Incubating Bird Eggs Is More Complex Than You Think

If you've ever seen a bird nudge its eggs with its beak, you may have wondered what all the fuss is about. Turns out, the behavior is a critical part of incubation, and each species may have its own egg-turning recipe to hatch a healthy chick, a new study shows.

Scientists believe that most birds rotate their eggs to ensure that the embryo gets enough albumen—the mixture of water and protein that makes up the "egg white" part of an embryo and provides nutrients to the developing chick. Too little albumen leads to an underdeveloped and usually sickly chick, research on domesticated birds shows. Few studies have investigated egg turning in wild birds, in no small part because the adults are, naturally, in the way. "Albatrosses shuffle their feet in the nest and it's hard to see what's going on," says Scott Shaffer, a biologist from San José State University in California.

To get an unprecedented look at what's happening in the nest, Shaffer tricked seabirds into treating plastic eggs with sensors hidden inside as the real thing. Researchers have previously slipped artificial eggs outfitted with sensors into the nests of unsuspecting birds, but Shaffer's team is the first to capture "a full turning" of an egg. That's because the loggers record 3D orientation (thanks to a combination of three-axis accelerometers and magnetometers), as well as temperature.

Shaffer and his team snuck the artificial eggs into the nests of three seabird species. They slipped 17 into the nests of laysan albatrosses at Kaena Point, Oahu, nestled another 17 among the eggs of western gulls on California's Año Nuevo Island, and put 35 into Cassin's auklets burrows on California's Southeast Farallon Island.

The loggers, which continuously recorded data for up to seven days, revealed several surprises. The birds turned their eggs at different rates, though the median was twice per hour. Unexpectedly, they all kept up the rotation throughout the night; Shaffer's team had assumed that only the auklet, known to be nocturnal, would be active after sundown. Perhaps more surprising was how they rotated the eggs. The greatest movement wasn't rolling them on their sides, as you might expect, but rather shuffling them from side to side around the nest. This approach likely allows the birds turn the eggs with their feet instead of their beaks, so they don't have to get up and risk the egg losing heat or being exposed to predators.

[video:215996|caption:Animation credit to Scott Shaffer, San José State University]

Next up, Shaffer wants to compare related species at different locations, like the albatrosses in Hawaii and their sub-Antarctic cousins, "to see if habitat temperature influences turning rates, egg attendance, and egg temperatures." He also hopes the egg loggers will help unravel other mysteries, like how disturbances from an adult being flushed off the nest affect the developing chick. Whatever he finds, one thing is clear: It takes a lot of work to keep an egg sitting pretty.