Can I send my Owl familiar on guard duty?

Question

I have the spell Find Familiar which partially reads:

You gain the service of a familiar, a spirit that takes an animal form you choose.

Your familiar acts independently of you, but it always obeys your commands.

This question suggests that your familiar will try to follow your commands to the best of its ability, but if the command is too complex for the simple mind of the animal to understand (DM decision based on their Intelligence) they can't follow through on it.

Would it be reasonable for a summoned Owl familiar to be on guard duty while my PC is asleep? Circling around the sleeping place or just sitting on a tree nearby and trying to wake up my PC when something out of the ordinary happens?

I could imagine this command to be worded like this:

Have a close look on our surroundings and if you see sudden movement or signs of other creatures like fire or sounds try to wake me up.

Things to consider:

1. The familiar obviously needs to sleep as well (as suggested by this question), this would happen the day after when my PC is awake by sending it to its pocket dimension for a reasonable time (8 hours probably).

2. The familiar is probably not able to determine what exactly is out of the ordinary so it might happen that it wakes the PC up for things that after a short investigation turn out to be nothing of big interest. Would these interruptions be too much for the rest to still be considered a long rest following the rules?

3. The familiar and the PC have been travelling together for a long time. The familiar is not really a wild animal anymore but dependent on the PC and has probably been trained to a certain degree like a pet. It will be rewarded for keeping guard and waking up the PC for positive reinforcement.

4. The owl has an intelligence of 2 which is not exactly to be considered overly intelligent. Comparing to real life this fits the fact that owls are not to be considered clever, even among birds. However there are trained owls that can at least do basic things like flying to certain places and returning in a given time-window.

5. As nwp pointed out the familiar is not actually an animal but a celestial/fey/fiend instead of a beast. Could this impact the intelligence? The spell description states the familiar has "the Statistics of the chosen form" which would include Intelligence.

Answer 1

Simple answer: yes

While your familiar is within 100 feet of you, you can communicate with it telepathically. (From spell description you cited)

Before you go to sleep, you can communicate clearly enough to leave the owl instructions to be alert and then do something if it detects something. (Wake me up, hoot as loudly as possible, etc). The text you quoted states that it will obey your instructions. Since it can act independently of you, you can be asleep and it can be awake.

Your familiar acts independently of you, but it always obeys your commands. (same citation)

• Quite frankly, your points 1 through 5 are an over-complication. If your table / DM goes into that level of detail and verisimilitude, then this particular detail needs to be discussed or resolved with your DM via the DM's ruling on how this works. It's best that you clear this up with your DM before your next play session if you want your owl to keep watch for you.

But there's a catch. The owl is not guaranteed to detect danger.

From the Basic Rules, DM, p. 39 (owl's stat block, excerpted)

Owl
STR 3 (−4) DEX 13 (+1) CON 8 (−1) INT2 (−4) WIS12 (+1) CHA 7 (−2)
Skills Perception +3, Stealth +3
Senses darkvision 120 ft., passive Perception 13

Keen Hearing and Sight. The owl has advantage on Wisdom (Perception) checks that rely on hearing or sight. (Emphasis mine)

While it's likely that the owl will detect danger that can be seen or heard, since it has darkvision and advantage on hearing/sight Wisdom(Perception) checks, a monster or NPC approaching by using Stealth may foil the owl's efforts at detection.

The owl may or may not detect danger, depending on whether or not the stealth check is successful as compared to the owl's perception check. If the stealth is more successful, then having the owl on watch won't result in an alert. This is the same risk as having a PC on watch: they may still get snuck up on if the approaching creature's Stealth score beats the PC's Perception score.

Experience

In our first campaign, my brother's wizard used an owl familiar that he assigned to help keep watch while he was trancing/sleeping while another character (like one of the 3 humans) was on watch. On a few occasions this helped alert us to approaching danger.

Answer 2

I can't see a good reason why this wouldn't work. It seems pretty obvious a guard-owl is going to recognise approaching enemies and act somehow.

The spell describes the familiar as being a spirit which has taken an animal form. This is open to some interpretation but it seems reasonable to say it's effectively an owl with some added weird stuff.
Not so much quacking like a duck as twit twooing like an owl. Or barking like next door's dogs. Regular mutts seem to have no problem warning when strangers approach. Geese also manage this. If regular animals can do it then a familiar ought to be able to.

This thing isn't very bright but it takes instructions so long as it understands them. There's some mental link so when you tell it to watch out for six guys in plate it will presumably indicate confusion back to you. Because an owl doesn't know what a guy is, what plate mail is and it can't count. You need to communicate in terms an animal is going to understand.
Danger is easily understood by animals, warning each other also seems reasonable animal behaviour and staying somewhere is pretty simple. All in all "Stay here and warn me of any danger" seems like a command an int 2 owl familiar is going to understand OK.

It's unclear whether this mental link can be initiated by the familiar. Maybe it can and the owl just thinks "danger". You wake up. Maybe it can't and it has to sit on your head or something.
That aspect could be clarified with your referee.

I can see some possible downsides.
Once you decide the familiar acts like an animal then they're prone to distraction. Not being very bright has down sides. If a mouse happens by then the owl could well go hunting for a while.
If you intend it sitting on a branch for 8 hours then maybe it's going to forget your instructions after 4 hours. If something happens, an owl is kind of small.
It could well take longer to wake you than a human guard.
Maybe it doesn't experience any time whilst in it's pocket dimension so you need to let it sleep some time.

Maybe it's not a great plan to just put an owl on guard duty all night.
There again guard duty is not exactly the stuff of heroic legend and unless it suits the story line for something bad to happen you'll probably be OK.