We've had Design Notes installments on species before - but I think Falkin give us an opportunity to go in-depth on the topic of subspecies.
If you haven't looked at the Falkin yet, take a gander! The base species offers limited flight, talons, and eagle eyes, which become baseline features all the subspecies (mostly) have. As with most species, this is a case where the base occupies most of the power budget, leaving a relatively small amount of power for the subspecies to work with. In Detect Balance terms, the base species here sums to about 27 points, enough that the subs don't necessarily need any features.
But obviously they deserve some. The point of Falkin is to bring the Aarakocra's power down to something more DMs can be comfortable allowing, and to pay players back for that by making them a little more interesting. As a side bonus, I can make subspecies that align with how I've long described aarakocra in my games - with mountain (the original), coastal/tropical, and arctic subspecies.
So what makes a good subspecies mechanically? Let's dig in.
Power Budget
Now, I am cautioning people constantly to treat Detect Balance as a post-creation check, not as a budgeting tool. But we're going to break that rule here, because it makes it way easier to talk about this.
We can start by looking at how powerful subspecies generally are. Let's catalog some efforts:
Dwarves: 20 point base, 9-10 point subs
Elves: 17 point base, 10-11 point subs
Gnomes: 16 point base, 7-8 point subs
Halflings: 15 point base, 8-9 point subs
Genasi: 8 point base, 9-11 point subs
Aasimar: 20 point base, 11-12 point subs
Shifter: 17 point base, 7-9 point subs
Generally, WotC gives subspecies about 1/3 of the total power budget.
Atxayotl: 22-point base, 5-7 point subs
Drowned: 21-point base, 5-8 point subs
Minotaur: 21-point base, 6-9 point subs
Clearly I'm a little stingier on my subspecies than WotC, with mine getting as low as 18% of the total budget.
Cnidaran: 19-point base, 10-13 point subs
Rakin: 17-point base, 9-11 point subs.
Loot Tavern is more subspecies-oriented, with pretty high budgets, about 40%.
Returning to Falkin, we can already see that they're going to have a very slim budget for the subspecies (spoiler, it's 5-6 points). Unfortunate, but unavoidable. With flight on the base species, there's really not much I can trim out.
Subspecies Philosophy
Subspecies can follow a few different themes. One is that they're adapted to different circumstances: Falkin are doing this, but so do my minotaurs and WotC's gnomes and elves. Another is that they're created through through different means: this would be genasi, aasimar, or my drowned. This has ramifications for the abilities we choose.
"Adaptation" subspecies should be built for a specific environment. Arctic Falkin get cold resistance; this makes an awful lot of sense. "Creation" subspecies instead have their features geared to represent how they came into being. Genasi don't particularly live on the elemental planes (I mean, they might, but adventurer genasi don't unless you're running a very specific campaign). Drowned subspecies are literally differentiated by their animating force; one of them has features to reflect being host to a sentient kelp colony.
The third style you can see in Loot Tavern's examples above, though it's not an approach I personally like or employ - it's the "umbrella" species. Rakin basically represent three different types of animal people (IIRC it's racoons, possums, and tanukis). If you were approaching these from the biological definition of species, they aren't; these aren't creatures that would be able to interbreed and produce fertile offspring. The base species isn't really designed to showcase a species, per se, just to wrap in similar traits expressed within the subspecies.
(And, yes, in a fantasy setting where interbreeding is widely possible, this definition already doesn't make sense. Half-orcs and half-elves already put the lie to this definition, and I don't know if LT's lore has different rakin interbreeding. I don't even allow half-elves in my main setting, though, don't @ me.)
Falkin Features
Circling back to our example at hand: Falkin adhere to an "adaptation" philosophy, where they really are the same type of creature, just with different plumage and traits for their environment. Given our very slim budget, they all look pretty similar. Each subspecies, mountain, coastal, and arctic, gets 1-2 proficiencies and one environmental feature. Coastal can hold their breath, mountain can meditate under the open sky, and arctic get cold resistance and advantage vs. blindness (since they're adapted for snow blindness).
But wait, you say. What about earthbound falkin? Yes, I've avoided mentioning them entirely up to now. Because they're a different case. While they're still philosophically an "adaptation" example, their adaptation is more extreme, because they've lost full flight. This is a tradeoff design, where the sub actually diminishes the base species features to make room for more stuff.
Offhand, I don't think WotC's done this, though I know other creators have. Earthbound falkin can only glide, not fly. In return, they can glide while wearing armor, they get stronger talons, they get faster walking, and they get a wing buffet ability. The lack of full flight adjusts their budget: they still net out to 6 points, but in this case it's a -4 for gliding and +10 for the other features.
The important thing with earthbound falkin is that they still feel like falkin. I had one supporter ask if they could get even cooler and wilder abilities if they lost flight entirely, and yeah! They could! But at that point, I don't think they'd feel like cousins to the rest. D&D has always loved reflavoring: if you want an underdark falkin that can't fly at all anymore, I say reflavor a dwarf or a drow and go to town. But I think the subspecies wouldn't really make sense if I did that myself.
What Is This Subspecies You Speak of?
Last, a quick note: WotC has obviously discarded the concept of subspecies entirely. From Monsters of the Multiverse onward, they break subspecies out into "full species" setups, with the linkages between them becoming implicit instead of explicit. Honestly, it feels a bit silly to me. I don't know if they're just trying to avoid the word "subspecies" (very clunky and scientific-sounding). Or maybe they just want to chart a very wide course away from potential eugenics implications (extremely valid! If you write a subspecies that's explicitly less intelligent than the others, that is Big Problems). On the whole, I think it's similar to WotC's move away from species ASIs in favor of the universal "just add +1/+2 anywhere" approach they put in Tasha's and MotM. They're so concerned about messing up that they discarded a useful design and worldbuilding tool entirely.
And it is a worldbuilding tool, not just a design one. The existence of coastal and arctic falkin implies something about the species' migratory patterns, they way they've spread out across the world. If your game has coastal falkin being populous and widespread and arctic being rare, that says something! You can even make arctic discoverable within your world - perhaps they're unknown to other societies until the party explores that far northern mountain range. Similarly, the existence of drow implies the existence of the Underdark; a world without an underdark doesn't really have logical room for drow to exist. Make these things separate full species and the implications here get weaker. I envision falkin as being four different-looking types of falcon-people, but if you write them as four separate species, there's no reason mountain falkin can't be eagles and arctic falkin are snowy owls. The mechanical connections reinforce the worldbuilding ones.