Marbled Murrelets Forced to Change Their Diet

Marbled murrelets may be having difficulty finding enough food for producing eggs because commercial fisheries have depleted the birds' critical food supplies.

The nutritious sardines and anchovies that the endangered seabirds in central California used to mainly eat have become scarce, forcing the birds to rely more on prey that's less concentrated in energy.

In the last century, the murrelets increased their krill intake by 26 percent, while eating 42 percent less of sardines and anchovies during the weeks before breeding. These results are for years when ocean temperatures were relatively cool. Similarly though, during warm years murrelets ate 23 percent more krill while prey from higher up the food chain declined by 23 percent.

The birds need to eat 80 krill to get the same amount of energy that's supplied by one sardine. California's sardine fishery collapsed due to overharvesting and ocean cooling in the late 1940s and has not recovered. Other energy-rich prey such as anchovies have also declined markedly in recent decades.

This study unveils an example of overfishing forcing an animal to feed at a less than optimal trophic level. The consequences of a collapsed fish stock can be far-reaching, as in this case involving an imperilled seabird that nests in old-growth forests.

Eating at a lower trophic level could be costly for murrelets. It takes a hearty diet for females to lay their single huge egg every year, since it amounts to one-quarter of their body weight. A shortage of rich food may in part be why each year 50 to 90 percent of marbled murrelets in central California don't breed.

Scientists deduced the murrelets' diets by measuring the concentrations of nitrogen and carbon isotopes in bird feathers. Some feathers were taken from birds in central California, at the southern end of their range, between 1998 and 2002. These were compared with feathers from murrelets in museums that were collected from the same region between 1895 and 1911.