As a (mostly) nocturnal bird of prey, the owl is inevitably seen as sinister by Muggles, but it has been a faithful servant and helpmeet to witches and wizards for many centuries. In spite of the many alternatives available for magical communication across long distances (including Patronuses, Floo powder, and enchanted devices such as mirrors and even coins), the faithful and reliable owl remains the most common method used by wizardkind across the world.
The advantages of owls as messengers are those very qualities that make Muggles view them with suspicion: they operate under cover of darkness, to which Muggles have a superstitious aversion; they have exceptionally well-developed night vision; and are agile, stealthy and capable of aggression when challenged. So numerous are the owls employed by wizards worldwide that it is generally safe to assume that virtually all of them are either the property of the Owl Postal Service of their country, or of an individual witch or wizard.
Whether because they possess an innate bent for magic (just as pigs are reputed to be innately non-magical), or because generations of their ancestors have been domesticated and trained by wizards and they have inherited the traits that make this easy, owls learn very quickly, and seem to thrive on their task of tracing and tracking the witch or wizard for whom their letters are intended.
The mystical association between the name and the human who bears it has long been understood by witches and wizards of all cultures. While the process remains mysterious even to those who train up owlets to become wizarding pets or postal owls, the birds appear to be able to make such a connection between the name and its possessor that enables them to trace the witch or wizard concerned wherever he or she may be. An owl does not need to know an address, although witches and wizards generally add the place to the envelope on the off-chance that the owl is intercepted and the letter falls into other hands.
Should a witch or wizard not wish to be sent letters (or tracked in any other way), he or she will have to resort to Repelling, Disguising or Masking Spells, of which there are a great range. It is possible to protect yourself from all correspondence, or all but that carried by a specific owl. If a witch or wizard is determined not to be contactable by a persistent creditor or ex-boy or girlfriend, they might try a masking spell specific to that person, but this ploy is easily circumnavigated by asking somebody else to send the owl. In general, it takes strong protective magic, and a willingness to forego a lot of birthday cards, to avoid the attentions of Owl Post.
Trained owls are expensive, and it is quite usual for a wizarding family to share a single owl, or else only use Postal owls.
My love of, and fascination with, owls long pre-dates the first idea for Harry Potter. I trace it to a cuddly owl toy that my mother made me when I was six or seven, which I adored.
Of course, owls have been associated with magic for a long time, and feature in many old illustrations of witches and wizards, second only to cats as Most Magical Creature. The owl’s association with wisdom was established in Roman times, for it is the emblem of Minerva, goddess of wisdom.
Owl breeds shown within the Harry Potter books include the eagle owl (large, tufted and fierce-looking, owned by Draco Malfoy); the Little Owl (tiny, cute, but perhaps not very impressive, like Pigwidgeon, owned by Ron); and the Snowy Owl, which is also known as the Ghost Owl (Harry’s Hedwig).
I made a few elementary mistakes when it came to my depiction of Hedwig. Firstly, Snowy Owls are diurnal (ie, they fly by day). Secondly, they are virtually mute, so Hedwig’s frequent hoots and chirrups of approval and comfort should be taken as signs of her magically enhanced abilities. Thirdly, as countless well-meaning owl-lovers and experts kept writing to me in the early days, owls do not eat bacon (Hedwig enjoys a bit of bacon rind when she delivers post at breakfast).