Day 30: Bird moms thwart bad bacteria on eggs

Pearly-eyed Thrasher (Margarops fuscatus) in Puerto Rico. Credit: Lee Karney/U.S. Fish and Wildlife Service/Wikimedia Commons

It’s fascinating enough that human moms pass immunity to their kids by nursing. But as it turns out, even mothers of the egg-laying variety have ways to stop nasty pathogens from harming their kids. A new study in the open access journal PLoS ONE reveals new insights about the anti-bacterial methods of the pearly-eyed thrasher, a secretive, scrappy dweller of mountain forests and coffee plantations in central American islands.

Matthew Shawkey, formerly an ornithologist at the University of California at Berkeley (he’s now at the University of Akron), and his colleagues took a look at the amounts and types of bacteria on the birds’ eggs just after laying and 12 days later. Their study eggs were divided into two groups:  those that had been incubated, and those that had not. Incubation, it turns out, keeps bacterial populations in check that might otherwise violate the shell and harm the developing embryo inside. 

Nice peepers, pearly-eyed thrasher! Credit: Ellen Muller, 2001, on BonaireTalk.com

Incubation now joins the lineup of several cool animal strategies for inhibiting harmful bacterial growth. Past studies have revealed that the contents of both bird and frog eggs contain anti-bacterial compounds. Fish apply antimicrobial peptides to their eggs, and crustaceans are known to boost the growth of fungi-attacking bacteria on theirs. Some birds’ preen glands harbor symbiotic bacteria that help fight off the disease-causing kinds. 

A 2005 study led by Berkeley’s Mark Cook had proposed the incubation of birds’ eggs  specifically limited infectious bacteria. But the authors of the newer study say the 2005 methodology was limiting.

“Cook et al. did not address the effects of incubation on complete microbial assemblages because they used standard culture-based microbiological methods, which identify less than 1 percent of environmental microbes,” they wrote.

The new study, which explored a much broader assemblage of bacteria, reports that incubation “appeared to inhibit not only harmful bacteria but also all other bacteria that grew on unincubated eggs.” 

The way this works remains unclear, especially because incubation keeps egg temperatures at a sweet spot for bacterial growth, roughly 93 to 98.6 degrees F (34–37 degrees C). Keeping the eggs dry might be one benefit; the authors noted that unincubated eggs were wetter, which could turn the eggs into fertile ground for the growth of hardy and destructive pathogens. 

“Many other mechanisms, including turning of eggs by parents, the physical abrasion or loss of oxygen potentially caused by incubation could also explain our results,” the authors wrote. “Identifying the relative importance of these and others will be fertile ground for future research.”

The complete PLoS study appears here, and I first saw it on Blog Around the Clock.