03-23-2021, 08:31 PM
OWL POST
A muggle can't send owl post other than as a direct reply with the same owl that brought the letter/parcel to them as the owl will then automatically return to the sender. Sending an owl requires magical power so that the bird can know who to track down, a charm happens between the sender and the bird which makes it possible. It's not a charm cast by the sender but a psychic one that happens between the magical bird and the magical human.
One can't use just any owl/bird but it has to be one of the magical sub-type that only look like the normal kind of the speacies. So one must be rented or bought from the wizarding community from a breeder or a store – or sent off from a post office. These magical types have their own magic that can't directly be compared to the wizardkind's magic much like the house-elves do. These birds have higher intelligence than their non-magical cousins, and a magical power to pick up and track down an individual human's soul's essence. That's why the bird can't find a person who is dead or under a strong anti-tracking spell, in which case the bird returns very soon with the letter/parcel because it couldn't pick up the essence.
Still, a witch or a wizard either can't send owl post to just anybody anywhere at any time. The sender must have some sort of an image of what the person looks like and who they are so that the bird can pick up the right person's essence. If the sender can't provide this information to the bird in their intent as they're sending it off, the bird simply will not leave. So no one can send owl post to someone they don't know at all but must use muggle mail or send the bird to someone they do know who can forward the letter/parcel to the right person.
Subscription to newspapers and magazines in the wizarding society must always be done in person in that paper's/magazine's office so that they can assign a specific bird to deliver it to the person in question. When that owl gets sick or dies the paper/magazine will be delivered to the subscriber's given address by a person and then a new owl is assigned.
The warning letter about underage magic use are delivered by an owl which has been trained to deliver mail to the specific address where a muggle-born or another magical person living with muggles, is known to live. They don't send warnings to all-wizarding houses, and if the alarmed location is nowhere near any specific house they will arrive to investigate in person.
The Hogwarts invitation letters are delivered by Hogwarts' own owls. There is the ancient magical quill that detects every single magical child in Great-Britain at the moment when the child's magical powers activate (around age 2) as well as the child's name which it then writes down in the Book of Admission. The names are written on the envelopes with that same magical quill, which enables the owl to pick up and track down the child in question. (The magical quill is ancient and the charm cast on it long lost, thus it can't be replicated in order to make owl post function more easily.)
Owls are not fast flyers even on their own, much less if they carry something.
A healthy and average size owl without a wind, carrying a letter, can fly maximum of 65 km an hour but not for long. The average pace is 30-50 km an hour. Small owls of course fly much slower and they're usually used only for local mail delivery.
In example, a letter between London and Hogwarts carried by a healthy average size owl travels about 24 hours, probably a few hours more - assuming the bird rests at times. (The distance between London and Hogwarts is 725 km in a straight line.)
Within London an owl post travels from a few minutes to a couple of hours, as London is about 50 km across in a straight line.
Here is a handy and quick tool to figure out the distance. Just type the city and country where the owl leaves and the same for where it's going.