Chirpy Chirpy Cheep Cheep: Geochemistry for the Birds

Angelina Souren, SmarterScience, Portsmouth, UK.

In this age of bird flu and global change, pinpointing bird migration routes and monitoring any changes in them has become of considerable interest. In the past, all sorts of methods have been used as tools in this type of research, but Laura Font may have found a new and better way.

Dr. Laura Font is currently based at the Faculty of Earth and Life Sciences at the Vrije Universiteit in Amsterdam, where she is a Marie Curie post-doctoral fellow in the Petrology Department. She started her project in February 2008. Prior to that, Laura was employed at Durham University, which she joined in 2004 on a NERC fellowship. She graduated from the Universitat de Barcelona in 1997, then worked in France, obtained her Ph.D. from the University of Southampton in the U.K., and moved to Italy as a post-doctoral fellow before returning to the U.K.

In Durham, Laura's research involved measuring Sr and Pb isotopes in plagioclase from volcanic samples. It was at Durham where Laura started developing her method to use Sr and Pb isotopes in bird feathers to track bird migration routes.

The problem with (previous) isotope methods is that their discriminatory power is relatively limited, and fractionation mechanisms in birds complicate matters in some cases.

Previously, other isotopes (H, C and O) as well as certain trace elements have been used to infer migration routes. Sulfur isotopes in combination with hydrogen isotopes were also tried, in an attempt to distinguish between birds foraging in the marine realm and terrestrial feeders. The problem with such methods is that their discriminatory power is relatively limited, and fractionation mechanisms in birds complicate matters in some cases. Other methods such as the use of tags or leg bands and the counting of birds at specific locations are labor-intensive and do not carry an overwhelming success rate. In addition, censuses do not provide any data on origin.

Using feathers to obtain information about bird migration is relatively easy and its impact on birds is minimal. While many bird feathers do have a blood supply when still attached to the bird, these veins do not run along the entire shaft. A large part of a feather could therefore be clipped off without impacting a bird's flight at all. However, this process is often not even necessary as birds naturally lose enough feathers that can simply be collected on-site. To use the composition of bird feathers as a much less invasive and less labor-intensive method is therefore quite promising.

Sr isotopes do not appear to fractionate in birds and therefore reflect a bird's natural environment

Previous studies involving bones, claws and shells had already shown that Sr isotopes do not appear to fractionate in birds and therefore reflect a bird's natural environment (there are geological situations involving preferential release of Sr during weathering that require special care, but for most geological samples, the relationship is also quite straightforward). However, what is seen most notably in bones tends to reflect the bird's environment over its entire lifetime--birds are relatively long-lived. Bird migration research wants to acquire data on the bird's environment at, for example, prior to and after migration (i.e. much shorter time-scales). Feathers represent such specific points or shorter periods because they periodically renew their feathers. Bird feathers consist largely of keratin, like human hair and finger nails, and reflect what the organism has consumed. The analysis of Sr isotopes in bird feathers is not a new idea in itself but Dr. Font's method is a different story.

First of all, a method had to be devised to remove all dust from the feathers as this dust would constitute serious sample contamination. Tests were carried out in which a bird's dust bath was simulated by sweeping feathers through a sandy soil, followed by various cleaning methods and SEM imaging to inspect the feathers' cleanliness. Using several blasts of nitrogen gas turned out to be the best way to remove both dust and wax deposits.

The next problem is that the Sr content in most feather samples is incredibly low. Multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) is not able to measure quantities as low as < 12 ng Sr because of Kr interferences (Kr is present in the Ar plasma). Dr. Font's new method uses ultra-low blank ion-exchange chemistry followed by thermal ionization mass spectrometry (TIMS). Sample preparation is necessary to preconcentrate the Sr and to remove as much organic material as possible from the sample. The organics would interfere with the column chemistry and if you load a "dirty" sample on a TIMS Re filament, you are not going to get good results (if any at all).

To optimize ionisation, the feather-derived samples are loaded onto a very small area of the filament by running a 1-A current through it and melting two strips of parafilm onto the filament to define the loading area. After loading a mix of sample and TaF5 activator, the parafilm is burned off with a 2-A current. At the end, more activator is loaded on top of the sample to seal the sample onto the filament and prevent any sample loss.

Dr. Font is now developing a new method for the analysis of Pb isotopes in bird feathers that will use the same fraction as used for the Sr analysis (collected from the columns after the Sr fraction). In 2007, feathers had already been collected at various sites in Europe and Africa for the new project. In 2008, Dr. Font collected feathers from Pied Flycatchers and Collared Flycatchers in Gotland in Sweden as well as soil, water, bird prey and feces samples. Hopefully this method with continue to reveal new exciting behaviors that the Sr method did before it. In the end, it's nice to see work that oozes so much elegance and thoroughness in all its details, not to mention an innovative and exploring spirit.

For more details on this method, see Font et al. (2007) Sr isotope analysis of bird feathers by TIMS: a tool to trace bird migration paths and breeding sites. Journal of Analytical Atomic Spectrometry, 22, 513-522.