I chose those two species because they’re so closely related but, as soon as you get to observe them, completely distinguishable. Ask any falconer! I could have added a member of the third falcon group, the kestrels, whose form and lifestyle are equally unique.
Anatomically, there are diferences such as the specialised nostrils of the peregrine enabling it to stoop at 200mph while breathing (and emulated by jet fighters). You can just see the lack of those in your hobby photo (though it’s the Australian rather than the British model).
Comparably, the wing geometry of the two is different, and it’s no surprise that the more swift-like wings of the hobby are employed in a flying style that is very similar to swifts and swallows - which are its prey.
The behaviour, as a package, is entirely distinct: most characteristically the peregrines will appear as a pair once in a few weeks - ranging a mile or so from home, at which distance their unique vision can spot prey - soaring high and, if they attack, coordinating their stoops from a great height to make a killing blow. If the tiercel misses, the falcon has a go, because they’re too fast to turn quickly. The hobbies, on the other hand, one encounters singly, typically finding them flying at head height in front of your car the way and following the corners the way swallows do - they take out dragonflies and swallows in level flight. Hobbies fly for fun - peregrines save their energy for the hunt.
The flight of the Hobby is unique. When hunting it will
fly along at a fair height, gliding and winnowing alternately.
If it sees a prospective quarry it hurls itself after it, following
the turns and twists of its prey with rare skill and great
Nothing like a peregrine at all.
Nesting (including brooding, feather and feeding development), calls, flight, diet, flocking are all different, but the overall impression is that you are seeing two different specialists - to the extent that after awhile, a mere glimpse is enough to distinguish them. I’m reminded of Arthur Jones who, doing his PhD work on Cichlid fish, was able to spot a species he’d not seen before in an aquarium out of the corner of his eye, from the way it behaved.
These are just the gross differences - ornithologists will note very many differences in character and behaviour that make up their unique roles in the world.
Now, one way of viewing this would be to say that, finding itself lumbered with unique nostrils, wing-shape, habits etc by neutral evolution, the peregrine soon found out it would starve trying to chase swallows, and elbowed itself the vacant niche of fastest bird in the world. Net result - superb adaptation of the entire lifestyle to a mainly accidental body plan. And such lucky accidents all within 10m years! Owing to a series of manufacturing errors, the Ford factory ends up with an F1 instead of a people-carrier and decides, consequently, to race, the company being equipped with “minds that can adapt their behavior to their form.”
Likewise for the hobby which, suffocating if it tried to stoop at high speed, was lucky enough to find itself with more manoeuvrable wings and body (remarkably similar to the most aerial birds in the world, the swifts, which touch ground only to breed); and with a convenient supply of swallows and swifts now within grasp. It took to nesting in trees rather than on cliffs (because it found itself with a compulsion to co-opt old nests instead of preferring laying on bare ledges).
All this, according to neutral theory, occurs because of the overload of adaptive selection in small populations, and a lot of weeding out of rubbish by purifying selection… fortunately the random changes keep the birds’ mental agility intact so they have the wit to turn the outrageous blows of cruel fortune into what - to all appearances - resembles supreme design for a special role.
And that’s how the elephant got his trunk.