Carrier Pigeons in the Mists

[Wolfglide of the Fraternity]

Last but not least: Homing pigeons are second to 'Sending' spell. Sending can go anywhere and won't be intercepted. Homing pigeons are not needed in places with a reliable number of clerics or mages. Most domains of Ravenloft have clerics that can serve those rich enough to send pigeons. Not all though. So there could be some use of homing pigeons, if a druid or ranger trained them.
Speaking of training them: Animal messenger is even lower level than sending...

On the other pinion, would a sending spell find someone in a location warded against scrying and divination magic?

Yes; sending is an evocation spell, not a form of divination. Some sort of antimagic field would work, but otherwise there isn't much that can stop a sending. Since domains are effectively separate planes, the spell is subject to a 5% failure chance between domains.

On the other hand, does Ravenloft have a magnetic North and South for the pigeons to home in? Do compasses work in all the domains?

Yes, Ravenloft has a magnetic field. In the RLDMG, on page 210, there is a magical compass, and the entry mentions that such an item is overpriced and unnecessary in Renaissance domains, where nonmagical compasses are more readily available.

[LeMaxul]

Well, if polar/magnetic tracking seems too convoluted, what if the pigeons in the misty domains don't use magnetic tracking, but have learned to follow the sound of rivers? (Maybe they're different birds that do that irl). Beings and "things" pass from domain to domain by swimming through the Musarde, right? So, maybe it's not weird that (certain) birds have learned to fly over it?

That'd allow for some messenger birds to be commonplace, albeit mostly useful near rivers and tributaries that flow from one domain through to another.

[Baron Von Stanton]

Well, if polar/magnetic tracking seems too convoluted, what if the pigeons in the misty domains don't use magnetic tracking, but have learned to follow the sound of rivers? (Maybe they're different birds that do that irl). Beings and "things" pass from domain to domain by swimming through the Musarde, right? So, maybe it's not weird that (certain) birds have learned to fly over it?

That'd allow for some messenger birds to be commonplace, albeit mostly useful near rivers and tributaries that flow from one domain through to another.

Birds are visually oriented animals: when they need to, pigeons locate rivers via sight or magnetic fields.

[alhoon]

We are not sure HOW pigeons do what they do, but in a fantasy world, does it really matter?
What I mean is that when tests were conducted, altering the magnetic field of an area confused pigeons of variety A but didn't affect much pigeons of variety B. Covering and altering the features of the area confused some pigeons and not others. Etc.

BUT... this is a fantasy world and actually an artificial one, not one that was made by something and then let to evolve magico-naturally. It is a piecemeal world added piece by piece by the Dark Powers as needed to troll a specific individual.
Thus, what pigeons can and cannot do and how in our world doesn't have to affect how the pigeons of Ravenloft act.