The use of stable-isotope ratios in ornithology

The use of mass spectrometry to analyse the stable-isotope ratios of bird tissues has become an important new tool for research ornithologists in the last 20 years. Because stable isotopes vary geographically and according to specific biological processes in the environment, they provide a unique forensic means of understanding more about avian biology and ecology than we can learn using conventional techniques alone.The stable-isotope ratios present in different tissues of birds reflect the ratios in the environment at the time those tissues were constructed. However, because of the rapid turnover of some tissues compared with others, an individual bird will bear within its body constituents a record of its present and past exposure to different isotopic environments. Because the stable-isotope ratios of specific elements vary geographically (e.g. hydrogen along oceanic to continental gradients) and between habitats (e.g. nitrogen and carbon in marine versus terrestrial ecosystems), they offer a unique means of studying the ways migratory birds move between different parts of the planet and of understanding the habitats they exploit.This review looks at some of the innovative ways in which the technique has been used in a variety of recent studies of birds.The technique is not restricted to breaking new ground in high science; in terms of understanding migration strategies, the importance of breeding, migratory and wintering habitat and the feeding ecology of birds, the study of stable-isotope ratios is becoming ever more important in supporting conservation actions. to occur in one or more forms, called isotopes. These isotopes differ in the number of neutrons present in each atom; the more neutrons, the heavier the atom and the greater the atomic mass. Stable isotopes persist over prolonged periods of time – unlike radioactive ones, which are unstable and decay rapidly so that their ratios in the environment change more quickly over relatively short time periods. The important point here is that the ratios of many stable isotopes of common elements in the environment reflect well-defined natural processes which often differ in time and space. For example, hydrogen is an abundant element that has two stable isotopes, the normal ‘light’ form (conventionally denoted as 1H, because it has one proton and no neutrons in its atom, so there is just one subatomic particle in the nucleus) and the ‘heavy’ form (also known as deuterium, and denoted 2H because it has two particles in its nucleus, one proton and one neutron). Deuterium is relatively rare in nature – sea water contains one deuterium atom to approximately 6,500 light atoms of hydrogen. Nevertheless, despite the fact that deuterium is always much the rarer member of the pair, the actual ratio of these two stable isotopes varies substantially around the globe, for instance with ambient temperature. It is this type of variation in stable-isotope ratios that offers such a remarkable range of possibilities, many of which are discussed below, which we can use to improve our understanding of the natural world around us. The atomic mass of the hydrogen responsible for forming water makes a difference to its physical properties. ‘Heavy water’ is formed from oxygen combined with the isotope deuterium (atomic mass 2) and is denser than ‘light’ (ordinary) water, which is formed from oxygen and hydrogen (atomic mass 1). Heavy water tends to be precipitated out of clouds before light water, so as moist air travels inland, the amount of heavy water is increasingly depleted. This means that highly continental areas (such as the middle of the Russian Arctic) receive much lower levels of deuterium in precipitation compared with Europe’s western seaboard, which experiences a much higher relative proportion of heavy water in precipitation. A similar process happens with increasing altitude, meaning that in mountainous areas, we can also expect deuterium to be less abundant in precipitation than in comparable lowland areas. All this gives us good reason to believe that 113 British Birds 101 • March 2008 • 112–130 The use of stable-isotope ratios in ornithology Box 1. Measuring stable-isotope ratios using mass spectrometry Stable-isotope ratios are measured using a mass spectrometer. This equipment measures the ratio of the electric charge of a particle in relation to its mass. This is achieved thanks to Newton’s second law of motion, namely that the acceleration of a particle is inversely related to its mass. The mass spectrometer vaporises the sample and converts the constituent parts into ions, charged particles that are then accelerated from a chamber and concentrated into a narrow beam which is subjected to a strong magnetic field. Because each isotope atom has a unique combination of mass and charge when ionised, the magnetic field bends the stream of particles to a different degree dependent on these two characteristics. For instance, light hydrogen will be deflected less by the magnetic field than heavy hydrogen, so the stream of concentrated ions comprising a mixture of these two isotopes will separate in the chamber and arrive at two different points in the detection part of the apparatus, dependent on their mass. Detectors can be positioned in the detector chamber, according to the degree to which the ion stream of a specific element of specific mass and charge would be expected to be deflected. To determine the relative abundance of one isotope to another, the machine measures the relative charge arriving at the different positions expected for each isotope. For hydrogen and many common elements, this means collecting the electric current from two streams originating from the two different isotopic forms, but for some elements with more isotopes, more detectors are required (see below). Although the theory is straightforward, in reality the differences in mass between isotopes of the same element are typically incredibly small and the less abundant isotopes in a sample usually very rare, so modern mass spectrometers have to be extremely sensitive to obtain accurate measurements. The wider application of stable-isotope research in ornithology in recent years has undoubtedly been due to both the increased availability of such machines in universities and research establishments throughout the world and the technological developments allowing rapid throughput of large numbers of samples. the stable-isotope ratios of hydrogen in rainwater (and hence in groundwater) show some predictable geographical patterns – but how does this affect birds? All biological activity relies on water, and living organisms not only take up water from their environment but are totally dependent on it for maintaining all of their cellular activity. Hence, the hydrogen stableisotope ratio of the bodies of all living things will reflect that of the environment in which that organism grew up. For example, Bullfinches Pyrrhula pyrrhula in Britain (subspecies pileata) and Denmark (subspecies coccinea) have hydrogen stable isotopes in their feathers that are characteristic of the oceanic groundwaters of western Europe, whereas those of the large nominate subspecies pyrrhula, originating from northern Scandinavia and continental Russia, show more depleted levels of heavy hydrogen in their feathers (see fig. 1 and Newton et al. 2006). The stable-isotope ratio of oxygen shows similar patterns, so the ratio of 18O/16O (the two commonly occurring natural isotopes of oxygen) shows very strong gradients in groundwater that correlate with temperature and oceanic–continental and upland– lowland gradients similar to those of hydrogen. 114 British Birds 101 • March 2008 • 112–130 The use of stable-isotope ratios in ornithology 66. Female Northern Bullfinch Pyrrhula pyrrhula pyrrhula, Sweden, November 2004. M ar ku s V ar es vu o Fig. 1. Mean (± standard errors) hydrogen stable-isotope determination ratios of feather samples taken from Bullfinches Pyrrhula pyrrhula caught in Nimtofte, eastern Denmark. Birds were either non-breeding migrants from Scandinavia and Russia (subspecies pyrrhula, shown by diamond symbols) or resident, locally bred birds (subspecies coccinea, shown by triangle symbols); filled symbols indicate females, open symbols indicate males, for both samples.The data are those presented in Newton et al. (2006). Given the hydrogen stable-isotope gradients in groundwater, the predicted range of values in feathers grown in Denmark and Scandinavia/Russia are shown as shaded areas for the two regions as indicated, based on contour maps in Hobson et al. (2004a) (see also fig. 3). -80