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They're probably the most successful birds in the history of birds. But what do you really know about pigeons? Hank shares three weird facts about the birds, from their amazing chick-raising trick to their history of heroism in wartime.

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Sources for this episode:
http://www.australiangeographic.com.au/journal/birds-produce-milk-similar-to-mammals.htm
http://www.ccmr.cornell.edu/education/ask/?quid=472 - head bobbing when walking
http://www.loc.gov/rr/scitech/mysteries/pigeon.html
http://blogs.nlb.gov.sg/ask/uncategorized/2298
http://animaldiversity.ummz.umich.edu/site/accounts/information/Columbidae.html
http://news.nationalgeographic.com/news/2002/02/0227_0228_dodo.html
http://www.pigeoncenter.org/militarypigeons.html
http://www.popsci.com/science/article/2013-01/geologist-has-probably-figured-out-how-homing-pigeons-find-their-way-home
http://news.nationalgeographic.com/news/2013/13/130130-homing-pigeon-navigation-animal-behavior-science/
http://www.sciencedaily.com/releases/2011/09/110919074253.htm
http://news.sciencemag.org/physics/2013/01/sound-maps-may-help-pigeons-navigate
Hank Green: We all know the white dove is a symbol of peace and purity, so, what do pigeons symbolize?  Dirt and disease?  Well, problem here, pigeons and doves?  Same thing.  Just turns out one is a slightly better dresser.

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Doves and pigeons are both members of the same family, Columbidae, whose 308 species can be found pretty much any place on earth except Antarctica, and they come in a variety of sizes and colors.  Since the dodo disappeared, yes, dodos were pigeons, the turkey sized ground pigeon is the biggest of the family, while the sparrow sized new world crowned dove is the smallest.

Now you have most definitely seen some kind of pigeon in your life, but did you know pigeons produce milk?  They don't lactate like mammals, but they do produce a similar milk-like substance to feed their chicks.  Now, if this sounds bizarre, it's because it is.  All pigeons do it, but they're joined only by flamingos and male emperor penguins in this ability.  Crop milk is a fat and protein rich substance produced by both male and female pigeon parents.  The milky white stuff is churned up in the bird's crop or throat pouch, usually used for food storage.  The crop changes in response to hormones when eggs hatch, and like a mammary gland, enters a sort of lactation period.  Pigeon milk contains tons of antioxidants and plays a key role in boosting a chick's immune system, much like mammalian breast milk does.  

Also, pigeons bob their heads to see better.  Lots of birds bob their heads as they strut around looking like Mick Jagger, and pigeons are no exception.  The head bobbing probably helps the birds stay balanced on their legs, which spring out fairly far back behind their bodies, but researchers think this kind of jiving has more to do with stabilizing their vision.  We humans can stabilize our vision with our eyeballs, they stay in the same place when I move my head around.  And unless we're really feeling the Dr. Dre, we don't need to bob our heads while we walk, but pigeons have a harder time multitasking in the busy world.  It's easier to observe a moving object when your head is still, so when a pigeon bobs its head, it's actually holding its head in place temporarily while its body moves and then thrusts its head forward again.  This keeps the head stable for as long as possible, so the pigeon can keep an eye out for squirmy insects or swooping hawks.  We know this because in the late 1970s, creative ornithologist Barry Frost put some pigeons on a treadmill to see what happened.  And in the controlled surroundings of the lab with no bugs or birds or prey to watch out for, the birds' heads didn't bob.

Third thing, pigeons are decorated war heroes with excellent hearing.  People have been using homing pigeons to deliver messages for centuries--Persian kings used them, Julius Caesar used them, and in World War I, solders on the front used them to relay hundreds of thousands of messages.  One famous flyer named Cher Ami single-wingedly saved a battalion of 600 trapped French soldiers, flying home with a missing eye, a bullet in its breast, and a leg dangling by a thread.  Cher eventually healed and was awarded with the prestigious service cross.  When he finally died years later, he was stuffed and mounted and now resides at the Smithsonian Institution.  

Humans have never quite understood homing pigeons' ability to find their way home across large distances, but a recent theory may have solved the mystery.  USGS researcher Jonathan Hagstrom believes that the birds use low-frequency sound waves to create a sort of acoustic map by which to navigate home.  Pigeons can hear down to the faintest of infrasound noises, down to even about 0.1 hertz.  Whereas even under the best laboratory conditions, humans can't hope to hear under 12 hertz.  Hagstrom got the idea that the birds used sound frequencies when he noticed racing pigeons going astray whenever the supersonic Concorde jet was flying nearby.  The jet's sonic interference was disorienting the pigeons.  Weather, landscape, and atmospheric changes can also lead the birds astray.

So yeah, pigeons, turns out they're pretty rad.  Be nice.  And thanks for watching this episode of SciShow, if you have any questions or comments or ideas for us, we're on Facebook and Twitter and down in the comments below, and if you want to keep getting smarter with us here at SciShow, you can go to youtube.com/scishow and subscribe.

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