The chemistry between parent and chick: stable isotopes show the origin of egg nutrients in arctic breeding birds

Birds produce thier eggs from recently ingested nutrients ('income breeders') or from boby stores ('capital breeders'). Since arctic summers are short, ecologists have long thought that arctic migrants need to bring the nutrients for egg production from afar, to be able to start breeding immediately upon arrival. For many arctic breeding birds the wentering grounds should therefore not only provide the essential nutrients for migratory preparation but also for breeding. However, using carbon stable isotope ratios of eggs, natal down, and juvenile and adult feathers it was demonstrated that wader eggs are produced from nutrients originating from lundra habitats. Thus, for these arctic breeding waders the fitness costs of transporting extra body stores to the breeding grounds outweigh the potential benefits. However, the capital strategy may still be used by large species of arctic breeding migrants, such as geese. It is likely that larger species need relatively smaller body stores for egg production. Also, the larger the species, the longer it needs to complete breeding. Using the same methodology of carbon stable isotope ratios, tentative data on Lesser Snow Geese show that they also use a capital breeding strategy, but that this may vary from year to year. Thus, the many arctic breeding waders inhabiting estuaries and wetlands around the globe, only use these areas for migratory preparation whereas the larger waterfowl may be storing additional fuel there for breeding. Besides carbon-isotope ratios also isotopes ratios of other elements may provide information on diet, but possibly also on the geographic location of the site where a feather or other body tissue was produced. The use of stable isotopes may thus assist in ringing research and additionally opens up an avenue for new tantalising research.