Hi, I watch Game of Thrones and mostly, in fantasy worlds, they use ravens, falcons, or pigeons to deliver messages. I am wondering how realistic this is.
Some additional questions: who often uses messenger birds (military, nobility)? Were there moments when battles were decided simply because messages were intercepted between camps?
Thucydides bless you. We don't get enough bird questions on this sub.
The short answer: Yes, you absolutely can use birds to deliver messages. No, you cannot use just any bird. Yes, this is an ancient practice, though depending on the era it was more common in certain societies and regions (or societal strata) than others.
Disclaimer: Most of what I know about pigeons is through commentary on them in a variety of falconry manuals and texts, so while a comprehensive history of their use as messenger birds is definitely beyond me, I hope at least some of this is useful.
Why bother using birds? The obvious attraction of using a flighted bird to convey messages is that they don't give a damn about washed-out roads, cutthroats in roadside inns, or most natural obstacles. However, most birds can't be used as messengers for the simple reason that we really don't have any way of telling them where to go. Instead, what we do is exploit their instinct to return to a familiar roost and their amazing ability to find their way home even from very far away. (Though we still don't know exactly how they do this, and there are multiple theories.) People who train racing or messenger pigeons (not that there's many of the latter these days, although they do still exist) don't really have to train them to do either of these things. What they do train are the bird's flight muscles and tolerance for long trips by continually releasing them to fly home over greater and greater distances.
So if you're going to use a bird as a messenger, you need to start with a species that: a). has the instinct to return to its home roost: b). can be relied on to do exactly that when needed, and: c). won't lose the message.
Ravens are problematic at best. While they see and accept certain places as "home" and prefer familiar areas, they don't have the overriding instinct that pigeons have to return to that one specific spot. Moreover, they're not reliable messengers. Ravens and crows are inconveniently smart birds and have no problem disobeying "orders" to go investigate something they find more interesting or amusing than their "job." They're also accomplished at getting into or damaging containers, and though you could design a raven-proof message attachment for their legs, it's just a nuisance you don't have to overcome with pigeons.
So in summary, it's within the realm of possibility that you could train a raven or crow to act as a messenger bird, and I'm sure you could find people out there who've done it, but they're a far less attractive option than pigeons. The last thing you want if your brigade is under attack is to watch while your messenger raven farts around with a snail or decides it's too much work to fly into a headwind.
Falcons (and birds of prey more generally) are an even harder sell. In falconry, you can successfully train a number of raptor species to hunt, but this relies on a combination of stringent weight control, exploiting their hunger, and training them to a lure that resembles the animal you want hunted. Ideally, you want them slightly underweight on the days you want to hunt because you need them hungry enough to fly at game, but not so underweight that they're weakened and unable to make a kill. Raptors did not evolve to recognize or prize human emotions, do not really accept orders as such, and even experienced falconers can and do lose birds to the wild despite their best efforts. So even though raptors, like ravens, see and accept certain places as "home," they don't have a homing instinct and cannot recognize a human's desire or order that they fly home.
Out of all the species currently used for falconry, the Harris hawk is the only one I know of that can be counted upon to follow you in the field and will return to you if it fails to make a kill. Notably, it is also the only raptor that evolved to hunt in packs and can learn to accept humans as a pseudo-pack mate. (It's for this reason that, if you ever sign up for a course at a falconry school, the odds are around 99.9% that you'll be given a Harris to fly.) However, Harrises are native to the Americas and obviously played no role in the societies or wars of Eurasia and Africa, and I have never seen any account of anyone using them as a messenger bird (which, again, is probably owed to the lack of a homing instinct). As with ravens, I'm sure someone out there's tried it, but there are better options elsewhere.
So that brings us to pigeons, which are superlative messengers (at least within a few contexts) and saw a wide variety of civilian, military, and diplomatic uses throughout history. There were some regions where they were so commonly used that military falconers trained raptors to hunt and kill pigeons in the hopes of nabbing messenger birds en route (no great stretch as it's common even today to train birds on specific prey types). This counter-pigeon strategy was being employed (albeit with middling success) as late as the Franco-Prussian War during the siege of Paris.
However, the problem with using messenger pigeons is that they really only go in one direction: Home. Therefore, any civilian or military body that wished to make use of pigeons had to have a way of continually transporting the birds anywhere that needed to be able to send messages. Once you ran out of pigeons, your message vector was Gonesville. Moreover, it was common to retain stocks of pigeons "homed" to different places so you weren't restricted to sending messages to a single destination, so someone had to keep track of this on top of feeding and caring for the birds. This was typically done by way of attaching color-coded bands to one leg (e.g., such-and-such arrangement of colors and stripes meant the pigeon was homed to such-and-such military office) and using the other to attach the messages. However, any additional weight to the birds was tricky; pigeons aren't big to begin with, and the size and length of messages was necessarily restricted to anything the bird was able to carry.
Were there moments when battles were decided simply because messages were intercepted between camps? Battles I'm not sure about (I've seen a few potential events referenced online but cannot confirm any of them with the books available to me at present), but I can tell you about the famous example of a pigeon who was credited with saving an American battalion in World War I. One of the few messenger pigeons in history to have made a name for herself, Cher Ami, successfully relayed a fairly irate message from Major Charles Whittlesay to 77th Division headquarters in slightly more than an hour, despite being practically shot to pieces. Had she not made it back, it is entirely possible that the battalion would have been wiped out; they overextended, got trapped behind German lines, suffered a 62% casualty rate, and to add insult to injury, became the target of their own artillery.
Cher Ami is now on display in the National Museum of American History, which is slightly morbid but a not-uncommon fate for animals that performed great services in war.
Sources: