The relative importance and biological significance of non-stop and intermittent flight strategies in bird migrants crossing the Sahara

IX Summary The aim of this thesis was to investigate the flight and resting strategies of birds crossing the Sahara on migration. In order to overcome the severe limitations of ground observations, we chose radar to obtain continuous data of the temporal and spatial distribution of bird migration and to answer the core question whether migrants cross the largest desert by a long non-stop flight or rather by intermittent flying and resting. Study site and methods The Swiss Ornithological Institute carried out extensive radar and field studies at three different study sites in Mauritania, covering three migration seasons in 2003 and 2004. One station (20°56’ N; 11°35’ W), close to the oasis Ouadâne and about 500 km east of the Atlantic coast, was run permanently for about two moths in spring and autumn 2003 and in spring 2004. It provided continuous data on the course of bird migration across this part of the Sahara. The two other radar sites were operated about 300 km further to the East in the plain sand desert close to the border of Mali. At Mohammed Lemna (18°35’ N; 08°38’ W) radar data was collected for eleven days in September 2003. At Bîr Amrâne (22°47’ N; 08°43’ W), located about 450 km further to the North of Mohammed Lemna, the observations covered about three weeks in April 2004 (chapter I). Quantification of bird migration by means of radar is still a difficult task, in spite of almost sixty years of growing experience in radar ornithology. We present for the first time a method how radar data can be analysed to achieve reliable estimates of the number of migrating birds in a given air space. To reach this goal three basic steps of quantification must be considered: (1) echo detection, (2) echo identification and (3) quantification. First, the radar must be calibrated to allow the calculation of standardized echo sizes that are independent of distance. Then a distance-dependent threshold to reduce the detection of small targets at close range has to be applied. After this basic step, the remaining echoes must be assigned to bird or insect targets, eventually eliminating the insects. In our case, we were also able to determine the birds’ body axis relative to the radar (aspect) and to differentiate detection probabilities according to echo sizes. For the final quantification, we estimated the surveyed air space with respect to the specific detection range per echo size class. Using this method we calculated the temporal and spatial distribution of bird migration crossing the western Sahara (chapter II). Composition of daytime migration Billions of songbirds breeding in the western Palaearctic cross the Sahara desert twice a year. While crossing temperate latitudes most of them use an intermittent flight strategy, i.e. fly at night and rest or feed during the day. As resting and feeding in the Sahara is much less rewarding than in Europe, we investigated whether in the Sahara migration during daytime comprises the normal diurnal migrant species or the nocturnal ones prolonging their flight into the day. At the radar station Ouadâne, spring passage of passerine migrants showed high variation from night to night, but in general passerine densities decreased towards sunrise. However, under favourable wind conditions passerine migration continued far into the day, mostly at high altitudes (around 2000 m above ground level). These relatively high densities of passerines flying during the day were in contrast to very low proportions of real diurnal migrants detected on daily transect counts in the study area. For nocturnal migrants, however, the transect counts showed high correlation IX-

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

Background Predicting 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. Results Here, 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 15,000 km 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. Conclusion Northern 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

The physicochemical habitat of Sclerolinum sp., at Hook Ridge hydrothermal vent, Bransfield Strait, Antarctica

At Hook Ridge hydrothermal vent, a new species of Sclerolinum (Monilifera, Siboglinidae) was found at a water depth of 1,045 m. On the basis of investigations of multicores and gravity cores, the species habitat is characterized. Sclerolinum does not occur in sediments that are most strongly influenced by hydrothermal fluids, probably because of high temperature (up to 49°C) and precipitation of siliceous crusts. About 800 individuals m-2 occur in sediments that are only weakly exposed to hydrothermal flow and have the following characteristics: 20°C (15 cm sediment depth) to 21.5°C (bottom water), 18-40 cm yr-1 advection rates, pH 5.5, <25 µmol L-1 methane, <170 µmol L-1 sulfide, and <0.0054 mol m-2 yr-1 sulfide flux. Comparison with geochemical data from other reducing sediments indicates that the two groups of Siboglinidae, Monilifera and Frenulata, occur in sediments with low sulfide concentration and flux. In contrast, sulfurbased chemosynthetic organisms that typically occur at hydrothermal vents and cold seeps (e.g., Vestimentifera, vesicomyid clams, and bacterial mats) occur in sediments with higher sulfide availability; threshold values are around 500 µmol L-1 sulfide and 0.1 mol m-2 yr-1 sulfide fluxes. We did not find typical hydrothermal vent species at Hook Ridge hydrothermal vent, which might be explained by the unfavorable physicochemical habitat: At sites inhabited by Sclerolinum, sulfide availability appears to be too low, whereas at sites with higher sulfide availability, the temperatures might be too high, siliceous crust precipitation could preclude their occurrence, or both

Inherent limits of light-level geolocation may lead to over-interpretation

In their 2015 Current Biology paper, Streby et al. [1] reported that Golden-winged Warblers (Vermivora chrysoptera), which had just migrated to their breeding location in eastern Tennessee, performed a facultative and up to “>1,500 km roundtrip” to the Gulf of Mexico to avoid a severe tornadic storm. From light-level geolocator data, wherein geographical locations are estimated via the timing of sunrise and sunset, Streby et al. [1] concluded that the warblers had evacuated their breeding area approximately 24 hours before the storm and returned about five days later. The authors presented this finding as evidence that migratory birds avoid severe storms by temporarily moving long-distances. However, the tracking method employed by Streby et al. [1] is prone to considerable error and uncertainty. Here, we argue that this interpretation of the data oversteps the limits of the used tracking technique. By calculating the expected geographical error range for the tracked birds, we demonstrate that the hypothesized movements fell well within the geolocators’ inherent error range for this species and that such deviations in latitude occur frequently even if individuals remain stationary

Unravelling migration connectivity reveals unsustainable hunting of the declining ortolan bunting

In France, illegal hunting of the endangered ortolan bunting Emberiza hortulana has been defended for the sake of tradition and gastronomy. Hunters argued that ortolan buntings trapped in southwest France originate from large and stable populations across the whole of Europe. Yet, the European Commission referred France to the Court of Justice of the European Union (EU) in December 2016 for infringements to legislation (IP/16/4213). To better assess the impact of hunting in France, we combined Pan-European data from archival light loggers, stable isotopes, and genetics to determine the migration strategy of the species across continents. Ortolan buntings migrating through France come from northern and western populations, which are small, fragmented and declining. Population viability modeling further revealed that harvesting in southwest France is far from sustainable and increases extinction risk. These results provide the sufficient scientific evidence for justifying the ban on ortolan harvesting in France.Peer reviewe

Fluid venting in the eastern Aleutian subduction zone

Fluid venting has been observed along 800 km of the Alaska convergent margin. The fluid venting sites are located near the deformation front, are controlled by subsurface structures, and exhibit the characteristics of cold seeps seen in other convergent margins. The more important characteristics include (1) methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges; (2) prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps, over‐steepened folds or outcrops of bedding planes; (3) calcium carbonate and barite precipitates at the surface and subsurface of vents; and (4) carbon isotope evidence from tissue and skeletal hard parts of biota, as well as from carbonate precipitates, that vents expel either methane‐ or sulfide‐dominated fluids. A biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5.5±0.7 L m−2 d−1 for tectonics‐induced water flow [Wallmann et al., 1997b]. A geophysical estimate of dewatering from the same area [von Huene et al., 1997] based on sediment porosity reduction shows a fluid loss of 0.02 L m−2 d−1 for a 5.5 km wide converged segment near the deformation front. Our video‐guided surveys have documented vent biota across a minimum of 0.1% of the area of the convergent segment off Kodiak Island; hence an average rate of 0.006 L m−2 d−1 is estimated from the biogeochemical approach. The two estimates for tectonics‐induced water flow from the accretionary prism are in surprisingly good agreement

Wind-governed flight altitudes of nocturnal spring migrants over the Sahara

Flight costs make up a large proportion of energy expenditure during migration and are strongly dependent on atmospheric conditions aloft. Birds crossing the Sahara can take advantage of the fairly reliable trade-wind regime. In our study, we investigated whether birds adapt flight altitude to minimise energy or water loss. Data from free-flying birds were collected during spring migration with a tracking radar in an oasis in Mauritania, about 500km east of the Atlantic coast. Density measurements revealed the height distribution of the ongoing migration up to 4km above ground level (agl). Daily radiosondes provided information on temperature, humidity, pressure and wind profiles up to 4km agl. We compared height distributions of nocturnal migrants with predictions based on the atmospheric conditions. The two models used predicted maximum flight ranges and maximum flight durations either for energy or water constrained birds. Nocturnal migrants were flying mainly 2km agl, where predicted maximum flight ranges were generally largest. There was little difference between the two models, whereas both models were strongly shaped by the tailwind component. In addition, altitudes where water loss per flight time was minimal could explain some of the remaining variance. The results confirm that wind is the most important factor determining flight altitudes in bird migration and, at least in spring, water stress above the desert seems to play a minor role.Ostrich 2007, 78(2): 337–34

A quasi-experimental approach using telemetry to assess migration-strategy-specific differences in the decision-making processes at stopover

Abstract Background Migrant birds travel between their breeding areas and wintering grounds by alternating energetically and physiologically demanding flights with periods of rest and fuelling, so-called stopovers. An important intrinsic factor influencing the decision to resume migration is the amount of energy stores available for the next flight. Correlative studies with free-flying birds and experimental studies with caged birds have shown that the amount of energy stores affects the day-to-day, within-day and the directional decision of departure. The methodological advantages of both the correlative and experimental approach are combined when radio-tagging many individuals on the same day and subsequently determining the departure decisions at a high spatiotemporal resolution. Making use of such a quasi-experimental approach with an automated radio-tracking system at stopover, we studied the effect of energy stores on departure decisions and whether they vary between species of different migration strategies experiencing contrasting time constraints. For this, we chose a long-distance migrant, the common redstart (Phoenicurus phoenicurus), and a medium-distance migrant, the European robin (Erithacus rubecula), because the former has to travel at relatively higher speed to reach its wintering ground in a reasonable time at the expense of relatively higher energetic costs for travelling than the latter. Results Common redstarts with higher energy stores were more likely to resume migration than their conspecifics with lower energy stores, whereas this pattern was absent in the European robins. The amount of energy stores significantly affected the timing of departure within the day, with large energy stores yielding early departures in both species. Departure directions from the stopover site during the first night after capture were oriented towards the seasonally appropriate direction but were not affected by variation in energy stores. Conclusions We demonstrate the importance of variation in energy stores on the departure decisions and that it may affect species with different migration strategies dissimilarly in autumn. Nevertheless, knowledge of other intrinsic factors, such as feeding conditions, health status and physiological consequences of previous flights, is additionally required to better understand the departure decisions of migrants, as this is the key to providing an overall assessment of the decision-making process