of Song Thrushes at different periods around sunset were recorded in northern Italy using orientation cages and release experiments, mostly under overcast skies. The aim of the experiments was to examine the importance of timing and energetic condition for the orientation and relative migratory activity of Song Thrushes located close to the wintering area. The data demonstrate that the birds tested during sunset showed a mean orientation that coincided with the average sunset azimuth. The birds that were tested after sunset chose headings towards the south, significantly different from tests at sunset, showing oriented Zugunruhe in visually cueless situations. Lean migrants in funnel cages displayed relatively more activity than migrants with larger fat stores, but they also showed more scattered directional choices. A bird's energetic state did seem to affect the likelihood of undertaking night migration. Test birds with relatively small fat reserves at release were more likely to perform landing flights than birds with larger fat deposits. Although nocturnal passerine migrants generally show well-oriented responses only shortly after sunset, both cage and release experiments 1-2 h after sunset indicate that timing may not be critical for the initial orientation of Song Thrushes. However, cage tests performed before sunset resulted in more unclear responses by the test birds than tests performed after sunset.

Orientation scatter of free-flying nocturnal passerine migrants: components and causes

We investigated the variation in concentration of orientation among nocturnally migrating passerine birds. Using tracking radar, we recorded flight tracks of birds during spring and autumn migration and, with the aid of concurrent wind recordings, we calculated heading directions. The concentrations of track and heading directions were compared between seasons and also between different categories of migrants that were defined by flight speed and wing beat frequency. Wind drift was a dominant cause of the large scatter of track directions, especially for autumn migration. When wind effects were compensated for, we found only small differences in the concentration of heading directions between different categories of migrants. This shows that between-group variation is not a major source of the overall variation in orientation when groups are distinguished on the basis of airspeed and wing beat frequency. Although the total concentration of heading directions was almost exactly the same for spring and autumn migrants, there was an element of partial compensation for wind drift in spring but not in autumn. When we removed the effect of this compensatory behaviour by considering situations with low wind speeds, the concentration of headings during spring tended to exceed that during the autumn. This suggests a more accurate orientation of the birds during spring than in autumn, when a large proportion consists of naı̈ve migrants on their first migratory journeys. The high concentration of heading directions of free-flying migrants are in clear contrast to the widely scattered distributions generally observed in orientation experiments with caged birds. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

Polarization patterns of the summer sky and its neutral points measured by full–sky imaging polarimetry in Finnish Lapland north of the Arctic Circle

Using 180° field–of–view (full–sky) imaging polarimetry, the patterns of the degree and angle of polarization of the entire summer sky were measured on 25 June 1999 at a location north of the Arctic Circle in Finnish Lapland as a function of the angular solar zenith distance. A detailed description of the used full–sky imaging polarimeter and its calibration is given. A series of the degree and angle of polarization pattern of the full sky is presented in the form of high–resolution circular maps measured in the blue (450 nm) spectral range as a function of the solar zenith distance. Graphs of the spectral dependence of the degree and angle of polarization of skylight at 90° from the Sun along the antisolar meridian are shown. The celestial regions of negative polarization and the consequence of the existence of this anomalous polarization, the neutral points, are visualized. The measured values of the angular zenith distance of the Arago and Babinet neutral points are presented as a function of the zenith distance of the Sun for the red (650 nm), green (550 nm) and blue (450 nm) ranges of the spectrum. The major aim of this work is to give a clear and comprehensive picture, with the help of full–sky imaging polarimetry, of what is going on in the entire polarized skydome. We demonstrate how variable the degree of polarization of skylight and the position of the neutral points can be within 24 h on a sunny, almost cloudless, visually clear day.

Orientation in relation to exposure to the setting sun in some passerine trans-saharan migrants

Four species of nocturnal trans-saharan passerine migrants (Ficedula hypoleuca, Phylloscopus trochilus, Phoenicurus phoenicurus, Sylvia borin) were tested in Emlen funnels to assess the relative importance of the setting sun and stars in determining the nocturnal course. The experiments took place during spring migration at a Mediterranean stop-over site (Sardinia, Italy). The birds were caught during the day in mist-nets and kept in opaque plastic cages preventing them seeing outside, until the time of the experiment, on the same night; each bird was tested only once. Two experimental groups were exposed to the sunset, the first including the sight of the sun disc, the other after the sun had set. A third group was kept in the closed cages until the time of the test. A fourth group was directly tested in the funnels at sunset time, after the sun disc had set. The first three groups were tested in the funnels later, in complete darkness. Exposure and tests occurred under the same sky conditions, either clear or overcast. Under a clear sky, 10 out of 16 distributions are oriented in the expected migratory direction, or have a tendency towards it, irrespective of their exposure to the setting sun. This shows that the cues present at sunset are not necessary for the orientation of these birds, whose ability to orient correctly in the absence of any celestial cues is also confirmed by the results of the tests performed in overcast conditions with no previous exposure to visual cues. Unexpectedly, exposure to the setting sun with an overcast sky, followed by a test in the same overcast conditions lead to disorientation, suggesting that exposure to visual cues makes the birds look, at least at first, for other similar cues before turning to non-visual compass information.

Flight Initiation of Nocturnal Passerine Migrants in Relation to Celestial Orientation Conditions at Twilight

The time of departure of nocturnal passerine migrants was studied by radiotelemetry and tracking radar in South Sweden in spring and autumn. Our objective was to analyse the time of flight initiation in relation to celestial orientation cues during the twilight period at dusk. The telemetry and radar results were compared with data on nocturnal flight departures reported in the literature. There was a considerable variation in time of flight initiation throughout the twilight period. A large fraction of the birds (mostly thrushes) departed mainly during the civil and nautical twilight periods, with a sun elevation between 0O and 120, while several migrants did not leave until much later in the night. This variation in time of departures speaks against the hypothesis that departure time is connected with a critical skylight situation with respect to the visual accessibility of celestial orientation cues. Characteristic times of departure, as reflected by the associated elevations of the sun below the horizon, seem to differ between different passerine species, latitudes and times of migratory season.