The setting is a post-scarcity civilization, where a galactic government exists, but due to the limitations on faster-than-light travel, each world is relatively self-governed.
I want a city on one of the habitable worlds to be known for the use and breeding of homing pigeons. How can I justify the use of these homing pigeons, when people can send digital messages?
Simple, because even in 2023, unless you're basically inside your local internet exchange, the pigeon is faster for bulk data transfer.
Raspberry PI fanatic Jeff Geerling decided to race his gigabit internet connection against some pigeons carrying USB thumbdrives... the pigeons won - and by a reasonable amount too! Well, provided you didn't need to send the data more than a good days drive away... (you'll have to watch the video to find out where Pijeff fits in)
Here's the speed/distance plot he came up with:
Edit: As for why no-one in our world uses pigeons, that's simply because light-weight, high-capacity data storage came a bit late to the party relative to international air freight. However, that's not to say that the concept of strapping some kind of data storage device to some kind of moving object (animal or otherwise) isn't still alive and well.
It's quite common for large science facilities to simply post each other boxes of drives when they need to share a couple petabytes with one another. In fact, this is so common that a number of commercial solutions already exist like Amazon's "snow mobile" (which is literally just a truck carrying 100'000 terabytes worth of drives) - for you know, when you need to upload the entire library of congress by next week...
So TLDR; Pigeons are much faster than the internet over short distances, we just don't use them ourselves because parcel post is easier. However, if your world has high-capacity storage but slow networks and snail mail or if you're in a dictatorship or war zone and thus can't trust the network or the post then it just might make sense to dust off your old copies of RFC-1149 and RFC-2549.
Plus, as RFC-2549 puts it: "One major benefit to using Avian Carriers is that this is the only networking technology that earns frequent flyer miles"
Government Spying
The people send messages on encrypted chips that they don't want the government to spy on.
Even if they get a hold of the message, the pigeon can't tell them where it came from, where it's going or who it's working with.
My wife wrote me a handwritten letter last year. That was 2022. I felt like asking if she knew [expletive] cuneiform too, but decided not to since that was just her way of being romantic.
Did you know that a lot of people still use pen and paper today? That's as far removed from modernity as pigeons are from satellites and even ansibles. And it is exactly because of that, that pigeons will still be used. Otherwise what are the teenagers of your world going to mock their respective millenials for? Using telephones to actually call other people?
Homing pigeons can be faster.
It all depends on how much they can carry. If they can carry 12 grams, that is a gram-mol of carbon, or 6*1023 atoms. If you can get one bit per benzine ring, then you have 125 Zettabytes, or about twice the internet in 2020. That would take some time to transmit.
I cannot find a sensible figure for what a pigeon can carry, but with training they can deliver 75 grams according to Wikipedia, so 12 grams is probably a sensible amount.
Throughput vs Latency
As others have mentioned - a Homing Pidgeon, carrying a storage drive can have a faster throughput than a networked link.
Security
This is a combination of two issues:
1: as a physical transfer of data - unless the pidgeon is intercepted, the Data transfer is secure.
2: A High-tech adversary might not notice/care/consider the use of old-school techniques.
This is where SneakerNets have worked quite well.
The Landscape is not conducive to Wireless or Wired communication
Electromagnetic interference, constantly shifting plate tectonics etc. Anything that prevents a signal being transmitted wirelessly or having a physical line would be a good
Because you can
The first implementation of RFC1149 (IP over Avian Carriers) was by a bunch of Linux nerds, because they could.
There are fears that a future combination of AI and quantum computing will make make it possible to decrypt most things encrypted with a known algorithm in a relatively short period of time, making encryption much less reliable than it is today.
There are ways to encrypt safely using something like a one-time pad, but that can involve the other party having a key that is just as long as the data, and use it only once! So for anyone wanting to send large digital files, and evade monitoring on a public communication system, physically transferring the data may be the best option for true secrecy.
We don't want to hand deliver every digital file, but we worry about interception if we just use pigeons. The solution: We send the message and the key through different mediums. We encrypt the data with a one-time pad, send the encrypted file over the internet, and send the key by pigeon. Either one is useless without the other.
It turns out that the mathematical underpinnings of encryption is totally wrong and that there is no viable replacement.
So instead physically delivering media becomes the norm.
Not that I expect Homing Pidgeons would fare especially well there but, think about Tatooine from the Star Wars universe. Despite being technically a part of the empire, the distribution of advanced technology there is clearly behind pace compared to somewhere like Coruscant or the Bespin Cloud City.
And even within individual planets, we see that Luke is far removed from nearly any type of civilization on his little moisture farm, versus Mos Eisley which has its own spaceport and thriving market.
The point being, even in a society where a galactic empire has control over a large swathe of territory, and regional governors have direct control over their planets, we can find very disparate forms of technology among the population.
All we have to do is find a place where the Galactic Network has not been established, either due to infrastructure problems [nowhere to build the giant receiving towers], political problems [internal warring factions on the planet that make establishing the network difficult], or even just resource problems [copper wire? Our planet doesn't even have rocks!] and you'll find a pocket of society that, while it may thrive in some ways, lags behind in technological development.
Local resources/economics
This is partially a frame challenge against the post-scarcity detail. If you have a galactic civilization without faster-than-light communication, such that the communication bottleneck requires planets to be mostly self-governed, there will inevitably be an even bigger bottleneck distributing goods. Goods always move slower than data. (With the boundary case of shipping physical media around, in which case data moves as a slow as goods.) Maybe local resources on the planet are extracted efficiently and distributed fairly, but anything you need to ship in from the next star system is going to come at a premium.
So, maybe this planet has lots of pigeons and no copper.
Cultural interia
Maybe they finally did manage to get sufficient copper imports, but
Why do they use pigeons? Because they want to. Because keeping pigeons makes them happy, has become a source of national (global?) pride, and the personal touch is considered to be much more respectful and significant. This is a post-scarcity world, so while the pigeons are obviously less efficient than the internet for any realistic real world purpose, it just doesn't matter. You don't need your communications to be efficient any more, so less efficient methods can eke out an effective niche.
Verify the sender
Perhaps it is almost impossible to verify who digital messages are actually coming from in your world, and the best way to do so, if something is important, is to send a known pigeon with personal signatures, seals, etc. I mean, nowadays it's already basically impossible to know if you're talking to an AI or not.
Astronomy!
In real life, the imaging of the M87 blackhole was an interesting use case for "homing pigeons", planes!
Instead of spending weeks to send petabytes of data over the internet, the massive quantity of data collected by the radio antennae had to be flown on airplanes to central data centers.
I reckon if your "homing pigeons" were very large and tamable birds, they could be used to transfer massive amounts of data for large data scale projects, like in our case the black hole imaging.
“There’s no internet that can compete with 5 petabytes of data on a plane.” https://www.inverse.com/science/54833-m87-black-hole-photo-data-storage-feat
