In the Baltic Sea, birds called common guillemots raise their young on herringlike fish called sprat. In the 1990s, local sprat became unusually abundant after populations of their main predator, cod, plunged because of overfishing and climatic changes.
Yet during that time, guillemot chicks grew poorly. Why?
The answer may lie in the "junk foodÂ hypothesis," which holds that poor-qualityÂ food can hamper the reproductive success of marine predators just as badly as low-quantityÂ food.
Henrik Ã–sterblom, the biologist from the Baltic Nest Institute at the University of Stockholm who studied the guillemots, noted that sprat were leaner when they were abundant and had to compete for limited supplies of zooplankton. The lean sprat made less-nutritiousÂ meals for the guillemot chicks. The chicks' parents tried to compensate by bringing home more sprats, but because they catch and carry just a single fish at a time, it was hard to keep up.