We depended on the ducklings and chicks to tell us, although if there's just one, I would err on the side of caution if it doesn't have any siblings to share heat with. The other thing would be to make certain there is no possible source for a draft. because that can cause them to loose body heat a lot faster than cooler temps alone.
We'd check on them every few hours through the night and early morning to see if they were moving further from the center of the heat lamp's beam, or closer to it. By that means we were able to gradually gauge how quickly we could move the lamp back so that it had less and less felt impact.
The move outside is kind of the same idea. Out for 15 to 20 minutes a few times a week, then every day and a little bit longer, til finally, once they are fully feathered - which starts happening really fast - they're pretty much fine all day. By the time ours were probably about 8 weeks old they were happy running around the yard most of the day. When they'd get tired they'd just plop down in some shade and take a snooze. Also depends on whether you have water available for them to play in. We have Mallards and started them out with a shallow bowl when they were about two weeks old. They don't have any of the oil in their feathers yet at that age, so you have to be careful not to let them get a chill, or stay wet too long, but they love it. Plus it supposedly hastens the development of the glands that release the oil which makes them "water proof". Anytimw it was sunny out we had a small rigid pond liner that we would fill and put a ramp on for the little pigs to climb up into. After a good swim they were like our kids and would flake right out they'd get themselves so tired.
It's said that they'll get used to anything and call it routine, and I'd have to say that's true. The ducks in particular seem to be much more habitual in their behavior. Our three were raised in a 75 gal. aquarium when they were young. We had some egg crate in the bottom aand would change the pine shavings every day, and they had water and food available at all times (no easy task since they love emptying waterers as fast as you can fill them. I finally rigged ours rigidly so that it hung about even with their backs and they couldn't tip it up and empty it. Funny thing is, they now have a duck house and a covered 1200 sq. ft. run filled with raised bed gardens that they can play in and spend the night, plus they have the yard and several ponds available all day, but they still prefer the little 3' long pond liner that they grew up playing in, even though there are plenty of others with waterfalls and fountains and little fish that they could resort to. At the end of the day, they also still waddle up and knock at the back door rather than walk up into the run. It's hilarious. If you leave the door open they'll just walk in and stand in front of the aquarium quacking their heads off til ya put 'em back up in their big fish bowl. They're getting better about staying outside at night, but they clearly have a preference for coming back indoors.
They were also raised where they would be out playing in the yard and running around under my feet while I was finishing their run and planting the gardens, so they kind of expect someone to stay out in the yard with them all day. If I run inside to use the computer or anything, they'll follow me back to the house and stand outside the door quacking at me til I come back out (like right now). It's pretty darn funny. They fly real good now, but they just fly to get from one level of our yard to another (it's all terraced hillside), but they never go beyond the fence lines... it's pretty bizarre. Now they take off, fly a tight circle and then crash land into their pond like kamikaze's. I could watch'em all day.
I'd seriously consider getting another for company 'cuz they are definitely gregarious, and they will imprint on the strangest things. Plus having more than one allows them to help keep each other warm when they're still feathering out.
