Solar heating may explain extreme diel flight altitude changes in migrating birds.

Great reed warblers, Acrocephalus arundinaceus,1 and great snipes, Gallinago media,2 exhibit a diel cycle in flight altitudes-flying much higher during the day than the night-when performing migratory flights covering both night and day. One hypothesis proposed to explain this behavior is that the birds face additional heating by solar radiation during daytime and hence must climb to very high, and thus also very cold, altitudes to avoid overheating during daytime flights.1,2 Yet, solar heat gain in birds has been shown to drastically decrease with wind speed,3,4 and the quantitative heating effect by solar radiation on a bird flying with an airspeed of 10 m/s or more is unknown. We analyzed temperature data from multisensor data loggers (MDLs)5,6 placed without direct exposure to solar radiation on great reed warblers (the logger covered by feathers on the back) and great snipes (the logger on the leg, covered from the sun by the tail). We found that logger temperatures were significantly higher (5.9°C-8.8°C in great reed warblers and 4.8°C-5.4°C in great snipes) during the day than during the night in birds flying at the same altitudes (and thus also the same expected ambient air temperatures). These results strongly indicate that the heat balance of the flying birds is indeed affected by solar radiation, which is in accordance with the hypothesis that solar radiation is a key factor causing the remarkable diel cycles in flight altitude observed in these two long-distance migrant bird species.1,2.

Activity patterns throughout the annual cycle in a long-distance migratory songbird, the red-backed shrike Lanius collurio.

BACKGROUND: Long-distance migratory birds undergo complex annual cycles during which they must adjust their behaviour according to the needs and conditions encountered throughout the year. Yet, variation in activity throughout the entire annual cycle has rarely been studied in wild migratory birds. METHODS: We used multisensor data loggers to evaluate the patterns of activity throughout the complete annual cycle of a long-distance migratory bird, the red-backed shrike Lanius collurio. Accelerometer data was used to identify life-history stages and to estimate levels of activity during various phases of the annual cycle. In this study, we analysed the variation in daytime activity along the annual cycle and between migratory and non-migratory days. RESULTS: The birds' daytime activity varied throughout the annual cycle while night-time activity was almost exclusively restricted to migratory flights. The highest daytime activity levels were observed during the breeding season, while it remained low during autumn migration and the winter period. Daytime activity differed between sexes during the breeding period, when the males showed the highest level in activity. During migratory periods, both sexes exhibited a higher daytime activity in spring compared to autumn migration, being particularly high in the final migratory leg towards the breeding ground. The birds showed a lower daytime activity on migratory days (days when a migratory flight took place during the succeeding night) than on non-migratory days during both migratory seasons. CONCLUSIONS: Activity measured during daytime results from a combination of several behaviours, and a high daytime activity during spring migration and the breeding period is possibly reflecting particularly energy-demanding periods in the annual cycle of migratory birds. The use of multisensor data loggers to track annual activity provides us with a full annual perspective on variation in activity in long-distance migratory species, an essential approach for understanding possible critical life-history stages and migration ecology.

Flight altitude dynamics of migrating European nightjars across regions and seasons.

Avian migrants may fly at a range of altitudes, but usually concentrate near strata where a combination of flight conditions is favourable. The aerial environment can have a large impact on the performance of the migrant and is usually highly dynamic, making it beneficial for a bird to regularly check the flight conditions at alternative altitudes. We recorded the migrations between northern Europe and sub-Saharan Africa of European nightjars Caprimulgus europaeus to explore their altitudinal space use during spring and autumn flights and to test whether their climbs and descents were performed according to predictions from flight mechanical theory. Spring migration across all regions was associated with more exploratory vertical flights involving major climbs, a higher degree of vertical displacement within flights, and less time spent in level flight, although flight altitude per se was only higher during the Sahara crossing. The nightjars commonly operated at ascent rates below the theoretical maximum, and periods of descent were commonly undertaken by active flight, and rarely by gliding flight, which has been assumed to be a cheaper locomotion mode during descents. The surprisingly frequent shifts in flight altitude further suggest that nightjars can perform vertical displacements at a relatively low cost, which is expected if the birds can allocate potential energy gained during climbs to thrust forward movement during descents. The results should inspire future studies on the potential costs associated with frequent altitude changes and their trade-offs against anticipated flight condition improvements for aerial migrants.

Extreme altitude changes between night and day during marathon flights of great snipes.

Several factors affect the flight altitude of migratory birds, such as topography, ambient temperature, wind conditions, air humidity, predation avoidance, landmark orientation, and avoiding over-heating from direct sunlight.1-6 Recent tracking of migratory birds over long distances has shown that migrants change flight altitude more commonly and dramatically than previously thought.4-8 The reasons behind these altitude changes are not well understood. In their seasonal migrations between Sweden and sub-Saharan Africa, great snipes Gallinago media make non-stop flights of 4,000-7,000 km, lasting 60-90 h.9,10 Activity and air pressure data from multisensor dataloggers showed that great snipes repeatedly changed altitudes around dawn and dusk, between average cruising heights about 2,000 m (above sea level) at night and around 4,000 m during daytime. Frequency and autocorrelation analyses corroborated a conspicuous diel cycle in flight altitude. Most birds regularly flew at 6,000 m and one bird reached 8,700 m, possibly the highest altitude ever recorded for an identified migrating bird. The diel altitude changes took place independently of climate zone, topography, and habitat overflown. Ambient temperature, wind condition, and humidity have no important diel variation at the high altitudes chosen by great snipes. Instead, improved view for orientation by landmarks, predator avoidance, and not least, seeking cold altitudes at day to counteract heating from direct sunlight are the most plausible explanations for the diel altitude cycle. Together with similar recent findings for a small songbird,6 the great snipes' altitudinal performance sheds new light on the complexity and challenges of migratory flights.

Extreme altitudes during diurnal flights in a nocturnal songbird migrant.

Billions of nocturnally migrating songbirds fly across oceans and deserts on their annual journeys. Using multisensor data loggers, we show that great reed warblers (Acrocephalus arundinaceus) regularly prolong their otherwise strictly nocturnal flights into daytime when crossing the Mediterranean Sea and the Sahara Desert. Unexpectedly, when prolonging their flights, they climbed steeply at dawn, from a mean of 2394 meters above sea level to reach extreme cruising altitudes (mean 5367 and maximum 6267 meters above sea level) during daytime flights. This previously unknown behavior of using exceedingly high flight altitudes when migrating during daytime could be caused by diel variation in ambient temperature, winds, predation, vision range, and solar radiation. Our finding of this notable behavior provides new perspectives on constraints in bird flight and might help to explain the evolution of nocturnal migration.

The lunar cycle drives migration of a nocturnal bird.

Every year, billions of seasonal migrants connect continents by transporting nutrients, energy, and pathogens between distant communities and ecosystems. For animals that power their movements by endogenous energy stores, the daily energy intake rates strongly influence the speed of migration. If access to food resources varies cyclically over the season, migrants sensitive to changes in daily energy intake rates may adjust timing of migration accordingly. As an effect, individuals adjusting to a common temporal cycle are expected to approach synchrony in foraging and movement. A large-scale periodic pattern, such as the dark-light cycle of the moon, could thus synchronize migrations across animal populations. However, such cyclic effects on the temporal regulation of migration has not been considered. Here, we show the temporal influence of the lunar cycle on the movement activity and migration tactics in a visual hunting nocturnal insectivore and long-distance migrant, the European nightjar, Caprimulgus europeaus. We found that the daily foraging activity more than doubled during moonlit nights, likely driven by an increase in light-dependent fuelling opportunities. This resulted in a clear cyclicity also in the intensity of migratory movements, with occasionally up to 100% of the birds migrating simultaneously following periods of full moon. We conclude that cyclic influences on migrants can act as an important regulator of the progression of individuals and synchronize pulses of migratory populations, with possible downstream effects on associated communities and ecosystems.

Actogram analysis of free-flying migratory birds: new perspectives based on acceleration logging.

The use of accelerometers has become an important part of biologging techniques for large-sized birds with accelerometer data providing information about flight mode, wing-beat pattern, behaviour and energy expenditure. Such data show that birds using much energy-saving soaring/gliding flight like frigatebirds and swifts can stay airborne without landing for several months. Successful accelerometer studies have recently been conducted also for free-flying small songbirds during their entire annual cycle. Here we review the principles and possibilities for accelerometer studies in bird migration. We use the first annual actograms (for red-backed shrike Lanius collurio) to explore new analyses and insights that become possible with accelerometer data. Actogram data allow precise estimates of numbers of flights, flight durations as well as departure/landing times during the annual cycle. Annual and diurnal rhythms of migratory flights, as well as prolonged nocturnal flights across desert barriers are illustrated. The shifting balance between flight, rest and different intensities of activity throughout the year as revealed by actogram data can be used to analyse exertion levels during different phases of the life cycle. Accelerometer recording of the annual activity patterns of individual birds will open up a new dimension in bird migration research.

Annual 10-Month Aerial Life Phase in the Common Swift Apus apus.

The common swift (Apus apus) is adapted to an aerial lifestyle, where food and nest material are captured in the air. Observations have prompted scientists to hypothesize that swifts stay airborne for their entire non-breeding period [1, 2], including migration into sub-Saharan Africa [3-5]. It is mainly juvenile common swifts that occasionally roost in trees or buildings before autumn migration when weather is bad [1, 6]. In contrast, the North American chimney swift (Chaetura pelagica) and Vaux's swift (C. vauxi) regularly settle to roost in places like chimneys and buildings during migration and winter [7, 8]. Observations of common swifts during the winter months are scarce, and roost sites have never been found in sub-Saharan Africa. In the breeding season, non-breeding individuals usually spend the night airborne [9], whereas adult nesting birds roost in the nest [1]. We equipped common swifts with a micro data logger with an accelerometer to record flight activity (years 1-2) and with a light-level sensor for geolocation (year 2). Our data show that swifts are airborne for >99% of the time during their 10-month non-breeding period; some individuals never settled, but occasional events of flight inactivity occurred in most individuals. Apparent flight activity was lower during the daytime than during the nighttime, most likely due to prolonged gliding episodes during the daytime when soaring in thermals. Our data also revealed that twilight ascents, previously observed during the summer [10], occur throughout the year. The results have important implications for understanding physiological adaptations to endure prolonged periods of flight, including the need to sleep while airborne.

Further evidence for amyloid deposition in clinical pancreatic islet grafts.

BACKGROUND: The reasons for the long-term complete or partial loss of islet graft function are unknown, but there are obviously other reasons than just pure allogeneic graft rejection. Earlier studies have shown that deposition of islet amyloid polypeptide amyloid in transplanted islets may indicate a mechanism for loss of ß cells. MATERIALS AND METHODS: Sections from liver material from four deceased islet-bearing recipients have been scrutinized for the presence of amyloid. Clinical data and certain aspects of the islet graft pathology of these patients have been published previously. RESULT: With this extended histological analysis, we demonstrate the occurrence of amyloid deposits in islets transplanted into the liver in three of four patients with type 1 diabetes. CONCLUSION: The finding adds evidence to the assumption that aggregation of islet amyloid polypeptide might be an important cause of progressing ß-cell dysfunction in clinically transplanted islets.

Islet amyloid polypeptide, islet amyloid, and diabetes mellitus.

Islet amyloid polypeptide (IAPP, or amylin) is one of the major secretory products of ß-cells of the pancreatic islets of Langerhans. It is a regulatory peptide with putative function both locally in the islets, where it inhibits insulin and glucagon secretion, and at distant targets. It has binding sites in the brain, possibly contributing also to satiety regulation and inhibits gastric emptying. Effects on several other organs have also been described. IAPP was discovered through its ability to aggregate into pancreatic islet amyloid deposits, which are seen particularly in association with type 2 diabetes in humans and with diabetes in a few other mammalian species, especially monkeys and cats. Aggregated IAPP has cytotoxic properties and is believed to be of critical importance for the loss of ß-cells in type 2 diabetes and also in pancreatic islets transplanted into individuals with type 1 diabetes. This review deals both with physiological aspects of IAPP and with the pathophysiological role of aggregated forms of IAPP, including mechanisms whereby human IAPP forms toxic aggregates and amyloid fibrils.