Experimental variation in the spatial deposition of trace metals in feathers revealed using synchrotron X-ray fluorescence

Feathers can be used to investigate exposure to pollution in birds because they are a secondary route for the excretion of trace elements. Evidence based on analytical imaging and spectroscopy suggests that the spatial distribution of the essential trace element zinc within feathers is related to melanin pigmentation. However, our understanding of how trace elements are deposited into growing feathers is poor and has been hampered by a lack of analytical tools to examine the localization of trace elements within a feather. Here, synchrotron micro X-ray fluorescence spectroscopy was used to map zinc directly within the barb and barbules of lesser scaup (Aythya affinis) feathers grown after experimental increases in dietary zinc. The results showed distinct spatial variation in zinc within barbs and barbules, with higher levels observed in the latter. Furthermore, increases in dietary zinc were found to increase the relative levels of zinc throughout the barbules from the base to the tip of the feather. Finally, analysis of feather cross sections revealed that regions of the feather barb and barbules with higher melanosome density also contained higher levels of zinc. These results provide a more detailed understanding of zinc and melanosome arrangement within the feather barb and barbules. Moreover, these results provide further support for the use of feathers as a noninvasive tool to study exposure to trace elements and highlight the utility of X-ray spectroscopy in studies investigating impacts of a rapidly changing environment on wild bird health.