Request – help us by collecting swift feathers!

Can stable isotopes in feathers from Common Swifts be used for identifying breeding and wintering areas?

For further information contact susanne.akesson@zooekol.lu.se

Until recent years, ringing has been the only method of finding the migration routes and wintering areas of birds. During the last decennium, traditional ringing has to some extent been challenged by new methods, appearing from research particularly within communication technology and molecular biology. The most detailed and interesting information has been obtained from satellite transmitters which can report the daily position of individual birds during migration between distant breeding grounds and wintering quarters. Unfortunately, satellite transmitters are expensive and still too heavy for use on most tropical migrating passerines, the group of birds whose migration routes and wintering areas we know least about. New molecular genetic methods have now partly given hope for the ability to distinguish different populations within a species on a DNA-level, and to establish the wintering areas for different populations by analyzing DNA from birds in the wintering quarters. Unfortunately, this has proved to be more difficult than some imagined initially, often because of the small genetic differences between different populations.

Stable isotopes in feathers

Recently studies on North American passerines have used an entirely new method to link breeding and wintering areas. The method is based on the fact that feathers carry chemical information from the area where they were moulted. By use of a mass spectrometer it is now possible to analyze the building blocks of a feather, and these building blocks can be used as a chemical fingerprint. In short, the method is as follows. A bird's feather is mainly composed of protein (keratin), which is built out of amino acids, which in turn mainly consists of the elements carbon, hydrogen, nitrogen, and oxygen. In nature, these elements exist in two or more variants, so called isotopes. These isotopes differ in the number of neutrons in the atom nucleus. The most common carbon (C) has 6 protons and 6 neutrons and is noted ¹²C, but there is also carbon with 7 neutrons (¹³C). The ratio of ¹³C in relation to ¹²C can be found by burning a small piece of a feather in a mass spectrometer type IRMS (Isotope Ratio Mass Spectrometer). Recently, Lund University has acquired and installed such equipment in the Ecology Building.

 When a bird feather grows, the two carbon isotopes are stored in the growing feather in a ratio corresponding to their presence in the food taken by the bird. If the food is an insect, the feather is built with the insect's ratio of carbon isotopes. The insect also has got its carbon isotope ratio from its food, for instance a plant. Isotopes of the various elements are stored in plants in different ratios depending on latitude, rain, plant species, and ground conditions. For certain elements also, a systematic concentration of the heavy isotope (the one with most neutrons) takes place in the food chain. For example, predators have more ¹⁵N in relation to ¹⁴N compared to plant eaters. Studies of North American wood warblers have shown great differences, especially in hydrogen and carbon isotopes, in feathers moulted in different places in North America, and these facts have been used for interpreting breeding areas among wintering birds in Middle America with a surprisingly high precision (Chamberlain et al. 1997, Hobson & Wassenaar 1997).

The Willow Warbler as a model species

In order to study the usefulness of stable isotopes as a method to find migration routes and wintering areas in European passerines, Staffan Bensch and Susanne Åkesson have chosen the Willow Warbler as a model species. The Willow Warbler is perfect for this kind of study as it moults twice a year (summer and winter) which means that a worn feather collected in the breeding quarters can reveal the winter moulting location, while a worn feather collected in Africa can reveal the summer moulting location. A couple of dozen ringing recoveries (from more than 800,000 ringed birds) show that southern willow warblers of the race trochilus winter in West Africa, while northern willow warblers of the race acredula winter in East and South Africa (Hedenström & Pettersson 1987, Bensch et al. 1999). Analyses of carbon and nitrogen isotopes in feathers from males holding territories showed great differences between southern (trochilus) and northern (acredula) willow warblers (Chamberlain et al. 2000). The isotope ratios also suggest that the migration barrier between the races is an area only about 100 km wide, close to latitude 62 degrees north, i.e. in Sweden through Hälsingland and Härjedalen. This is a surprisingly narrow overlap zone, considering the relatively great dispersing migratory patterns of the Willow Warbler.

 It is important to investigate a few fundamental questions before the method is put to use on a large scale. How great is the variation between individuals which have grown their feathers at the same place? How great differences can be found between places close to each other? Do species that use similar food also have similar isotope ratios? Can we find differences in isotope composition in the same individuals between years? Presently we are collecting feathers from moulting Willow Warblers in their African winter quarters in order to establish that the pattern we have observed in the breeding area really has a ground in Africa. Apart from samples from locally active ringers and expeditions, we also intend to use samples from museum collections. Material from museum specimens should be suitable since these isotopes are stable and remain unchanged in the feathers. We want to extend our investigation of these questions, and we have now decided to examine feathers from four bird species: Willow Warbler, Blue Tit, Bluethroat, and Common Swift.

The Common Swift

The Common Swift is a species with a very wide and relatively continuous distribution, and its feeding areas, both in breeding and wintering quarters, are relatively extensive. Its aerial plankton food can be expected to reflect average isotope ratios for these areas, and thus reflect relatively large-scale variations. The Common Swift has a somewhat complex wing moult, retaining the primaries the first year, and showing interrupted moult in some birds (De Roo 1966). Body feathers, however, are moulted in Africa each year, and can be safely sampled in Europe (Chantler & Driessens 1995). Body feathers from juvenile birds can be expected to reflect the isotope ratios of breeding areas, while body feathers from adult birds can be expected to reflect the isotope ratios of wintering areas.

 For the Common Swift, we wish to obtain feather samples from as many locations as possible, distributed around Europe and in Africa. Apart from extensive geographical coverage, we also hope to obtain samples from populations with different behavior, for example from swifts nesting in woodpecker holes, crevices in cliffs, and nest boxes placed in trees. We wish to get samples from four categories: 1) juvenile birds within the nest cavity, 2) nest seeking nonbreeding birds, 3) breeding birds, and 4) birds wintering in Africa (probably obtainable only from museum specimens). It is probably most easy to obtain feathers from juveniles, for example in connection with ringing, which then should be postponed until feathers are well developed (age 25-30 days). Nonbreeders can probably be netted as they are making fly-ins exploring potential nest sites (Kaiser 1992). One must be aware of the risks for nest desertion in the Common Swift; especially, adults which should not be disturbed within the nest cavity. However, the risk for desertion is small (the adults will probably return next year) if they are handled in the nest after the young have left (Perrins 1971). Feathers can be taken from dead birds even if they have been dead for a long time. However, feathers from swifts in Europe which cannot with certainty be related to a geographical area cannot be used (e.g. feathers obtained from birds in weather movements killed in car accidents or netted, or from newly fledged birds on migration, easily recognizable by their white 'face' and light fringes on the feathers).

Sampling procedure

Each sample should be collected by picking just one feather and 3-4 downs from a bird's rear back, and these should be put into an envelope (that will be supplied), marked with sample number, species, place, and name of collector. Feathers should be plucked out, not cut off, which will prompt the early replacement of the feathers, and the small amount of blood in the quill can be also used to extract DNA. The following information should be registered separately (on a list that will be supplied): name and address of collector, sample number, ring number (if the bird is ringed), date, name and coordinates of the location, age or category of the bird (juvenile, nest seeker, breeder), and a short description of the circumstances (e.g. nest in building, nest box in tree, netted close to colony, etc). The envelopes can be kept in room temperature until they are mailed to us. Optimally, we would be grateful to receive samples from five birds of each category at each location (15 samples per location) in Europe. In case you cannot achieve this goal, any less complete collection of samples will be welcomed and appreciated.

 We hope you want to participate as a feather collector! In case you have questions, please don't hesitate to ask. As a first step, please confirm your interest to participate by sending your name, address, telephone number, and e-mail address (in case you have one) to Susanne Åkesson, at the mail or e-mail address below. Please let us know if you also can help us with blood samples; we intend also to perform DNA-analyses. In good time before the coming breeding season, we will send you feather envelopes and lists for registration of the samples.

 A warm THANK YOU for your help in collecting swift feathers! You will be kept informed about the progress of the project, and you will be gratefully mentioned in the publications produced within the project.

With best regards,

Susanne Åkesson, Department of Animal Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden, Tel: +46 46 222 3614, Fax: +46 46 222 4716, E-mail: susanne.akesson@zooekol.lu.se, Bird Migration Ecology homepage: http://orn-lab.ekol.lu.se/birdmigration/ .

 Jan Holmgren, Rödhakevägen 23, SE-274 33 Skurup, Sweden, Tel: +46 411 42093, E-mail: j.holmgren@telia.com .

References

Bensch, S., Andersson, T. & Åkesson, S. 1999. Morphological and molecular variation across a migratory divide in willow warblers, Phylloscopus trochilus. Evolution 53:1925-1935.

 Chamberlain, C.P., Bensch, S., Feng, X., Åkesson, S., & Andersson, T. 2000. Stable isotopes examined across a migratory divide in Scandinavian willow warblers (Phylloscopus trochilus trochilus and Phylloscopus trochilus acredula) reflect their African winter quarters. Proc. R. Soc. Lond. B 267:43-48.

 Chamberlain, C.P., Blum, J.D., Holmes, R.T., Feng, X., Sherry, T.W. & Graves, G.R. 1997. The use of isotope tracers for identifying populations of migratory birds. Oecologica 109: 132-141.

 Chantler, P. & Driessens, G. 1995. Swifts: A Guide to the Swifts and Treeswifts of the World. Pica Press.

 De Roo, A. 1966. Age-characteristics in adult and subadult swifts, Apus a. apus (L.), based on interrupted and delayed wing-moult. Gerfaut 56:113-131.

 Hedenström, A. & Pettersson, J. 1987. Migration routes and wintering areas of willow warblers Phylloscopus trochilus (L.) ringed in Fennoscandia. Ornis Fennica 45:1-7.

 Hobson, K.A. & Wassenaar, L.I. 1997. Linking breeding and wintering grounds of neotropical migrant songbirds using stable hydrogen isotopic analysis of feathers. Oecologica 109:142-148.

 Kaiser, E. 1992. Population dynamics in a colony of common swifts Apus apus with special reference to nonbreeding birds. Vogelwelt 113:71-81.

 Perrins, C. 1971. Age of first breeding and adult survival rates in the Swift. Bird Study 18:61-70.

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