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 food hypothesis," which holds that poor-quality food can hamper the reproductive success of marine predators just as badly as low-quantity food.

Henrik Österblom, 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-nutritious 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.

The guillemots aren't alone: recent experiments have shown that many marine fish-eaters, including Steller's sea lions and kittiwakes, either can't raise healthy young or can't maintain their own weight when fed low-energy food, however plentiful.

With colleagues, Österblom reviewed all the papers he could find on the subject and concluded that the junk food hypothesis could explain, at least in part, recent cases of breeding failure among northern marine predators.

The research was detailed in the journal Oikos.