Researchers study egg incubation - BERKELEY, Calif. - A new study by University of California at Berkeley scientists on the risk of microbial infection on unincubated eggs may shed light on why some birds start incubating their eggs early, before the whole clutch has been laid.

According to UC Berkeley Chair of Environmental Sciences, Policy and Management Steven Beissinger, the study's principal investigator, by incubating earlier-laid eggs first, parent birds may help prevent microbes from penetrating the egg shell and invading the developing embryo.

"In the egg white there are a lot of antibiotic properties but they don't become active until they're warmed enough for the antibiotic reactions to become catalyzed," says Beissinger. "Incubation helps sterilize the egg."

The question of why some bird species begin incubating earlier-laid eggs first, causing the eggs within a clutch to hatch at different times, has perplexed scientists in the past.

"If they start incubating before all the eggs are laid, some of the eggs will hatch ahead of the others and the chicks that hatch later will be handicapped because they're smaller and can't compete with bigger siblings," Beissinger says. "They oftentimes starve to death, get trampled or die from other accidents."

Beissinger worked with postdoctoral researcher Mark Cook and scientists from the U.S. Forest Service and the University of Puerto Rico studying how climate and microbe presence impact the shelf-lives of eggs that are not incubated.

To investigate the impact of temperature on egg viability, the team performed an experiment in a Puerto Rican forest using freshly laid eggs from the native Pearly-eyed Thrasher.

Half of the eggs were moved to a cool cloud at the top of a mountain in the forest, where cold temperatures would force them to enter suspended animation, and the other half was moved to the bottom of the mountain, within temperature ranges that would cause abnormal development.

When the eggs were brought back to be hatched, however, eggs placed at the top of the mountain failed as often as eggs placed at the bottom.

"We began to think that there was another mechanism involved and we thought that mechanism might be bacteria and fungi which might be getting into the eggs," Beissinger says.

The scientists decided on another experiment to test whether microbes could penetrate across the shell to the inside of the egg and whether this would affect hatching success.

Fresh chicken eggs were placed at both the top and bottom of the mountain for seven days. The eggs were then brought back to the lab to be tested for microbes.

"We were surprised at how frequently they (microbes) were able to get in," Beissinger says. "By five days the microbes had reached the yolk and continued to grow."

The team then placed another set of eggs at the top and bottom of the mountain but cleaned half of the number placed at each location. After five days the eggs were brought back and placed in incubators to hatch.

"We got really clear results," Beissinger says. "Eggs held in the cool cloud forest and cleaned had very high hatching success because embryos were maintaining viability at the cool temperatures." Those that were not cleaned were invaded by microbes.

The research results suggest that by incubating earlier-laid eggs first, parents birds activate the antibiotics in the egg white that help reduce the risk of microbe infection. In turn, chicks from eggs incubated later are put at a disadvantage, in a sort of evolutionary compromise.

So perhaps what we're seeing is that the incubation patterns of some birds may be shaped by the need to keep microbes from overwhelming the embryos, Beissinger says.

Source: M2 Communications Ltd - 3rd December 2003