The High Efficiency of Avian Lungs

Photo by: Rick Lewis
This Great Blue Heron always has fresh air entering its lungs, regardless if it’s inhaling or exhaling, because the air is streamlined and stored in multiple air sacs. Photo by: Rick Lewis

Can you remember the last time you thought about your lungs?

For me, most days pass without giving a pause of consideration to them. That is, until I recently visited one of our restoration sites at Eden Landing Ecological Reserve and saw a murmuration of American Avocets smoothly gliding above the marsh. Their comfort and ease navigating the sky renewed my awe for our feathered friends’ coveted gift of flight. Beyond all of the physiological reasons I can’t make it on my own into the sky, just the idea of flying has me winded.

Birds have adapted to experience breath very differently than us. Like other living beings, they rely on good ole air to stay kicking. But what’s special about the avian species is a highly efficient ability to get the most out of each breath.

Ok, take a deep breath in. I’m about to get technical.

Let’s start with us. Human (mammalian) lungs are bidirectional. This means that the pathway for air to enter our lungs is the same pathway used to leave our lungs. Picture our lungs working like balloons that inflate and deflate.  In contrast, the avian species have unidirectional flow, made possible by separate entry and exit points to the lungs. Rather than a balloon, picture a bird’s lung as a windsock that has air continually flowing through in one direction.

Let’s take a Great Blue Heron for example. When the bird breathes in, the inhaled air passes down the throat just like it does for us, but right before it reaches the lungs, it splits. Some of the air enters the lung, where the oxygen is absorbed, and some of the air enters multiple adjacent cavities called air sacs.  These air sacs do not absorb the oxygen and function as storage tanks for the air. When the Great Blue Heron exhales, the old air in the lungs is released into different air sacs and eventually out through the beak and nostrils, and the fresh air stored in the first air sacs enters the lung. In this way, the Great Blue Heron always has fresh air entering its lungs, regardless if it’s inhaling or exhaling.

And here’s another twist, birds don’t have diaphragms.  So to push all this air around they rely on the simple physics of air pressure. The multiple air sacs (up to nine in some species!) exist to offer changes in air pressure, which moves air into the lungs at regulated frequencies that maximize oxygen absorption. Relative to their body mass, birds do not breathe as fast as we do.  However, this respiratory system allows birds to have consistent oxygen absorption.  It’s this unwavering air flow that contributes to their ability to sustain the high energy costs of flying.

Now breathe out. How do your lungs feel?

To learn more about avian respiratory systems check out this site.

San Francisco Bay is home to many bird species and offers habitat to migratory birds making their way along the west coast on the Pacific Flyway. Watch this video featuring the Birds of San Francisco Bay.


Surveying Birds at Eden Landing Ecological Reserve

The freeways might be less crowded at sunrise, but our restoration sites aren’t.

Early morning and late evening are typically the busiest times of day for bird activity. That’s why I’ve been heading out to Eden Landing Ecological Reserve (ELER) at dawn once a week since starting to work with Save The Bay’s restoration team. Save The Bay volunteers and staff have planted thousands of plants in an effort to give native plants the advantage in colonizing this newly reclaimed land. Now it’s time to take a preliminary look at how this habitat is being used—specifically, how birds are using the sites in which Save The Bay has begun work, compared with unrestored sites.

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Bird surveys involve observing a certain area for a specified amount of time. Count the birds in this area, observe behavior, note interactions with habitat and other organisms, and take down some details about weather conditions. Translation: soak in the beauty of Eden for 20 un-interruptible minutes at a time. While I marvel in the artistic contrast of dark water-worn mud channels against bright dawn-lit clouds and creeping tidewater, the egrets and sandpipers read the landscape in a concentrated search for food.  With hundreds of shorebirds calling all around me plus the occasional curious jackrabbit, focusing on just the survey area can be a challenge! But my immediate reward might come in the form of catching a Song Sparrow sneaking around with a large beakful of invertebrates, a couple of gulls bullying an American Avocet at the edge of the water, or a House Finch pair pulling silky nesting material from a seeding thistle. I surveyed birds at 4 sites at ELER: 2 sites in which restoration planting is well underway, one future site now just bare ground, and one overrun with invasive plants. All of my survey sites were located in the transition zone, the ground between the highest tide line and the upland, where Save The Bay does restoration work.

There is a lot more to learn, many more questions to ask, but a look at the results of this preliminary study is exciting. I definitely notice more bird activity in the vegetated sites. Songs sparrows pop in and out of the marsh gumplant volunteers planted in the ground two years ago. White-crowned Sparrows line up along the fence bordering the restoration site. A Yellowlegs might wander along between water and vegetation looking for breakfast. Zipping above the plants, Barn Swallows and a lone Black Phoebe catch insects mid-air. In fact, I saw an average of 5-6 birds per survey in the vegetated sites but only 1.5 birds per survey in the un-vegetated site.

Bare soil, by contrast, is less hospitable for many transition zone species.  Song Sparrows, for example, need shrubs and grasses in which to build their nests, and the seeds and fruits of plants comprise a large part of their diet. The transition zone is also an important refuge for small mammals and birds during storms and the highest tides. The site I surveyed without vegetation only turned up 7 species during 6 surveys, most of them aerial foragers who flew over the site.

Species diversity appears to be greater on both of the restored sites compared to the area with mostly invasive plants and the un-vegetated area. I saw total of 11-12 different species of birds in the locations where restoration work has begun, compared with only 5-7 species in the unrestored sites.

Peaceful mornings monitoring birds are interspersed with busy volunteer workdays in my schedule at Save The Bay. As the community comes together to improve tidal marsh transition zone habitat for species like Song Sparrows, it’s exciting to be able to share with them my findings from bird surveys: Yes, your work is making a difference.

If you enjoy bird-watching and are able to identify local birds, you can contribute your findings to inform research! Visit to learn more.


SLIDESHOW: Birds of the Redwood City Salt Ponds

We have written in the past about Cargill’s attempt to mislead the public and government agencies about the ecological value of the Redwood City salt ponds. While Cargill and its development partner, DMB Pacific, have withdrawn their original plan to build as many as 12,000 houses on the site, the companies consistently say they intend to submit another plan to fill the below-sea-level, restorable salt ponds with housing. As Cargill is busy lobbying federal agencies to exempt the ponds from the Clean Water Act and other important environmental regulations that protect the Bay, now seems like a good time to remind ourselves of the beauty and diversity of bird life found on these salt ponds.

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The San Francisco Bay salt ponds support hundreds of thousands of migratory shorebirds who rely on the Bay as a key stop on their route along the Pacific Flyway. The San Francisco Bay, in fact, is a recognized site of hemispheric importance for migratory shorebirds, and the Bay’s salt ponds provide important habitat for dozens of species, including several that are threatened or endangered.

Studies from Point Blue Conservation Science (formerly Point Reyes Bird Observatory), a leader in studying shorebirds along the Pacific Flyway, document that the Redwood City salt ponds are home to at least 24,800 shorebirds annually, including the federally threatened Western Snowy Plover, a species whose surviving Pacific coast population now numbers just 1,500-2,000 birds. In addition, Point Blue describes the Redwood City ponds as having “among the highest [bird] counts from the West side of the Bay between the Bay and Dumbarton bridges” making up more than a quarter of the total shorebird population of the region.

The Environmental Protection Agency has called the Redwood City salt ponds a “critically important aquatic resource that warrants special protection,” as has the San Francisco Bay Regional Water Quality Control Board. Even Cargill’s own environmental consultants have observed over 70 different species at the Redwood City salt ponds, and have documented the federally-threatened Western Snowy Plover breeding on site.

Save The Bay has shared photos of the large numbers of birds that live on the Redwood City salt ponds in the past, but to really appreciate the beauty and fascinating behavior of these birds, you have to see them up close and personal.

Cargill has restricted access to the site, so we have turned to Bob Cossins and other talented local photographers for a good look at a few of the species that have been observed on the Redwood City salt ponds. Take a look at the slideshow and learn a little bit more about the shorebirds that are at risk of losing their home if Cargill is successful in their plans to pave over these 1,400 acres of San Francisco Bay. Help us protect the Redwood City salt ponds from development – sign our petition telling Cargill “Don’t Pave My Bay” and spread the word with your friends and neighbors!


(Special thanks to former Save The Bay policy volunteer Leland Malkus for his substantial support in the publication of this article and slideshow. All bird descriptions are courtesy of the Cornell Lab of Ornithology)

Cargill Misleads Gov’t Agencies about Salt Pond Wildlife

Video of shorebirds on the Redwood City salt ponds
Click the image above to view a video of shorebirds on the Redwood City salt ponds

One of Cargill’s most consistent efforts in its campaign to pave as many as 1,436 acres of restorable salt ponds in Redwood City has been an attempt to mislead the general public, especially voters in Redwood City, into believing that that the salt ponds have little value to wildlife.

Cargill/DMB representatives have repeatedly denied the significant wildlife use of these salt ponds – saying, for example, that there is “nothing alive” on the salt ponds and that birds would “burn their fannies” if they tried to land on these two square miles of the Bay. Eneas Kane, the CEO of developer DMB Pacific has even gone so far as to describe the salt ponds as “inhospitable to man or beast.”

This is a theme that is repeated in Cargill’s official 370-page submission to the U.S. Army Corps of Engineers and the U.S. Environmental Protection Agency, where the company argues that the Redwood City salt ponds should be declared “exempt” from the Clean Water Act and other federal regulations that protect the Bay.

One of the attachments in their submission to the federal agencies is a 2002 “Significant Nexus Analysis” performed by Cargill’s long-time consultant, Mike Josselyn, that outrageously claims that the Redwood City salt ponds are only used on a “relatively limited basis by birds” and that the salt pond site “does not contribute to the integrity of the surrounding watershed.”

We understand why Cargill would prefer to ignore, downplay and outright deny wildlife use of the Redwood City salt ponds, but the annual presence of thousands of migratory shorebirds on the site is simply indisputable.

PRBO Conservation Science, a leader in studying birds along the Pacific Flyway, notes that San Francisco Bay, including the salt ponds, is a recognized site of hemispheric importance for migratory shorebirds. PRBO’s studies document that the Redwood City salt ponds are home to at least 24,800 shorebirds annually, including several threatened species. They describe the Redwood City ponds as having “among the highest [bird] counts from the West side of the Bay between the Bay and Dumbarton bridges” making up more than a quarter of the total shorebird population of the region. They also believe these numbers are an underestimate.

We could cite additional reports, but it doesn’t take reams of scientific data to prove that Cargill has been misleading state and federal permit agencies about the habitat value of the site. Just take a look at the video in the top right of this blog post, or any of the images in Save The Bay’s photo set of shorebirds on the Redwood City salt ponds to see for yourself. Do you see birds “burning their fannies?”