Do Deep Diving Seabirds Have to Equalize Like Humans Do?

Question: How do seabirds dive so far underwater? Do they have to equalize like humans do?”—Sioned S., Devon, U.K.

Answer: Some aspects of seabird diving physiology are actually pretty similar to how humans equalize, but there are some important differences as well.

Before we get into how this works in seabirds, let’s make sure everyone understands what really happens at a physiological level when human divers equalize. Basically, the goal of equalizing is to make the pressure inside your ear equal to the pressure of the surrounding environment, which changes rapidly as you descend. What we commonly call “equalizing,” holding your nose tight and gently blowing through it, opens your eustachian tubes, allowing pressure inside your middle ear to become more similar to pressure outside.

So what about seabirds? Some seabirds can dive much deeper than humans. Emperor penguins can reach depths up to more than a quarter mile. When seabirds ascend and return to the surface, they tend to do so much, much faster than humans are supposed to, according to Dr. Kyle Elliott, an assistant professor and seabird expert at McGill University who was kind enough to answer my questions for this piece.

“As birds ascend from depth and air volume enlarges, they become more buoyant, leading to acceleration near the surface like a 'runaway ascent' in a scuba diver whose BCD is overinflated,” Elliott tells me. “To prevent such runaway ascents, many birds ascend at an angle as they near the surface and force out air trapped in the plumage, which helps them reduce the risk of decompression sickness by slowing the rate of ascent.”

Birds also breathe at the surface and then descend, which poses different physiological challenges than breathing compressed air at depth, making them more similar to a freediver than a scuba diver. However, Elliott says some birds still build up nitrogen levels in their blood that could be high enough to cause bubbles in the bloodstream, and scientists are not entirely sure why this doesn’t happen more.

“Marine mammals avoid decompression sickness by having collapsible lungs and small respiratory air stores (they store most of their oxygen in the blood or muscles),” Elliott says. “In contrast, birds have noncollapsible lungs and large respiratory air stores, which makes this enigma all the more mysterious. The best guess at the moment is that birds have shunts in their circulatory system that allows the blood more readily to mix with the muscles and organs, preventing nitrogen levels from getting too high by rapidly moving blood around to reduce nitrogen buildup; these shunts also explain how penguins can withstand levels of oxygen so slow that they would make us faint by controlling blood flow to essential organs. Seabirds deal with equalization in a somewhat similar manner: They have blood vessels around the ear, called the corpus cavernosum, which contract as the bird dives, reducing the air space in the ears and equalizing pressure.”

So how can seabirds dive so deep? They have some physiological adaptations, including shunts in their circulatory system and blood vessels around their ears, which explains some of it. But some of this remains a fascinating scientific mystery!

Ask a Marine Biologist is a monthly column where Dr. David Shiffman answers your questions about the underwater world. Topics are chosen from reader-submitted queries as well as data from common internet searches. If you have a question you’d like answered in a future Ask a Marine Biologist column, or if you have a question about the answer given in this column, email Shiffman at WhySharksMatter@gmail.com with subject line “Ask a marine biologist.”

Dr. David Shiffman is a marine conservation biologist specializing in the ecology and conservation of sharks. An award-winning public science educator, David has spoken to thousands of people around the world about marine biology and conservation and has bylines with the Washington Post, Scientific American, New Scientist, Gizmodo and more. Follow him on Twitter, Facebook and Instagram, where he’s always happy to answer any questions about sharks.

The views expressed in this article are those of David Shiffman, and not necessarily the views of Sport Diver or Scuba Diving magazines.