When nesting season rolls around, American coots (Fulica americana) transform into shameless parasites. Seemingly respectable hens sneak eggs into neighbors' nests in hopes of tricking them into raising their chicks. But the neighbors don't take this lying down. A new study suggests that coots can count their own eggs, helping them reject the donations of their freeloading fellows. Not everyone is convinced.
Biologist Bruce Lyon of the University of California, Santa Cruz, monitored eggs in 417 wild coot nests in British Columbia for 4 years. He found that nest parasitism among coots is both common and costly; parents lose one of their own chicks to starvation for every interloper they mistakenly accept. In response, the birds have developed a sophisticated defense. For starters, each hen lays eggs with a distinctive color and speckle pattern. Parasite eggs with the greatest color difference from the host's eggs were eventually buried deep in the nest, where they failed to hatch. Hens that accepted parasite eggs similar in appearance to their own often banished the interlopers to the nest's chilly periphery, as if hedging their bets. Chicks from these eggs still hatched, but they emerged later and were less likely to survive.
Coots can also avoid another pitfall of nest parasitism. Many birds produce smaller clutches when a parasite adds eggs to their nest. Researchers think this is because they use tactile cues--such as the feel of the surface area of eggs in the nest--to decide when to stop laying. Coots apparently use a different strategy. Birds that eventually rejected parasite eggs still laid an average-sized clutch of their own--even though the alien eggs remained at the top of the nest for several critical days before being buried. That means the coots can't be judging the total number of eggs based on feel alone, Lyon says. Instead, he argues in the 3 April issue of Nature, coots count their own eggs and ignore any others.
But something less cerebral could be going on, says avian nest parasitism expert Stephen Rothstein of the University of California, Santa Barbara. A coot detecting a parasitic egg or another hen near its nest, he says, may simply decide to ignore any environmental cues and lay its standard number of eggs.