Season 7, Episode 4.Daenerys fights the Lannister's army with Drogon and Dothraki khalasar.Contains spoilers.Owned by HBO.

Warning: This post contains spoilers for season seven of Game of Thrones.

“Ravens. We need to send ravens,” Bran Stark said at the beginning of “Eastwatch,” the action-packed fifth episode of this season’s Game of Thrones.

Bran had just warged and seen a flock of ravens flying north of the Wall — and when he opened his eyes back in Winterfell, he told Maester Wolkan they needed to dispatch ravens. The birds were then used to set up a series of big moments in Sunday’s episode, including Samwell Tarly’s hasty decision to leave the Citadel and a tricky maneuver by Littlefinger that could pit Arya and Sansa against each other.

And these ravens were just some of the many used to advance the plot in HBO’s hit fantasy show — a raven carried a message from Tyrion Lannister that invited Jon Snow to meet Daenerys Targaryen, and a rare white raven let Sansa Stark and Jon Snow know that winter had finally arrived. And that’s not even getting into the matter of the Three-Eyed Raven.

But, though the land over which those ravens have flown is one of fantasy, there’s a real historical basis to the idea of message-carrying birds. Homing pigeons have a storied history dating back far beyond the Middle Ages, the period from which Thrones author George R.R. Martin frequently draws inspiration. (Pigeons aren’t the only birds with special abilities — corvids like ravens are thought to be particularly smart — but they are the most famous for this particular purpose.) And many of history’s most famous homing pigeons — including one that saved nearly 200 soldiers in World War I — have earned their celebrity through their involvement in war.

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Also referred to as messenger or carrier pigeons, the birds are capable of traveling as fast as some cars — about a mile a minute — and traveling at least 500 miles a day while carrying letters attached to their legs. As a point of comparison, a regular pigeon might go a mile or two a day. Once the message is received, the pigeons will return back to its “home” — though experts disagree exactly how pigeons carry out the feat.

“If you believe the Italians, it’s all due to smells,” Charlie Walcott, a professor emeritus of neurobiology and behavior at Cornell University, told TIME. “And if you believe some of the Germans, it has to do with the earth’s magnetic field. And my personal view is it’s a combination of the two. If I were to blindfold you, I bet you could find your way home by hailing a cab or talking to someone. And I think that’s what’s happening with pigeons, is that they have these alternative strategies.”

The first known homing pigeons were used in ancient civilizations in Egypt, Greek and Rome, according to Hugh Gladstone’s 1919 treatise Birds and the War. For example, in 44 B.C., Marcus Junius Brutus used these pigeons to protect his city during the siege of Modena by sending messages to his allies. “Gradually, it was recognized that pigeons would prove of great military importance,” Gladstone wrote.

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Carrier pigeons continued to be used in the centuries that followed — and one tale claims that a fruit-loving Arab ruler during the Middle Ages used the birds for more than just delivering his correspondences. He also used them to bring him his fix of Lebanese cherries, receiving a single cherry inside a silk bag in each delivery, according to the The Pigeon Wars of Damascus.

In fact, homing pigeons remained a prevalent form of communicating, especially over long distances, until 1844, when Samuel Morse invented the telegraph.

But the winged messengers weren’t completely phased out after that, despite newer technology. An estimated 200,000 homing pigeons were used throughout World War I and World War II, and were essential to the strategies of various armies for their work disseminating important messages from the front lines of the battlefield.

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Perhaps the most famous such messenger was Cher Ami, a name that means “Dear Friend” in French. The purple- and blue-speckled bird transported a dozen messages for the U.S. Army Signal Corps in France, and is credited with saving nearly 200 American soldiers in World War I. Cher Ami’s swan song came during one of his missions in September of 1918. At the time, nearly 500 American troops, led by Major Charles Whittlesey, were trapped, surrounded by German forces on a battlefield in northern France. What was worse, they were also coming under friendly fire.Whittlesey needed more troops — at that point, there were only 200 survivors of his so-called “Lost Battalion” left — and he had already tried using two different pigeons to deliver that message. Both were shot down by the Germans.

He tried once more, this time deploying Cher Ami with a note that read, “We are along the road parallel to 276.4. Our own artillery is dropping a barrage directly on us. For heaven’s sake, stop it.”

Cher Ami was also struck by enemy fire, taking a bullet to his breast and leg. But that didn’t stop him from delivering the message, and his heroic commitment eventually led to the remaining 197 trapped soldiers getting saved. While Cher Ami died a year later, likely due to complications from his battle injuries, he received a medal of honor from the French army. And his legacy lives on at the Smithsonian Museum in Washington, D.C., where his stuffed body is part of the “Price of Freedom: Americans at War” exhibit.

And even today, while there are literally hundreds of apps people can choose from to share messages, some still rely on homing pigeons.

One of the most popular uses for the birds is pigeon racing, and Orlando Martinez is one of the most renowned names in the game. The New York-based pigeon racer has a flock of feathered friends that he uses to enter races that earn him upwards of $15,000, according to Smithsonian magazine. The birds are brought to a location hundreds of miles away from their homes, and then race to fly back. The pigeons are so goal-oriented that they rarely stop for food or to recharge — and Martinez learned that even a broken wing didn’t stop one of his birds from trekking back to his Brooklyn home after a 300-mile race.

It’s that level of dedication that makes these birds the favored form of communication throughout the Seven Kingdoms — and perhaps the most trusted beings in the realm.

EDIT POST

This is tough because a lot of numbers are thrown around on Game of Thrones and thousands more on the internet, but I think I've got the armies down. Using three sources: the show, the books, and George R.R. Martin, I've recorded the might of all the armies in the Known World. With Daenerys Targaryen headed for Westeros, there's going to be a reckoning of biblical proportions. How biblical? Here's a breakdown of the opposing forces:

**Spoilers ahead. Only proceed if you're up to date with Game of Thrones.**

House Targaryen

Queen Daenerys I Targaryen

Dragons: 3 (Drogon, Viserion, Rhaegal)

Unsullied: 8000 infantry sword and spearmen

Dothraki: 140,000 riders

The Reach (House Tyrell): 80,000 + ships

Dorne (House Martell): 20,000 + ships

Iron Fleet (Greyjoy rebels): 3000 men+ 100 ships

Numbers explained: It is known Queen Daenerys has her three fully-grown dragons, her Unsullied army (8000 strong before the Meereen takeover so the numbers are lower), and her Second Sons (left behind to police Meereen). What isn't known is exactly how big her new Dothraki army is.

Despite rumors he led a force of 100,000 riders, both Viserys Targaryen and Jorah Mormont state Khal Drogo's army is 40,000 strong in season one. There were seven khals at the Temple of the Dosh Khaleen when Dany burned it down so conservative estimates place at least 20,000 riders in each khal's army. That's 140,000 total and that number is still conservative. It's also what Thrones guru Jason Concepcion came upwith over at the Ringer so (sing it) CREDIBILITYYYY.

House Tyrell lords over the Reach. They managed to assemble an army of 100,000 for Renly Baratheon in season two so they should be able to do about the same for the Mother of Dragons (we'll say 80,000 to be safe). The Tyrells are rich and without any future thanks to the losses of Loras, Mace, and Margaery at the Sept. They'll be playing for keeps. Olenna wants her revenge. Martin has stated Highgarden also has the second or third greatest strength at sea.

Martin has also said the Dorne forces, despite rumors, are about the size of the North and the Vale each: 20,000 men. That's assuming the usurper Ellaria Sand and her Sand Snakes are firmly in control of House Martell. We'll assume they are considering the high stakes allegiance with the Tyrells shown in season six. Dorne has a strong presence at sea as well.

In the books, Yara and Theon Greyjoy steal the best ships of the Iron Fleet after Yara loses the kingsmoot to Euron. They pledge them all to Daenerys in exchange for her support of Yara's claim to the Salt Throne. Tyrion notes 'there are more than 100 ships in the Iron Fleet' meaning they've arrived with 100. But we don't know what kinds they have... Are they 20-oar raiding vessels, or 100-oar longships? And how many men are on each one? Assuming they're half and half let's estimate 30 men on average on each ship. That's 3000 fighting seamen of the Iron Fleet.

Daenerys also has what's left of the former masters' fleet. Tyrion tells her, with the help of the Greyjoys, all these ships will 'barely' be enough to get her army to Westeros. But the Iron Fleet and the masters' ships don't seem like enough to carry all those Dothraki. Perhaps Dany scored a fleet we don't yet know about.

Total number of fighting men: 251,000

Total number of ships: Unknown (100 from Iron Fleet + Tyrell + Martell + Masters' fleet)

Total ships needed: 2500

The North

King in the North Jon Snow the White Wolf

Lady of Winterfell Sansa Stark

Direwolves: 1 (Ghost, but watch for a Nymeria return when Arya shows up)

House Stark and its banners: 15,000 infantry and riders

Knights of the Vale (House Arryn): 20,000 riders and infantry

Free Folk (Wildlings): 1500 infantry

Numbers explained: With Jon declared King in the North, he's rallied all the banners who denied him before the Bastard Bowl. When they aided Robb Stark, the total count was 20,000. That number should be smaller thanks to the Red Wedding and the Battle of Winterfell, which wiped out the Boltons and Umbers (5000 men). Save for some reinforcements, we'll conservatively say Jon has 15,000 under his direct command right now.

Martin has said the Vale army is roughly the same size as all the Stark banners so rough estimates have Sansa's surrogate forces at 20,000 strong thanks to minimal losses after mop up duty at Winterfell. But those are book numbers. It's possible the Vale forces are much larger on the show.

The Wildlings also took some losses at Winterfell so Tormund's forces are down from the 2000 he had before the battle. I'm guessing 1500 fighters are left, but that may be generous.

As for the Stormlands, there aren't many warriors left. Renly Baratheon commanded 100,000 before he was murdered ('all the might of the Stormlands and the Reach'). After his death, the Tyrells returned to the Riverlands, taking most of that army. The rest of Renly's men flocked to Stannis, who promptly lost everyone at the Battle of Blackwater. In defeat, Stannis revealed he had 4000 men and 32 ships left during a visit to the Iron Bank. Those 4000 were lost on the way to and at Winterfell as Stannis tried to take on the Boltons without the help of the Night's Watch or the Wildlings. House Baratheon is wiped out until Gendry shows up.

Total number of fighting men: 36,500

House Lannister

Queen Cersei Lannister

House Lannister: 60,000 + ships

They're alone now, but they still have a pretty huge army behind the walls of King's Landing (not to mention untold amounts of Wildfire). The 'they' in that last sentence being Cersei and Jaime Lannister who now command the Lannister army, City Watch, and, not to be forgotten, the Mountain. Cersei controls them at least, for now. At the beginning of season two, Tywin Lannister tells Jaime he's giving him 30,000 men, or half their troops. So they had 60,000 at that time. Since then, the Lannisters have taken casualties at the hands of Robb Stark's army, fought off Stannis at Blackwater, and laid siege to Riverrun. With reinforcements and the City Watch (supposedly 6000 men), I'm betting they can gather 60,000 and probably more.

Total number of fighting men: 60,000

The White Walkers

Game Of Thrones Characters

The Night's King

Total number of fighting men: Untold thousands, everyone who has ever lived and died (and dead horses).

Wildcards

House Greyjoy: King Euron wants to build the greatest fleet the world has ever seen. Can he do it?

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The Hound: Roaming free again, whose side will he join?

Arya Stark: Last seen killing Walder Frey at the Twins, she's close to Jon and Sansa. Will she join them, finally... or remain free and alone? She wants to see the edge of the world. Don't put it past her to do just that.

Melisandre: Exiled from the North, the Red Woman still believes in Jon Snow. Will she find a way back to him, or perhaps to the Brotherhood Without Banners?

Pigeon Army In Game Of Thrones Online