Measures of stable carbon and nitrogen isotopes ?derived from food ingested at the time the birds create feathers and preserved into the feathers ?give clues to the birds' diet. More nutritious food items, such as sardines, anchovies and squid, are distinguished in the feathers by higher levels of nitrogen isotopes. Less valuable food sources, including marine invertebrates such as krill, are signaled by greater levels of carbon isotopes.
"It takes about 70 to 80 krill to equal the energy value of one sardine," said Beissinger. "When murrelets have to rely upon less nutritious foods, they need to spend more time diving underwater to catch and eat them, and have less energy to make and lay eggs. The availability of food commonly affects the reproductive success of seabirds."
The researchers were able to determine the time of year that individual feathers were grown. They pointed out that murrelets undergo two molts of feathers each year, one right before the onset of the March through April breeding season, and another after their nesting period ends between August and September. Brown-tipped breast feathers are only lost in the pre-breeding period, while all-white breast feathers and wing feathers are lost in the post-breeding period.
The researchers were thus able to relate the diet of the murrelet to the environmental and fishing conditions at the time the feather was created. "In this paper, we're trying to tease apart climate signals from human fishing impacts," said Ben Becker, the study's lead author. Becker was a UC Berkeley Ph.D. student at the time of the study and is now director and marine ecologist at the Pacific Coast Science and Learning Center at the Point Reyes National Seashore.
The research team f
Source:University of California - Berkeley