The use of markers present within the tissues of migratory birds to analyse broad-scale movement patterns offers an alternative approach in species that yield few or no useful ring recoveries, and are too small to carry PTTs. In particular, analyses of stable isotopes (different forms of the same element) in bird tissues can provide information on the broad provenance of trapped migrants. Stable isotopes of several abundant elements, including hydrogen (H), carbon (C), strontium (Sr) and others, have spatial distributions that vary consistently either across broad geographical regions or between bird habitats and food types (Hobson 1999). For example, in North America, the ratio of hydrogen to its isotope deuterium (6D) in precipitation varies across the continent, from deuterium-enriched in the southeast to deuterium-depleted in the northwest. These patterns are transferred through food webs from plants to higher organisms. Birds absorb isotopes from their food and deposit them in body tissues, giving isotope signatures which reflect either the region where the food was eaten, or the habitat and type of food. Birds that move between regions or food webs can retain information of previous feeding locations for periods that depend on the turnover rates of particular isotopes in their body tissues. Keratinous tissues, such as feathers, are metabolic-ally inert following synthesis, and maintain an isotopic signature reflecting the food eaten at the time and place of their formation. Other tissues are metabolic-ally active, and retain their signatures for periods ranging from a few days (in the case of liver or blood plasma) to several weeks (in the case of muscle or whole blood), to the lifetime of the individual in the case of bone collagen (Hobson 2003). Isotope ratios also change as the chemicals concerned pass from prey to predator, upwards through food webs, because of differential loss through excretion and respiration, but these changes are known and can be allowed for.
In practical terms, by catching a bird and pulling a single feather, analysing by mass spectrometry its isotope signature, and comparing this with known geographical patterns in isotope ratios, it is possible to find (in very broad terms) where the bird grew its feathers. Even without a baseline reference, one can tell whether different breeding populations have their own distinct wintering areas, or whether wintering populations have their own distinct breeding areas. It is thus not necessary to re-capture birds, and the method is equally applicable to museum specimens.
Many birds moult in their breeding areas, and retain the feathers grown there for up to a year. In migration studies, linkages between breeding and wintering sites have been established using this approach. For example, in Black-throated Blue Warblers Dendroica caerulescens, 613C, 6D and 687Sr values in feathers varied systematically across the breeding range, while equivalent values from wintering sites in the Caribbean region indicated that the birds there had been drawn from northern parts of the breeding range. The 6D and 613C values among individuals from local wintering sites showed greater variation than those among individuals from local breeding sites, which implied that migrants sampled at each wintering locality were drawn from more than one part of the breeding range (Chamberlain et al. 1997). Even more strikingly, 6D signatures for five species of Neotropical migrants sampled at a single locality in Guatemala represented individuals from across the breeding ranges of these species (Hobson & Wassenaar 1997); and 6D signatures for Northern Bullfinches Pyrrhula p. pyrrhula obtained in Scotland in 2004 indicated that these irruptive migrants derived from a large part of the European boreal region, as far east as the Urals (Newton et al. 2006). In contrast, material from Wilson's Warblers Wilsonia pusilla showed isotopic evidence for 'leap-frog migration', where more northern birds migrate to wintering areas further south than those of southern breeders (Kelly et al. 2002).
In contrast to the above species, Eurasian Willow Warblers Phylloscopus trochilus moult in summer and winter, and analysis of carbon and nitrogen isotopes of winter-grown feathers plucked on European breeding areas confirmed that the two subspecies (Phylloscopus t. trochilus and P. t. acredula) found breeding in different parts of Sweden winter in different (west and east) parts of Africa (see Figure 22.2; Chamberlain et al. 2000). Similarly, stable isotope analysis of Barn Swallow Hirundo rustica feathers (grown in winter) has indicated that Swiss and English breeding birds probably winter in different parts of Africa (the 613C signatures being more depleted in Swiss birds, which indicates wintering in more wooded areas than the
English birds, Evans et al. 2003a). More surprisingly, analyses of Barn Swallows breeding in Denmark revealed that isotopes in feathers grown in winter quarters had a bimodal distribution, suggesting two different wintering areas (M0ller & Hobson 2004). The two types of birds also differed in phenotype, as did their offspring. Depending on the season of moult, different species are suited for summer or winter population differentiation, while a twice-yearly moult makes some species useful for study in both summer and winter areas. The same is true for species with split moults, which grow some of their feathers in breeding areas and others in wintering areas (Chapter 11).
Sometimes migrants from northern areas winter within the range of more southern birds which are resident. It is then hard to tell the relative proportions of migrants and residents in the same wintering area. Using measurements of deuterium (8D) and 613C values in feathers of Loggerhead Shrikes Lanius ludo-vicianus, it was established that northern breeders made up about 10% of the Florida population in winter, 4% of the Texas population and 8% of the Mexican population (Hobson & Wassenaar 2001). The differences in proportions between States were not statistically significant, but the figures showed that northern migrants formed only a small part of these lower-latitude wintering populations. This would have been difficult to establish reliably by ringing.
Although useful in identifying the regional origins of migrants, and filling gaps in other information, the method of isotope analysis cannot provide anything near the geographical resolution that is possible with other approaches, such as ringing or radio-tracking. The levels of deuterium (6D), which follow patterns in rainfall, are perhaps the most useful in studies of migratory birds: they give good latitudinal precision, but less good longitudinal precision (Hobson 2005). However, 615N and 613C values are of less value in this respect because drought conditions can enrich both, and natural regional variations in both are increasingly modified by human activities, such as fertilizer use and atmospheric pollution, reducing their value as geographical markers. Nevertheless, the method of isotope analysis provides better-than-nothing information for species with low recovery rates in the regions concerned, especially where analysis of several elements rather than one can give greater discrimination power.
Isotope analyses of soft tissues have also been used to address other questions, such as: (1) whether eggs were formed from food eaten in the immediate breeding area or from food imported to the breeding areas as body reserves accumulated in migration or wintering areas (Chapter 5; Hobson et al. 2000, Klaassen et al. 2001); (2) whether birds breeding in one region had accumulated body reserves on the same or different stopover sites (Atwell 2000, cited by Hobson 2003); and (3) whether particular individuals examined in a breeding area had spent the winter in good or poor habitat (knowledge which can then be related to migration and breeding performance, Marra et al. 1998). In addition, analyses of muscle have provided information on the changes in feeding areas and diets that occur during the course of a single migration (see Minami et al. 1995 for shearwaters migrating from the Southern Ocean to the North Pacific).
Analyses of trace elements in feathers have also been used to indicate the broad geographical origins of birds. This is possible because the proportions of different elements in feathers vary from region to region, according to geological substrate. Sand Martins Riparia riparia breeding in different parts of Europe differed markedly in the elemental composition of their tail feathers, indicating that the birds from different breeding areas had moulted in different parts of Africa. Moreover, tail feathers from the same individuals in different years were similar in elemental composition, implying that individuals were consistent in their moulting areas from year to year (Szep et al. 2003). Other studies of this type have involved Peregrine Falcons Falco peregrinus (Parrish et al. 1983), and various species of geese (Hanson & Jones 1976).
Some other bird populations drawn from the same species can sometimes be distinguished by their DNA. Using as a reference DNA samples from different breeding areas, birds sampled on migration or in wintering areas could be assigned to one or more of these areas (for Dunlin Calidris alpina see Tiedemann 1999, Wennerberg 2001, for passerines see Smith et al. 2005 for review see Wink 2006). However, the use of DNA markers is limited among birds because of the weak genetic differentiation found in many populations. This in itself results partly from the mobility of birds, and the consequent genetic exchange between populations in different breeding areas, and partly from their recent (postglacial) colonisation of northern regions, which has given too short a time for spatial genetic variation to arise (Chapter 22). In addition, DNA-based methods tend to be expensive and time-consuming.
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