Volker Salewski

The evolution of bird migration—a synthesis

We approach the problem of the evolution of bird migration by asking whether migration evolves towards new breeding areas or towards survival areas in the non-breeding season. Thus, we avoid the ambiguity of the usually discussed “southern-home-theory” or “northern-home-theory”. We argue that migration evolved in birds that spread to seasonal habitats through gradual dispersal to enhance survival during the non-breeding season; this in contrast to the alternative idea suggesting that migration evolved towards new breeding areas to increase reproductive success. Our synthesis is based on the threshold model explaining how migratory traits can change rapidly through microevolutionary processes. Our model brings former theories together and explains how bird migration, with the appropriate direction and time program, evolves through selection after genetically non-directed events such as dispersal and colonization. The model does not need the former untested assumptions such as competition as a reason for migration and for the disappearance of sedentary populations or higher reproductive success in temperate breeding areas. Our theory offers answers to questions such as how birds with a southern origin may gradually reach northern latitudes, why migration routes may follow historical expansion routes and why birds leave an area for the non-breeding season and move back instead of breeding on their wintering grounds. The theory proposes gradual change through selection and not sudden changes such as long distance dispersal or mutations and can be applied to migration at all latitudes and in all directions. The scenario provides a reasonable concept to understand most of the existing migratory phenomena on the basis of the ecology and genetics of migratory behaviour.

Palearctic passerines in Afrotropical environments: a review

Previous ecological studies of Palearctic passerine migrants in Africa have claimed to reveal some general features with respect to habitat use, foraging ecology and interspecific relationships with Afrotropical residents. In this review we discuss apparent contradictions between earlier generalisations and more recent results from more detailed field studies and explore in which areas our ecological knowledge and theoretical understanding remain poor and have given rise to misconceptions. For example, it has been claimed that migrants use structurally more diverse and open habitats and that they forage higher and in more peripheral parts of the vegetation than their ecologically similar Afrotropical counterparts, yet in the past these characteristics were often not clearly defined and not always correlated in practice. It has also been stated that migrants are more flexible in habitat use, occupying a wider range of habitat types and employing a higher diversity of foraging techniques, both of which were assumed to be adaptations to permit coexistence with Afrotropical residents by using untapped resources that are only seasonally available. Yet results from studies of the role of competition in shaping migrant-resident communities remain largely unconvincing. While flexibility may facilitate migrant-resident coexistence, it may also favour the evolution of migration because specialists are less able to use their advantages in different environments. We note that definitions of flexibility and specialisation may themselves depend on the ecological or evolutionary approach adopted by researchers. We conclude that few generalisations can safely be made about the ecology of Palearctic migrants in Africa and that adaptive explanations for the behaviours observed are largely lacking, as are studies of the fitness consequences of different migrant strategies such as have been conducted in the Nearctic-Neotropical migration system.

Barrier crossing in small avian migrants: individual tracking reveals prolonged nocturnal flights into the day as a common migratory strategy

Over decades it has been unclear how individual migratory songbirds cross large ecological barriers such as seas or deserts. By deploying light-level geolocators on four songbird species weighing only about 12 g, we found that these otherwise mainly nocturnal migrants seem to regularly extend their nocturnal flights into the day when crossing the Sahara Desert and the Mediterranean Sea. The proportion of the proposed diurnally flying birds gradually declined over the day with similar landing patterns in autumn and spring. The prolonged flights were slightly more frequent in spring than in autumn, suggesting tighter migratory schedules when returning to breeding sites. Often we found several patterns for barrier crossing for the same individual in autumn compared to the spring journey. As only a small proportion of the birds flew strictly during the night and even some individuals might have flown non-stop, we suggest that prolonged endurance flights are not an exception even in small migratory species. We emphasise an individual’s ability to perform both diurnal and nocturnal migration when facing the challenge of crossing a large ecological barrier to successfully complete a migratory journey.

Numbers, foraging and refuelling of passerine migrants at a stopover site in the western Sahara: diverse strategies to cross a desert

Twice a year, songbirds breeding in the Western Palaearctic cross the largest desert of the world, the Sahara, to reach their African winter quarters. Recently, a radar study quantified this migration and demonstrated that almost all passerines cross the western Sahara with an intermittent strategy, i.e. they fly during the night and rest during the day. Before crossing the desert, most passerines accumulate fat stores because they will not find appropriate resting sites for feeding in the Sahara. However, it has also been reported that birds use the vegetation around oases for refuelling. Since birds resting at oases had smaller fat deposits than birds resting in the open desert, it was hypothesised that mainly lean birds or fall-outs use the oases for feeding. In this study, we investigated which species or individuals use oases in the western Sahara during spring migration and how they use them. We demonstrate that a minority of species adapted to dry vegetation may cross the Sahara with low energy stores and intermittent refuelling in vegetation patches. These birds avoid the costs of transporting large energy stores, in contrast to most other passerine migrants which fuel up before crossing the Sahara and adopt an intermittent strategy without refuelling. The birds which rely on refuelling at oases probably often have a slow refuelling rate and may even run the risk of not finding appropriate habitats. The available studies reveal that birds use a wide variety of strategies to cross the Sahara. The particular strategy adopted depends on the species, and is modulated according to weather conditions aloft at the time, existing energy stores, the availability of stopover sites, and the suitability (food availability, competitors) of stopover sites.

An unknown migration route of the ‘globally threatened’ Aquatic Warbler revealed by geolocators

AbstractThe globally-threatened Aquatic Warbler (Acrocephalus paludicola) breeds in Europe and spends the northern winter in sub-Saharan West Africa. We attached 30 geolocators to breeding birds in the central Ukraine in 2010. Three geolocators which collected data from the autumn migration were recovered in 2011. They revealed a previously-unknown migration route via southern Europe to stopover sites in south-western France and Spain. In West Africa, one bird spent some time well south of known non-breeding areas. For the conservation of some Aquatic Warbler populations, protection of hitherto unknown stopover sites in southern Europe and Africa may be crucial.ZusammenfassungGeolokation deckt unbekannten Zugweg des global bedrohten Seggenrohrsängers auf. Der global bedrohte Seggenrohrsänger (Acrocephalus paludicola) brütet in Europa und überwintert in Westafrika. Wir rüsteten 2010 30 Seggenrohrsänger in einem Brutgebiet in der Zentral-Ukraine mit Geolokatoren aus. Drei Geolokatoren, die Daten vom Herbstzug aufnahmen, wurden 2011 zurückgewonnen. Sie zeigten einen bis dahin unbekannten Zugweg der Seggenrohrsänger durch Südeuropa zu den ersten Hauptrastgebieten in Südfrankreich und Spanien an. In Westafrika hielt sich ein Vogel weit südlich der bekannten Überwinterungsgebiete auf. Für den Schutz der Seggenrohrsänger könnte es daher bedeutend sein, die noch unbekannten Rastgebiete in Südeuropa und Afrika zu berücksichtigen.

Plastic colour rings and the incidence of leg injury in flycatchers (Muscicapidae, Monarchidae)

Studies of birds involving colour ringing can provide much detailed information concerning bird movements and behaviour without the need for recapture. However, a fundamental premise of colour ringing, and indeed all ringing studies, is that the rings applied should neither cause harm to the birds concerned nor alter their behaviour or survival. Colour rings have been used safely for many studies on a wide range of species, and problems are rarely reported. Here, we report on problems associated with colour‐ringing several species of flycatcher, and discuss the nature and extent of the issue. Unacceptable levels of leg injury were reported when flycatchers were ringed using either celluloid or PVC colour rings, particularly when the colour ring was proximal to the foot. In the light of our evidence, we would urge those considering embarking upon colour‐ringing projects involving flycatchers to consider using anodised aluminium colour rings and to avoid the use of plastic colour rings.

Longer wings for faster springs – wing length relates to spring phenology in a long‐distance migrant across its range

Abstract In migratory birds, morphological adaptations for efficient migratory flight often oppose morphological adaptations for efficient behavior during resident periods. This includes adaptations in wing shape for either flying long distances or foraging in the vegetation and in climate‐driven variation of body size. In addition, the timing of migratory flights and particularly the timely arrival at local breeding sites is crucial because fitness prospects depend on site‐specific phenology. Thus, adaptations for efficient long‐distance flights might be also related to conditions at destination areas. For an obligatory long‐distance migrant, the common nightingale, we verified that wing length as the aerodynamically important trait, but not structural body size increased from the western to the eastern parts of the species range. In contrast with expectation from aerodynamic theory, however, wing length did not increase with increasing migration distances. Instead, wing length was associated with the phenology at breeding destinations, namely the speed of local spring green‐up. We argue that longer wings are beneficial for adjusting migration speed to local conditions for birds breeding in habitats with fast spring green‐up and thus short optimal arrival periods. We suggest that the speed of spring green‐up at breeding sites is a fundamental variable determining the timing of migration that fine tune phenotypes in migrants across their range.

Migrating passerines can lose more body mass reversibly than previously thought

Knowing the amount of body mass that migrants can lose is essential for the estimation of potential flight ranges, assessing the importance of stopover sites or modelling migration strategies. ‘Lean’ body mass, (the mass without fat stores) is often used to describe the limit of body‐mass loss but birds also use protein as fuel for flight. The mean body mass of 621 passerines mist‐netted in Mauritania between 2001 and 2004 with a fat score of 0 (no visible subcutaneous fat stores) and a muscle score of 0 (emaciated flight muscle) was much lower compared to the lean body mass values found in the literature. Recaptures showed that these birds were able to refuel again. The mean body mass of 122 Garden Warbler Sylvia borin with both a fat score and a muscle score of 0 was 12.8 g. The mean potential flight range of four Garden Warblers captured in the desert was 35% higher when 12.8 g was used for the estimation compared to the 15.0 g often used in previous studies. This has implications for future studies of the migration strategies of passerines, and suggests that, in the past, the fuel load of migrating passerines has been underestimated.

Phenology of Western Olivaceous Warbler Hippolais opaca and Eastern Olivaceous Warbler Hippolais pallida reiseri on stopover sites in Mauritania

Due to the problems of identifying the recently‐split Western Olivaceous Warbler Hippolais opaca and Eastern Olivaceous Warbler H pallida reiseri in the field, the migration phenology, passage routes and wintering areas of the two species remain unknown. Both species were mist‐netted regularly during autumn and spring migration on Mauritania and the migration phenology of the species in the western Sahara is described here for the first time. Autumn migration of Western Olivaceous Warblers through central Mauritania lasts from early August to early October. Passage at the coast started later and occurred during a shorter time period. The species does not seem to winter in central Mauritania. During spring migration, Western Olivaceous Warblers occurred from early March to early May. No birds were mist‐netted in spring at the coast although some were present. The Eastern Olivaceous Warblerwas mist‐netted at the inland sites only. Only a few birds were mist‐netted on autumn migration, which was between late August and early October. The species does not seem to winter in central Mauritania. During spring migration, Eastern Olivaceous Warblers were mist‐netted from mid March until early May. There were no indications that the species breeds in central Mauritania.

Microhabitat use and feeding strategies of the Pied Flycatcher and the Willow Warbler in their West-African winter quarters compared with resident species

Salewski, V. 2000. Microhabitat use and feeding strategies of the Pied Flycatcher and the Willow Warbler in their West-African winter quarters compared with resident species. Ostrich 71 (1 & 2): 191–193. Habitat choice, microhabitat and foraging behaviour of the palaearctic Pied Flycatcher and Willow Warbler are described in their West-African wintering areas and are compared with those of resident species. The migrants were more flexible in habitat choice and foraging techniques, but in general did not feed in more open habitat.