Songbird migration across the Sahara: the non-stop hypothesis rejected!

Billions of songbirds breeding in the Western Palaearctic cross the largest desert of the world, the Sahara, twice a year. While crossing Europe, the vast majority use an intermittent flight strategy, i.e. fly at night and rest or feed during the day. However, it was long assumed that they overcome the Sahara in a 40 h non-stop flight. In this study, we observed bird migration with radar in the plain sand desert of the Western Sahara (Mauritania) during autumn and spring migration and revealed a clear prevalence of intermittent migration. Massive departures of songbirds just after sunset independent of site and season suggests strongly that songbirds spent the day in the plain desert. Thus, most songbirds cross the Sahara predominantely by the intermittent flight strategy. Autumn migration took place mainly at low altitudes with high temperatures, its density decreased abruptly before sunrise, followed by very little daytime migration. Migration was highly restricted to night-time and matched perfectly the intermittent flight strategy. However, in spring, when migratory flights occurred at much higher altitudes than in autumn, in cool air, about 17% of the songbird migration occurred during the day. This suggests that flying in high temperatures and turbulent air, as is the case in autumn, may lead to an increase in water and/or energy loss and may prevent songbirds from prolonged flights into the day.     

沈阳桃仙国际机场鸟类夜间迁徙规律

在鸟类迁徙季节,夜间鸟击事故频发是机场鸟击发生的一个显著特点.了解鸟类的夜间迁徙规律对于改进夜间鸟击防范措施具有重要的指导意义.本研究综合采用网捕法和声音记录法对沈阳桃仙机场夜间鸟类迁徙物种组成和迁徙规律进行研究.结果表明: 56种鸟类(占比88.9%)具有夜间迁徙习性,且以后半夜迁徙为主;鸟类夜间迁徙具有明显的时间动态和迁徙次序,春季鸟类迁徙较为集中,迁徙高峰在5月中旬,主要鸟类由鹌鹑、红尾伯劳、栗耳鹀、黑喉石鵖、普通夜鹰、黄眉柳莺等组成,秋季迁徙较为分散,迁徙高峰出现在9月下旬至10月上旬,主要由鹌鹑、灰背鸫、红喉鹨、丘鹬、矛斑蝗莺和灰头鵐等组成.对夜间迁徙鸟类的危险等级评估发现,春季严重危险物种是鹌鹑和红尾伯劳,秋季严重危险物种是鹌鹑、纵纹腹小鸮、灰背鸫和丘鹬.分别从夜间迁徙鸟类组成、迁徙动态、时间节律和物种危险等级等角度提出了相应的鸟击防范对策,为桃仙机场鸟击防范提供参考.     

Further perspectives on the breeding distribution of migratory birds: South American austral migrant flycatchers

1. Patterns of distribution of breeding austral migrant tyrant-flycatchers in temperate South America were quantified and analysed in conjunction with a variety of ecological, biogeographical and climatic variables.
2. The pattern of proportion of migratory to total breeding tyrannids was most strongly associated with latitude and two temperature variables, mean temperature of the coldest month and relative annual range of temperature.
3. The strong associations of latitude and temperature with percentage of migrants are consistent with the results of most similar investigations of the breeding distributions of migratory birds, both for migrants breeding in North America and in Europe, but contradict the hypothesis that habitat complexity plays a major role in structuring the proportion of migrants in communities of breeding birds.
4. The consistency of results among studies of migrants on different continents suggests that temperature and latitude, presumably a surrogate for one or more climatic variables, are globally significant factors in the breeding distributions of migratory birds.
5. The results for austral migrant flycatchers are consistent with the hypothesis that the prevalence of migration at any particular locality is ultimately dependent on the abundance of resources in the breeding season and the severity of the winter season, or on the difference in resource levels between summer and winter.     

Evaluating the benefits of CAP reforms: Can afforestations restore bird diversity in Mediterranean Spain?

Current Common Agricultural Policy (CAP) subsidies in the Mediterranean region tend to prioritize afforestation on former arable land with oaks rather than pines because pine plantations would maintain lower biological diversities than native forests. Nevertheless, no thorough evaluations of the conservation values of pine plantations as compared to oak remnants have been carried out to date. We analyze the diversity and conservation value of bird assemblages breeding in 200 remnants of Holm oak Quercus ilex woodlands and 82 mature (>50-year-old) pine plantations in central Spain, a Mediterranean region mostly devoted to arable farming. Species–area relationships were compared between forest types. The conservation value of bird assemblages was assessed using the “Species of European Conservation Concern” (SPEC) classification of Burfield and van Bommel [(2004). Birds in Europe: Populations estimates, trends and conservation status. Cambridge: BirdLife International]. Overall numbers of bird species maintained by oak and pine archipelagoes were rather similar, but species–area relationships differed between forest types. Intercepts were higher in oak fragments, whereas slopes were steeper in pine plantations. Small oak fragments held more species (mainly Mediterranean Sylvia warblers) than plantations, whereas large plantations held more species than large oak remnants. Differences in species–area relationships seemed to be due to differences in vegetation structure, especially understorey shrub cover and tree height and cover. We recorded nine SPECs, all exclusive (6) or near-exclusive (3) to oak woodlands, although such woodlands do not appear to be critical for their conservation. Hence, we conclude that pine afforestations have played a role for maintaining and restoring forest bird communities in the farming landscapes of central Spain. Promoting large and shrubby plantations would enhance their conservation value for breeding birds, together with promoting growth, regeneration and expansion of Holm oak remnants by means of set-aside measures previous or alternative to oak reafforestation. The increasing importance of non-commercial as compared to commercial values of Mediterranean forests would justify subsidizing the proposed policy.     

Welcome aboard: are birds migrating across the Mediterranean Sea using ships as stopovers during adverse weather conditions?

Birds use stopovers during migration to interrupt endurance flight in order to minimize immediate and/or future fitness costs. Stopovers on ships is considered an exceptional and anecdotal event in the ornithological literature. This does not match the experience we had in the summer of 2021, during an oceanographic campaign in the Central Mediterranean, when we regularly observed on average 2.8 birds, of at least 13 species, stopping on board during the 25 days of the campaign. The median stopping time was 42 min, ranging from a few minutes to overnight stays on board. The probability of finding a bird stopping aboard increased with wind force and cloud cover. Birds also stopped more often in a headwind and did not stop when the wind came from different directions other than the headwind. The Central Mediterranean is one of the busiest sea routes in the world, combining the mean daily number of birds on board with the thousands of ships that pass through it during the 3 months of summer migration; we estimate that nearly 4 million birds could use ships as stopover sites. This behaviour may represent a modern-day strategy that uses ships as stopovers in the event of adverse weather conditions or could act as an ecological trap, increasing the mortality of migrants. This phenomenon deserves more research attention and further studies recording body condition and tagging of individuals on board would be informative.     

The insectivorous bat Pipistrellus nathusii uses a mixed-fuel strategy to power autumn migration

In contrast to birds, bats are possibly limited in their capacity to use body fat as an energy source for long migrations. Here, we studied the fuel choice of migratory Pipistrellus nathusii (approximate weight: 8 g) by analysing the stable carbon isotope ratio (δ(13)C(V-PDB)) of breath and potential energy sources. Breath δ(13)C(V-PDB) was intermediate between δ(13)C(V-PDB) of insect prey and adipocyte triacylglycerols, suggesting a mixed-fuel use of P. nathusii during autumn migration. To clarify the origin of oxidized fatty acids, we performed feeding experiments with captive P. nathusii. After an insect diet, bat breath was enriched in (13)C relative to the bulk and fat portion of insects, but not deviating from the non-fat portion of insects, suggesting that bats oxidized exogenous proteins and carbohydrates, but not exogenous fatty acids. A feeding experiment with (13)C-labelled substrates confirmed these findings. In conclusion, migratory P. nathusii oxidized dietary proteins directly from insects captured en route in combination with endogenous fatty acids from adipocytes, and replenished their body reserves by routing dietary fatty acids to their body reserves.     

Autumn orientation behaviour of paddyfield warblers, Acrocephalus agricola, from a recently expanded breeding range on the western Black Sea coast

The paddyfield warbler, Acrocephalus agricola, has extended its breeding range from Central Asia to the western Black Sea coast. The Balkan population offers a unique chance to test the effect of breeding range expansion on the genetically programmed migratory direction. We studied 21 paddyfield warblers at Durankulak Lake, NE Bulgaria, by recording their autumn migratory orientation in circular orientation cages. Our data show that the preferred migratory orientation is directed along a NE-SW axis. Paddyfield warblers seem to avoid direct crossing of the Black Sea by following the western coast. The mean bearing was parallel to the nearest coastline and corresponds to the direction of the historical breeding range expansion of the species. In our experiment many individuals showed south-western orientation in autumn, a course which would potentially lead the birds to exploratory movements outside the current breeding range. An axial orientation response has been often shown in circular cage tests, and can be due to factors such as coastal orientation or reverse orientation triggered by the physiological condition of some individuals. However, it might also be one of the driving mechanisms for range expansion. Hence, we can expect the future expansion of Balkan paddyfield warblers to continue towards south-west.     

Climate change leads to decreasing bird migration distances

Global climate change has led to warmer winters in NW Europe, shortening the distance between suitable overwintering areas and the breeding areas of many bird species. Here we show that winter recovery distances have decreased over the past seven decades, for birds ringed during the breeding season in the Netherlands between 1932 and 2004. Of the 24 species included in the analysis, we found in 12 a significant decrease of the distance to the wintering site. Species from dry, open areas shortened their distance the most, species from wet, open areas the least, while woodland species fall in between the other two habitats. The decline in migration distance is likely due to climate change, as migration distances are negatively correlated with the Dutch temperatures in the winter of recovery. With a shorter migration distance, species should be better able to predict the onset of spring at their breeding sites and this could explain the stronger advancement of arrival date found in several short distance species relative to long-distance migrants.