Shearwaters are migratory marine birds that travel in a figure-of-eight pattern between the coasts of Siberia and Japan to Tasmania.
Placing one of the undistinguished grey feathers from a shearwater into the brilliant light of the X-ray fluorescence microscopy beam reveals something unexpected. We see intricately patterned deposits of chemical elements that tell the story of how a feather grows.
Among other findings, the images show strikingly regular bands containing zinc. There are roughly the same number of bands as the estimated number of days of feather growth.
Different from simple feather growth bars, these patterns were not known before our study, published this month.
Like the annual growth rings of trees, birds’ feathers lay down growth bars during their moult. (Moulting is the process of shedding old feathers, making way for new ones to grow.)
While bars simply show growth, the patterns of chemical elements tell us about the bird’s life during the growth period of the feather. They can indicate environmental exposures in a bird population, perhaps before impacts such as illness and death are clear.
We think the zinc banding may be a natural diurnal (daily) time stamp locked up within the feather. If confirmed, it’s a finding that may be applicable for retrospective dating of the occurrence of stressful events – for example, the temporary exposure to environmental contaminants such as heavy metals – during the period when birds grow new feathers.
In addition to zinc, other elements detected in feathers include calcium, bromine, copper and iron, each with its own unique pattern of distribution.
The team, including field researcher Jennifer Lavers, analysed feathers painstakingly sampled from remote locations in Japan, as well as Lord Howe Island and New South Wales, and complemented by feathers from the Australia Museum collection in Sydney.
The bulk of the work investigated feathers from three species of shearwaters who migrate more than 60,000km over open ocean each year, to and from their breeding areas.
Foraging across huge areas, shearwaters are important indicators of environmental health. As described by Nobel Laureate and author Peter Doherty in his book Sentinel Chickens: