A new study says that seabirds experience a precipitous drop in birth rates when fish supplies dip beneath one-third of maximum levels, a finding that could provide critical insight into how overfishing imperils numerous bird species. In an analysis of research conducted on 14 bird species — from seagulls to penguins — in seven different ecosystems worldwide, an international team of scientists found that over long periods of time the ecosystems consistently followed the same basic law: When the amount of prey fish falls beneath that critical tipping point, the birds produce fewer offspring. The researchers selected only seabirds that feed on sardines, anchovies, herrings and other small fish targeted by fishermen and currently under threat. Those small fish, which are increasingly used to make meal and oil for fish farming, comprise about 30 percent of the global catch. The study, coordinated by Philippe Cury of the University of British Columbia, was published in the journal Science.