Nocturnal Migratory Flights Research Articles

Foraging on the wing for fish while migrating over changing landscapes: traveling behaviors vary with available aquatic habitat for Caspian terns

BackgroundBirds that forage while covering distance during migration should adjust traveling behaviors as the availability of foraging habitat changes. Particularly, the behavior of those species that depend on bodies of water to find food yet manage to migrate over changing landscapes may be limited by the substantial variation in feeding opportunities along the route.MethodsUsing GPS tracking data, we studied how traveling behaviors vary with available foraging habitat during the long-distance migration of Caspian terns (Hydroprogne caspia), a bird with a specialized diet based on fish that needs bodies of water to forage. We measured individual variation in five traveling behaviors related to foraging along the route and used linear mixed effects models to test the following variables as predictors of traveling behaviors: proportion of overlap with water bodies, weather conditions, days at previous stopover and days of migration. Also, we tested if during traveling days flight height and speed varied with time of day and if birds were in areas with greater proportion of water bodies compared to what would be expected by chance from the landscape.ResultsWe found variation in migratory traveling behaviors that was mainly related to the proportion of overlap with water bodies and experienced tailwinds. Suggesting a mixed migratory strategy with fly-and-foraging, Caspian terns reduced travel speed, flew fewer hours of the day, had lower flight heights and increased diurnal over nocturnal migratory flight hours as the proportion of overlap with water bodies increased. Birds had lower flight speeds and higher flight heights during the day, were in foraging habitats with greater proportions of water than expected by chance but avoided foraging detours. Instead, route tortuosity was associated with lower wind support and cloudier skies.ConclusionsOur findings show how birds may adjust individual behavior as foraging habitat availability changes during migration and contribute to the growing knowledge on mixed migratory strategies of stopover use and fly-and-forage.

Diel variation in corticosterone and departure decision making in migrating birds

Animals usually show distinct periods of diel activity and non-activity. Circulating baseline levels of glucocorticoid hormones (corticosterone and cortisol) often peak just before or at the transition from the non-active to the active period of the day. This upregulation of glucocorticoids may function to mobilize stored energy and prepare an animal for increased activity. Usually, the alternation of active and non-active periods is highly predictable; however, there is one group of animals for which this is not always the case. Many otherwise diurnal birds show nocturnal activity during the migration seasons. Nocturnal migratory flights are alternated with stopover periods during which the birds refuel and rest. Stopovers vary in length, meaning that nocturnal migrants are inactive in some nights (when they continue their stopover) but extremely active in other nights (when they depart and fly throughout the night). This provides an ideal natural situation for testing whether glucocorticoids are upregulated in preparation for an increase in activity, which we used in this study. We found that in northern wheatears (Oenanthe oenanthe), corticosterone levels peaked in the few hours before sunset in birds departing from stopover that night, and, importantly, that this peak was absent in birds continuing stopover. This indicates that corticosterone is upregulated in the face of an increase in energy demands, underlining corticosterone's preparative metabolic function (energy mobilization). The timing of upregulation of corticosterone also gives a first insight in when during the day nocturnally migrating birds decide whether or not to resume migration.

Species-specific attraction call of the blackbird (Turdus merula) in the periods of seasonal migration and breeding

We present the results of the analysis of 346 spectrograms of the species-specific attraction call tsiirr issued during daytime and night migration, at migratory stopovers and during breeding by Blackbirds (Turdus merula). The duration of this type signal varies within the range of 102–359 ms, and its frequency is in the intermediate and high frequency range from 5.1 to 9.9 kHz. In the spectrograms most tsiirr signals have one high-frequency and one lower-frequency modulated band. It is assumed that such a structure of this signal allows the birds to maintain contact at a great distance and to determine each other’s spatial position. Temporary and frequency characteristics of the species-specific attraction calls of Blackbirds during nocturnal and daytime migration did not differ significantly. The species-specific attraction calls during the migration flight had on average a much narrower range of sounding than at stopovers. This difference could have been caused by stronger degradation of the signal structure during propagation in migratory flight than at stopovers. The analysis of the spectrograms of the species-specific attraction calls in two pairs of adult Blackbirds during breeding revealed significant individual variation. Signals of migrating individuals at stopovers were shorter and broader than in adult breeding Blackbirds. Our studies of Blackbird acoustic signals, as well as of signals of two other thrush species, Song Thrushes (T. philomelos) and Redwings (T. iliacus), show that during diurnal and nocturnal migratory flights, they use only one type of signal, the speciesspecific attraction call. This call is also used by the birds for communication during migratory stopovers. No specialized migratory signals are given by thrushes during their seasonal movements.

Do flight-calls of Redwings differ during nocturnal and diurnal migration and daytime stopovers?

In the present study, we analyzed 173 spectrograms of acoustic signals of Redwings (Turdus iliacus). These were issued during diurnal and nocturnalmigratory flight and also during feeding, when resting and before take-off at daytime stopovers.During nocturnal and diurnal migration flights and daytime stopovers, Redwings use a single type of signal, a long tsii. It is classified as a species-specific attraction call. In flight Redwings emit shorter calls than during daytime stopovers.We did not find confirmation of the hypothesis that during nocturnalmigration in the absence of visual contact with each other, due to lowlight levels andmuch looser flocks in the course of daytime migration, Redwings emit longer signals with wider frequency spectrum than in the daytime.

Activity and migratory flights of individual free‐flying songbirds throughout the annual cycle: method and first case study

We describe a method and device (< 1.2 g) for recording, processing and storing data about activity and location of individuals of free‐living songbirds throughout the annual cycle.Activity level was determined every five minutes from five 100 ms samples of accelerometer data with 5 s between the sampling events. Activity levels were stored on an hourly basis throughout the annual cycle, allowing periods of resting/sleep, continuous flight and intermediate activity (foraging, breeding) to be distinguished. Measurements from a light sensor were stored from preprogrammed key stationary periods during the year to provide control information about geographic location.Successful results, including annual actogram, were obtained for a red‐backed shrikeLanius colluriocarrying out its annual loop migration between northern Europe and southern Africa. The shrike completed its annual migration by performing > 66 (max. 73) nocturnal migratory flights (29 flights in autumn and > 37, max. 44, in spring) adding up to a total of > 434 (max. 495) flight hours. Migratory flights lasted on average 6.6 h with maximum 15.9 h. These flights were aggregated into eight travel episodes (periods of 4–11 nights when flights took place on the majority of nights). Daytime resting levels were much higher during the winter period compared to breeding and final part of spring migration. Daytime resting showed peaks during days between successive nocturnal flights across Sahara, continental Africa and the Arabian Peninsula, indicating that the bird was mostly sleeping between these long migratory flights.Annual activity and flight data for free‐living songbirds will open up many new research possibilities. Main topics that can be addressed are e.g. migratory flight performance (total flight investment, numbers and characteristics of flights), timing of stationary periods, activity patterns (resting/sleep, activity level) in different phases of the annual cycle and variability in the annual activity patterns between and within individuals.

Geographic position and landscape composition explain regional patterns of migrating landbird distributions during spring stopover along the northern coast of the Gulf of Mexico

Annual migration of landbirds across the Gulf of Mexico (GOM) presents a unique opportunity to examine extrinsic processes operating at various spatial scales in determining animal distributions. Our objectives were to comprehensively quantify bird stopover densities across the northern GOM coast and model broad-scale factors explaining distributional patterns. We used weather surveillance radars to measure reflectivity of birds aloft at onset of nocturnal migratory flights and estimate bird stopover densities during four springs (2009–2012) for 6.7 million ha along the GOM. We aggregated bird densities to one longitudinal degree and 3 km of proximity to coast. Boosted Regression Tree models revealed that stopover density was related to year, longitude, proximity to coast, and amount of hardwood forest cover in the landscape. Average longitudinal patterns supported previous studies of broad-scale trans-Gulf migrant arrivals with highest density in Louisiana (92–93°W) and lowest in Alabama (88–89°W). Florida (83–84°W) supported a second peak in migrant density, suggesting an eastern trans-Gulf route or contribution from trans-Caribbean migrants. Longitudinal patterns in migrant distributions varied strongly between years and appear generally related to variability in GOM wind patterns. Densities increased with proximity to coast, highlighting constraints on migrants to travel inland, especially in Florida’s panhandle. Despite this, density was positively related to amount of forest cover more steeply along the immediate coast. Broad-scale stopover distributions of migrating landbirds along the GOM coast are heavily influenced by geographic constraints in the context of the GOM acting as a barrier to landbird migration.

Timing of nocturnal passerine migration in Arctic light conditions

The nocturnal migration of many passerines starts after sunset and reaches peak intensity during the dark hours of the night. Birds destined for high Arctic breeding grounds encounter a special situation, as they will experience continuous daylight when reaching the high latitudes during the final part of spring migration. How does this affect the pattern of nocturnal migration? We consider three alternative hypotheses; that the period of nocturnal flight activity may become compressed, remain unchanged or become disorganized under Arctic light conditions. We tracked migrating birds by radar north of the Arctic Circle (at Abisko, 68°21′N, 18°49′E, in Swedish Lapland) and show that the pattern during the night, with a migration peak around midnight, persisted even in continuous daylight when the sun remained above the horizon throughout the 24 h of the day. The flight intensity peak under continuous daylight in spring (midnight sun) was very similar to the corresponding peak in autumn, when the migration took place under twilight conditions. The duration of the flight period under continuous daylight in spring lasted 8–10 h and did not seem to be compressed. We hypothesize that the flight period under midnight sun conditions may in fact be more protracted than during short nights, because of release from twilight cues that tend to synchronize initiation and termination of migratory flights. These cues will thus capture and confine the flight period. The results of this provisional study suggest interesting dynamics in timing of nocturnal migratory flights under seasonally and latitudinally changing day length conditions, which will need detailed documentation by future studies of migration intensity at high-latitude sites.

Influence of fuel load and weather on timing of completing nocturnal flights in Reed warblers, Acrocephalus scirpaceus, during season migrations

Time of completing nocturnal migratory flights in passerines is essential for their safety during landing and stopover site selection. I studied landing time, fuel stores and weather at landing in Reed warblers migrating through the Courish Spit on the Baltic Sea in autumn and spring. I used data on 697 Reed warblers tape-lured into a habitat atypical of this species in 1999–2002. In both seasons, more than one-half of all birds (59% in autumn, 56% in spring) were captured in the last two hours before sunrise, while the rest in the beginning and middle of the night. I tested the hypothesis that early landing could be caused by take-offs in the beginning of the night with small fuel stores, not sufficient for the flight throughout the night. It was confirmed only for the young birds that migrated in the second half of the autumn season. Comparing wind assistance, cloud cover and precipitation in the days of landings in the beginning-middle of the night versus late night, I found no impact of these potentially important weather parameters on the temporal distribution of early and late landings. It is assumed that early landings of some Reed warblers could be caused by short-term nocturnal flights not directly related to the migration rush to the target.

Mapping Wintering Waterfowl Distributions Using Weather Surveillance Radar

The current network of weather surveillance radars within the United States readily detects flying birds and has proven to be a useful remote-sensing tool for ornithological study. Radar reflectivity measures serve as an index to bird density and have been used to quantitatively map landbird distributions during migratory stopover by sampling birds aloft at the onset of nocturnal migratory flights. Our objective was to further develop and validate a similar approach for mapping wintering waterfowl distributions using weather surveillance radar observations at the onset of evening flights. We evaluated data from the Sacramento, CA radar (KDAX) during winters 1998–1999 and 1999–2000. We determined an optimal sampling time by evaluating the accuracy and precision of radar observations at different times during the onset of evening flight relative to observed diurnal distributions of radio-marked birds on the ground. The mean time of evening flight initiation occurred 23 min after sunset with the strongest correlations between reflectivity and waterfowl density on the ground occurring almost immediately after flight initiation. Radar measures became more spatially homogeneous as evening flight progressed because birds dispersed from their departure locations. Radars effectively detected birds to a mean maximum range of 83 km during the first 20 min of evening flight. Using a sun elevation angle of −5° (28 min after sunset) as our optimal sampling time, we validated our approach using KDAX data and additional data from the Beale Air Force Base, CA (KBBX) radar during winter 1998–1999. Bias-adjusted radar reflectivity of waterfowl aloft was positively related to the observed diurnal density of radio-marked waterfowl locations on the ground. Thus, weather radars provide accurate measures of relative wintering waterfowl density that can be used to comprehensively map their distributions over large spatial extents.

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)

Migratory strategy of Bluethroats, Luscinia svecica, in eastern Fennoscandia. Part 2: response to acoustic markers and habitat selection at stopover

Response of bluethroats to the playback of species-specific song and the role of acoustic markers in habitat selection at autumn migratory stopovers were studied. The birds were mist-netted after the song playback and under the control conditions (no song). First captures and recaptures of birds were discussed. The song playback attracts bluethroats in the end of nocturnal migratory flights and the very beginning of a stopover and does not attract birds during stopover. Acoustic marker plays a significant role in habitat selection at the first stages of stopover: The tape-lured bluethroats concentrate around the sound source and then distribute over surrounding area. The possible reasons of response of migrating birds to the acoustic marker, as well as higher proportion of adults and lower proportion of fat birds among tape-lured bluethroats in comparison to the birds trapped under the control conditions have been discussed.

Wingbeat frequency and flap-pause ratio during natural migratory flight in thrushes

Powered flapping flight has evolved independently in many different taxa. For flapping fliers, wingbeat parameters such as frequency and amplitude are the primary determinants of these animals' energetic expenditure during flight. Here we present data on wingbeat frequency and amplitude for three New World thrush species during 15 entire nocturnal migratory flights over the Midwestern United States. Using continuous (non-pulsing) radio transmitters, we were able to measure wingbeat frequency and relative amplitude of wingbeats as well as the characteristics of flap-pauses. Contrary to previous telemetric findings, all of the individuals we followed used both flapping-only and flap-pause flight. During migratory flights, wingbeat frequency, effective wingbeat frequency, and amplitude were highest during initial ascent. Effective wingbeat frequency and amplitude were lowest during final descent. We show that identification of species based solely on characteristics of the wingbeat e.g., during radar studies, can be difficult because variables such as wingbeat frequency and amplitude, wingbeat pausing, and pattern of beats and pauses vary between individuals of the same species and even within individual flights. We also show that observed wingbeat frequencies were lower than those predicted by theoretical models. We speculate that this may be because theoretical predictions are generally based on (1) data from larger birds and (2) data from diurnal flights. We found that diurnal wingbeat frequencies of thrushes were generally higher than were those during nocturnal migratory flight. Finally, we suggest that rather than remaining at a single altitude during flight or climbing slightly as theoretical models predict, thrushes often moved up and down in the air column, perhaps searching for favorable atmospheric conditions.

Time of nocturnal departures in European robins, Erithacus rubecula, in relation to celestial cues, season, stopover duration and fat stores

To identify the mechanisms of control of temporal schedule of nocturnal migratory flights in small songbirds in nature, we studied the time of departure in a medium-distance nocturnal migrant, the European robin, on the Courish Spit (southeastern Baltic coast) using radiotelemetry. Exact measurements of departure time in 100 birds (58 in autumn and 42 in spring) showed no fixed time window of nocturnal departures during Evening Nautical Twilight. Take-offs occurred throughout the night and even during Morning Astronomical and Nautical Twilight Periods. Departure time showed no significant difference between clear skies and overcast conditions, and significantly differed between the seasons. The median departure time shifted significantly towards sunset during shorter nights in spring as compared to autumn. In both seasons (1) European robins initiating flight after short (1–2 days) stopovers showed no significant differences in departure time between individuals with small and large fuel stores at arrival; and (2) departure time was significantly related to stopover duration: the longer the birds stayed, the earlier they departed in relation to sunset. Only in spring and after longest stopovers, all departures occurred within a short time window soon after sunset.

Influence of body condition and weather on departures of first-year European robins, Erithacus rubecula, from an autumn migratory stopover site

How migratory birds decide when to leave a stopover site is important to the understanding of bird migration strategies. Our study looks at how body condition and the weather affect the decision to depart on nocturnal migratory flight. During two autumn migration seasons (2002–2003), we radio tracked 51 first-year European robins, Erithacus rubecula, at a stopover site on the Courish Spit (Eastern Baltic) from the first day after landing until their migratory departure. The tagged robins stopped over for 1–14 days. There was no clear relationship between stopover duration and energetic condition on arrival. Weather conditions (wind, precipitation, and cloud cover) on departure differed measurably between years. In 2002, robins took off mainly under following winds and clear skies. In 2003, there were mainly light head winds and partially cloudy or overcast skies. This could be explained by the year-specific role of weather factors in making the decision to depart. In both years, robins making short (1–2 days) stopovers took off in more varied weather situations than those individuals with long stopovers. This suggests that robins from the former group were more inclined to continue with migration than longer-stay birds that, apart from re-fuelling, could be waiting for favourable weather. The lack of a relationship between stopover duration and body condition and some departures under unfavourable weather conditions suggest that endogenous spatiotemporal programmes may play an important role in controlling stopover duration in robins.

Nocturnal migratory flight initiation in reed warblers Acrocephalus scirpaceus: effect of wind on orientation and timing of migration

We used radio‐telemetry to study autumn migratory flight initiation and orientation in relation to wind and air pressure in a nocturnal passerine migrant, the reed warbler Acrocephalus scirpaceus at Falsterbo, southwest Sweden. The majority of the reed warblers departed in the expected migratory direction towards south of southwest, while a low number of the birds took off in reverse directions between north and east. Flight directions at departure correlated with wind directions. These correlations were particularly prominent at higher wind speeds but were absent at wind speeds below 4 m/s. Birds departing in the expected migratory direction compensated completely for wind drift. The reed warblers preferred to depart during nights with tailwinds and when air pressure was increasing suggesting that reed warblers are sensitive to winds and air pressure and select favourable wind conditions for their migratory flights. Since air pressure as well as velocity and direction of the wind are correlated with the passage of cyclones, a combination of these weather variables is presumably important for the birds' decision to migrate and should therefore be considered in optimal migration models.

Reed warbler orientation: initiation of nocturnal migratory flights in relation to visibility of celestial cues at dusk

We used radiotelemetry to investigate the time of migratory flight initiation relative to available celestial orientation cues and departure direction of a nocturnal passerine migrant, the reed warbler, Acrocephalus scirpaceus, during autumn migration. The study was carried out at Falsterbo, a coastal site in southwest Sweden. The warblers initiated migration from times well after local sunset and well into the night, corresponding to sun elevations between −4° and −35°, coinciding with the occurrence of stars at night. They departed in the expected migratory direction towards south of southwest with a few initiating migration in reverse directions towards northeast to east. Flight directions under overcast conditions (7–8/8) were more scattered than under clear sky conditions (0–4/8). There were fewer clouds on departure nights than on nights when the birds did not initiate migration. For birds staying longer than one night at stopover the horizontal visibility was higher and precipitation was less likely on departure nights than on the previous night. The results show that the visibility of celestial cues, and stars in particular, are important for the decision to initiate migration in reed warblers. However, cloud cover, horizontal visibility and precipitation might be correlated with other weather variables (i.e. wind or air pressure) that are also likely to be important for the decision to migrate.

Bird orientation at high latitudes: flight routes between Siberia and North America across the Arctic Ocean

Bird migration and orientation at high latitudes are of special interest because of the difficulties associated with different compass systems in polar areas and because of the considerable differences between flight routes conforming to loxodromes (rhumblines) or orthodromes (great circle routes). Regular and widespread east-north-east migration of birds from the northern tundra of Siberia towards North America across the Arctic Ocean (without landmark influences) were recorded by ship-based tracking radar studies in July and August. Field observations indicated that waders, including species such as Phalaropusfulicarius and Calidris melanotos, dominated, but also terns and skuas may have been involved. Analysis of flight directions in relation to the wind showed that these movements are not caused by wind drift. Assuming possible orientation principles based on celestial or geomagnetic cues, different flight trajectories across the Arctic Ocean were calculated: geographical loxodromes, sun compass routes, magnetic loxodromes and magnetoclinic routes. The probabilities of these four alternatives are evaluated on the basis of both the availability of required orientation cues and the predicted flight paths. This evaluation supports orientation along sun compass routes. Because of the longitudinal time displacement sun compass routes show gradually changing compass courses in close agreement with orthodromes. It is suggested that an important migration link between Siberia and North American stopover sites 1000-2500km apart across the Arctic Ocean has evolved based on sun compass orientation along orthodrome-like routes.

High Plasma Triglyceride Levels in Small Birds during Migratory Flight: A New Pathway for Fuel Supply during Endurance Locomotion at Very High Mass-Specific Metabolic Rates?

Plasma fat metabolites were measured during the nocturnal migratory flight in three species of small passerines. The birds had higher free fatty acid (FFA) and glycerol levels than resting birds that had been fasted overnight. In contrast to exercising mammals and large birds, they also had elevated levels of triglyceride and the very low density lipoprotein (VLDL) fraction. It is hypothesized that FFA reesterification in the liver and delivery of triglyceride-VLDL to the flight muscles helps to circumvent constraints in energy supply during endurance locomotion in animals with very high mass-specific metabolic rates.

Migrating birds respond to Project Seafarer's electromagnetic field

Radar tracking of individual migrating birds flying over a large alternating-current antenna system showed that the birds turned or changed altitude more frequently when the antenna system was operating than when it was not. These results suggest that birds sense low-intensity alternating-current electromagnetic fields during nocturnal migratory flight.

An Astronomical Determination of the Heights of Birds during Nocturnal Migration

TWENTY-FIVE years ago W. E. D. Scott, in the 'Bulletin of the Nuttall Ornithological Club,'1 called attention to the telescope as a means for actually observing against the moon as a background the nocturnal migratory flight of birds. Realizing the possibilities of this method in throwing light on the unsettled question of elevation, Mr. Scott, with the aid of Professor Young of the Astronornical Department of Princeton University, made the following observations and calculations. A nine and one-half inch telescope was pointed toward the moon in the month of October, 1880. Numbers of birds were seen flying in a general southeasterly direction. Since these birds were clearly outlined they must have been at least one mile from the observers, for if nearer the telescope they would have been out of focus. A distance of one mile was, therefore, taken as the inferior limit of the field of observation; and on the assumption that birds would not flv at a greater height than ten thousand feet the superior limit of the field was fixed at a distance of four miles. These distances and the angle at which the telescope was inclined gave the data for the conclusion that the birds observed were flying from one-half2 to two miles above the earth. F. M. Chapman followed a few years later with a report in this journal3 of observations made with a six and one-half inch telescope during a night in September, 1887. With the assistance of an astronomical friend, John Tatlock, Jr., the writer prepared a table