Heiko Schmaljohann

Stopover optimization in a long-distance migrant: the role of fuel load and nocturnal take-off time in Alaskan northern wheatears (Oenanthe oenanthe).

IntroductionIn long-distance migrants, a considerably higher proportion of time and energy is allocated to stopovers rather than to flights. Stopover duration and departure decisions affect consequently subsequent flight stages and overall speed of migration. In Arctic nocturnal songbird migrants the trade-off between a relatively long migration distance and short nights available for travelling may impose a significant time pressure on migrants. Therefore, we hypothesize that Alaskan northern wheatears (Oenanthe oenanthe) use a time-minimizing migration strategy to reach their African wintering area 15,000 km away.ResultsWe estimated the factors influencing the birds’ daily departure probability from an Arctic stopover before crossing the Bering Strait by using a Cormack-Jolly-Seber model. To identify in which direction and when migration was resumed departing birds were radio-tracked. Here we show that Alaskan northern wheatears did not behave as strict time minimizers, because their departure fuel load was unrelated to fuel deposition rate. All birds departed with more fuel load than necessary for the sea crossing. Departure probability increased with stopover duration, evening fuel load and decreasing temperature. Birds took-off towards southwest and hence, followed in general the constant magnetic and geographic course but not the alternative great circle route. Nocturnal departure times were concentrated immediately after sunset.ConclusionAlthough birds did not behave like time-minimizers in respect of the optimal migration strategies their surplus of fuel load clearly contradicted an energy saving strategy in terms of the minimization of overall energy cost of transport. The observed low variation in nocturnal take-off time in relation to local night length compared to similar studies in the temperate zone revealed that migrants have an innate ability to respond to changes in the external cue of night length. Likely, birds maximized their potential nightly flight range by taking off early in the night which in turn maximizes their overall migration speed. Hence, nocturnal departure time may be a crucial parameter shaping the speed of migration indicating the significance of its integration in future migration models.

Response of a free-flying songbird to an experimental shift of the light polarization pattern around sunset

SUMMARY The magnetic field, the sun, the stars and the polarization pattern of visible light during twilight are important cues for orientation in nocturnally migrating songbirds. As these cues change with time and location on Earth, the polarization pattern was put forward as a likely key reference system calibrating the other compass systems. Whether this applies generally to migratory birds is, however, controversially discussed. We used an experimental approach in free-flying birds to study the role of polarization for their departure direction in autumn. Experimental birds experienced a 90 deg shift of the band of maximum polarization during sunset, whereas control birds experienced the polarization pattern as under natural conditions. Full view of the sunset cues near the horizon was provided during the cue conflict exposure. Here we show that both the experimental and the control birds being released after nautical twilight departed consistently towards south-southeast. Radiotelemetry allowed tracking of the first 15 km of the birds outward journey, thus the intrinsic migration direction as chosen by the birds was measured. We found no recalibration of the magnetic compass after pre-exposure to a cue conflict between the natural magnetic field and the artificially shifted polarization pattern at sunset. The lacking difference in the departure direction of both groups may suggest that birds did not recalibrate any of the compass systems during the experiment. As free-flying migrants can use all available orientation cues after release, it remains unknown whether our birds might have used the magnetic and/or star compass to determine their departure direction.

Towards a conceptual framework for explaining variation in nocturnal departure time of songbird migrants

Most songbird migrants travel between their breeding areas and wintering grounds by a series of nocturnal flights. The exact nocturnal departure time for these flights varies considerably between individuals even of the same species. Although the basic circannual and circadian rhythms of songbirds, their adaptation to migration, and the factors influencing the birds’ day-to-day departure decision are reasonably well studied, we do not understand how birds time their departures within the night. These decisions are crucial, because the nocturnal departure time defines the potential flight duration of the migratory night. The distances covered during the nocturnal migratory flights in the course of migration in turn directly affect the overall speed of migration. To understand the factors influencing the arrival of the birds in the breeding/wintering areas, we need to investigate the mechanisms that control nocturnal departure time. Here, we provide the first conceptual framework for explaining the variation commonly observed in this migratory trait. The basic schedule of nocturnal departure is likely regulated by both the circannual and circadian rhythms of the innate migration program. We postulate that the endogenously controlled schedule of nocturnal departures is modified by intrinsic and extrinsic factors. So far there is only correlative evidence that birds with a high fuel load or a considerable increase in fuel load and significant wind (flow) assistance towards their migratory goal depart early within the night. In contrast, birds migrating with little fuel and under unfavorable wind conditions show high variation in their nocturnal departure time. The latter may contain an unknown proportion of nocturnal movements not directly related to migratory flights. Excluding such movements is crucial to clearly identify the main drivers of the variation in nocturnal departure time. In general we assume that the observed variation in the nocturnal departure time is explained by individually different reactions norms of the innate migration program to both intrinsic and extrinsic factors.

Can wheatears weather the Atlantic? Modeling nonstop trans-Atlantic flights of a small migratory songbird

ABSTRACT Oceans represent extreme ecological barriers for land birds. Yet the Northern Wheatear (Oenanthe oenanthe leucorhoa), a 25-g songbird, negotiates the North Atlantic Ocean twice yearly between Canadian natal and sub-Saharan wintering grounds. Each autumn, these migrants appear to have 2 options: (1) a detour via Greenland, Iceland, and/or Europe to reduce the extent of open-ocean flights or (2) an astonishing nonstop flight of 4,000–5,000 km without resting opportunities between eastern Canada and northwestern Africa. We assessed the feasibility and reliability of nonstop trans-Atlantic migration of Northern Wheatears from Canada to Africa using an individual-based model incorporating flight costs and autumnal wind data from 1979 to 2011. Prevalent wind conditions were supportive of nonstop migration, especially at high altitudes and when winds at departure were favorable. For modeled individuals with high fuel loads, flying at altitudes of ∼3,000 m, successful nonstop trans-Atlantic flights reached Africa on 62% of departure days. On 24% of unsuccessful departure days, individuals could have first stopped in Europe before continuing to Africa. Durations of successful flights varied between 31 and 68 hr, with significantly shorter flights after mid-September. It remains unclear whether natural selection might favor nonstop ocean crossings by O. o. leucorhoa between North America and Africa, but we conclude that reliably supportive winds en route and potentially huge time savings render it a feasible migration strategy.

Flexible reaction norms to environmental variables along the migration route and the significance of stopover duration for total speed of migration in a songbird migrant

BackgroundPredicting the consequences of continuing anthropogenic changes in the environment for migratory behaviours such as phenology remains a major challenge. Predictions remain particularly difficult, because our knowledge is based on studies from single-snapshot observations at specific stopover sites along birds’ migration routes. However, a general understanding on how birds react to prevailing environmental conditions, e.g. their ‘phenotypic reaction norm’, throughout the annual cycle and along their entire migration routes is required to fully understand how migratory birds respond to rapid environmental change.ResultsHere, we provide direct evidence that northern wheatears (Oenanthe oenanthe) from a breeding population in Alaska adjusted their probability to resume migration as well as the distance covered per night, i.e. travel speed, to large-scale environmental conditions experienced along their 15,000xa0km migratory route on both northwards and southwards migrations. These adjustments were found to be flexible in space and time. At the beginning of autumn migration, northern wheatears showed high departure probabilities and high travel speeds at low surface air temperatures, while far away from Alaska both traits decreased with increasing air temperatures. In spring, northern wheatears increasingly exploited flow assistance with season, which is likely a behavioural adjustment to speed up migration by increasing the distance travelled per night. Furthermore, the variation in total stopover duration but not in travel speed had a significant effect on the total speed of migration, indicating the prime importance of total stopover duration in the overall phenology of bird migration.ConclusionNorthern wheatears from Alaska provide evidence that the phenotypic reaction norm to a set of environmental conditions cannot be generalized to universal and persistent behavioural reaction pattern across entire migratory pathways. This highlights the importance of full annual-cycle studies on migratory birds to better understand their response to the environment. Understanding the mechanisms behind phenotypic plasticity during migration is particularly important in the assessment of whether birds can keep pace with the potentially increasing phenological mismatches observed on the breeding grounds.

Proximate mechanisms affecting seasonal differences in migration speed of avian species

Faster migration in spring than in autumn seems to be a common pattern in birds. This has been ultimately explained by seasonally different selection pressures. Variation in migration speed is proximately caused by adjusting travel speed (distance covered during flight) and/or stopover duration (times when birds rest and refuel). Yet, it remains unclear whether individual seasonal differences in migration speed match the common pattern and what the precise role of the proximate, behavioural mechanisms for adjusting migration speed is. By reviewing 64 studies of 401 tracks, I show that in waders, gulls, swifts, and songbirds speeds were significantly higher in spring, while the opposite was the case in waterfowl and owls. Thus, the ultimate mechanisms selecting for a faster migration in spring might not consistently act across bird groups. Breeding latitude, migration strategy, migration distance, flight style, body mass, and sex did not explain seasonal differences in speed. The ratio between spring and autumn total stopover duration of 257 bird tracks significantly negatively affected the seasonal migration speed ratio of the same individuals in a comparative analysis accounting for shared ancestry. Seasonal variation in stopover duration appears thus to be the main biological mechanism regulating seasonal differences in migration speed.

Nocturnal departure timing in songbirds facing distinct migratory challenges

Most migratory songbirds travel between their breeding areas and wintering grounds through a series of nocturnal flights. The timing of their departures defines the potential flight duration and thus the distance covered during a migratory night. Yet, migratory songbirds show substantial variation in their nocturnal departure timing. With this study, we aim to assess whether the respective challenges of the migration route, namely its distance and nature, help to explain this variation. At a stopover site, we caught Northern Wheatears (Oenanthe oenanthe) of two subspecies that differ in distance and nature of their onward migration route in spring, but not in autumn. We determined the start of their nocturnal migratory restlessness during short-term captivity, and radiotracked their nocturnal departure timing after release in both migration seasons. Northern Wheatears started their nocturnal migratory restlessness earlier when facing a long remaining migration distance and an extended sea barrier in spring. Individual departure directions generally affected the nocturnal departure timing with early departures being directed towards the respective migratory destination. In spring, this pattern was predominantly found in birds carrying relatively large fuel stores, but was absent in lean birds. At the same time, birds facing a short remaining migration distance and no extended sea barrier strongly reacted to relatively large fuel stores by an early start of nocturnal migratory behaviour (migratory restlessness and departure timing), whereas this reaction was not found in birds facing a long remaining migration distance and sea barrier. These results suggest that the basic diel schedule of birds migratory activity is adapted to the onward migration route. Further, they suggest that birds adjust their behavioural response, that is start of nocturnal migratory behaviour, to fuel stores in relation to their impending migratory challenges. This is a substantial step in understanding variation of nocturnal departure timing and its adjustments in migratory songbirds. Further, it emphasizes the importance of interpreting birds nocturnal migratory behaviour in the respective ecological context.

Cues, corticosterone and departure decisions in a partial migrant.

Most migrating birds make multiple stopovers to fuel and/or rest. The decision to resume migration from stopover is based on various cues, such as time within the season and wind conditions. There are hints that the strength of these departure cues shapes corticosterone level, which in its turn appears to regulate the timing of departure. We here provide results that very strongly indicate that indeed departure cues jointly shape corticosterone level of migrants at stopover. We compared corticosterone level between migrating and sedentary common blackbirds (Turdus merula) sampled simultaneously at the same location during autumn migration. As expected, in migrating individuals corticosterone level was positively associated with time within the season and with current wind conditions. The latter was only apparent in adult birds and not in 1st year migrants, thus matching the observation that 1st year autumnal migrants are less wind selective than adults. In contrast to the migrants, in sedentary blackbirds these cues did not explain variation in corticosterone level. Furthermore, stopover departure seemed more likely and to occur earlier in the night in migrants with high corticosterone level. Our unique comparative study thus supports the newly developed concept that corticosterone mediates between departure cues and stopover departure timing in avian migrants.

Common Terns on the East Atlantic Flyway: temporal–spatial distribution during the non-breeding period

We studied the temporal–spatial distribution of Common Terns Sterna hirundo along the East Atlantic Flyway. In 2009 and 2010 experienced adults from a colony on the German North Sea coast were tagged with geolocators recording light intensity and saltwater contact. Main objectives were the inter-individual temporal–spatial variation of migration routes and wintering areas, wintering site fidelity, and time spent at sea across the annual cycle. Geolocators had no effects on various traits of breeders, but their reproductive output suffered from egg breakage. This can be avoided by artificially incubating the eggs. Twelve routes of nine individuals were tracked. Transponder readings at the breeding site showed that birds left the colony 4xa0weeks before starting autumn migration. In spring and autumn, Common Terns stopped over around the Canary Islands. Main wintering distribution was the upwelling seas alongside the West African coast and similar between years, but different among individuals. Three females wintered further north and more offshore than six males. Pair mates wintered at different locations. Spring migration was longer (56xa0±xa08xa0days) than autumn migration (37xa0±xa017xa0days). During both migration and wintering the terns spent more time on salt water than during breeding and post-breeding. In most individuals saltwater contact was higher during the day than at night, reduced at sunrise and sunset likely due to foraging, and peaked about noon possibly related to resting or thermoregulation. Detailed ecological and behavioral studies of common terns during wintering are needed to clarify the results based on geolocators.ZusammenfassungFlussseeschwalben entlang des Ostatlantischen Zugweges: Raumzeitliche Verteilung außerhalb der Brutperiode Wir untersuchten die raumzeitliche Verteilung von Flussseeschwalben Sterna hirundo entlang des Ostatlantischen Zugweges. 2009 und 2010 wurden erfahrene Brutvögel einer Kolonie an der deutschen Nordseeküste mit Geolokatoren versehen, die Lichtintensität und Salzwasserkontakt aufzeichneten. Ziele der Untersuchungen waren die interindividuelle raumzeitliche Variation der Zugrouten und Überwinterungsgebiete, die Winterortstreue und die Dauer des Seewasserkontakts im Jahreszyklus. Die Geolokatoren beeinträchtigten die Flussseeschwalben nicht, der Reproduktionserfolg jedoch war durch Bruch der Eier verringert, was durch Austausch und Ausbrüten der Eier in einem Inkubator vermieden werden kann. 12 Routen von 9 Individuen wurden verfolgt. Die Registrierungen der zusätzlich mit Transpondern gekennzeichneten Vögel zeigten, dass sie den Koloniestandort vier Wochen vor Beginn der Herbstwanderung verließen. Im Frühjahr und Herbst legten einige Flussseeschwalben bei den Kanarischen Inseln einen Zwischenhalt ein. Die Hauptverbreitung im Winter erstreckte sich entlang der Westafrikanischen Küste und war ähnlich zwischen den Jahren, aber verschieden zwischen Individuen. Die drei Weibchen überwinterten weiter nördlich als die sechs Männchen und die Paarpartner an verschiedenen Orten. Der Frühjahrszug dauerte länger (56xa0±xa08 d) als der Herbstzug (37xa0±xa017 d). Während der Wanderung und Überwinterung verbrachten die Flussseeschwalben mehr Zeit auf dem Salzwasser als im Brutgebiet. Die meisten Individuen hatten tagsüber längere Salzwasserkontakte als nachts, die bei Sonnenaufgang und -untergang stark reduziert waren, vermutlich aufgrund der Nahrungssuche. Während der Mittagszeit waren Salzwasserkontakte besonders intensiv, möglicherweise bedingt durch Rasten oder Thermoregulation. Detaillierte Studien zu Ökologie und Verhalten im Winter sollten folgen, um die auf den Geolokatoren basierten Ergebnisse zu klären.

Ghrelin, corticosterone and the resumption of migration from stopover, an automated telemetry study

The spectacular natural phenomenon of avian migration is evidently shaped by physical factors, but we know little about the underlying physiological regulation. This contrast is especially apparent for the process of departure on a migratory flight. The decision to resume migration is shaped by a suite of departure cues from innate rhythms, and intrinsic and extrinsic factors. It currently appears that these departure cues are translated into actual departure by the hormone corticosterone, but other hormones may play a role too and probably interact with corticosterone. We captured this concept here by investigating the role of the hormone ghrelin and its interaction with corticosterone for the departure decision in a migratory songbird. Ghrelin functions as an appetite-regulating hormone. It has also been suggested to be involved in the regulation of departure by upregulating corticosterone in migrants ready to depart, and by facilitating the breakdown of lipids to fuel migratory flight. We measured plasmatic ghrelin and corticosterone levels in migrating common blackbirds (Turdus merula) at an autumnal stopover site, and determined their departure timing with the use of a fully-automated radio-telemetry system. Against our expectations, ghrelin level was not correlated with the birds lipid stores or with corticosterone level. Furthermore, departure likelihood and nocturnal departure time were not associated with ghrelin levels. Our study thus does not support the idea that ghrelin is involved in the regulation of departure from stopover, at least not in common blackbirds. We discuss possible reasons for the lack of confirmation of our expectations.