My budgie is mourning his friend and won't go back in his cage.

If Harley is used to having a friend I think it would be a good idea to introduce a new buddy for him. Budgies are social birds and having a friend may help with his depression over the loss of his previous one. Make sure to introduce the birds slowly, and that both will have access to food, water, perch spaces, etc without competition to ensure it goes smoothly.

I'm sorry to hear about your loss! It is very normal for birds to mourn the loss of their friend, just as people do, so spending a little extra time together or offering up a favorite treat may help. Birds are extremely good at hiding illness, and if you suspect your other bird was ill at all, thoroughly cleaning the cage/shared dishes/etc can help prevent the spread of illness. Signs of illness to watch out for include listless behavior, sleeping all the time, fluffed feathers, difficulty breathing, changes in weight, appetite changes, drinking changes, feather loss, etc. If you do see these signs, or suspect Cameron may have caught something from your other bird, seeking out an exam in person is always best. If you're having trouble coping with the loss of Barney, I highly recommend checking out some resources such as The Rainbow Bridge where you can talk to other owners and even make a virtual pet memorial.

I think Jim may be enough so you're not overcrowding the cage for now. Birds, like people, can experience grief, so Jolly is likely missing her friend Joy. You are doing the right thing by keeping her company and giving her a little extra love an attention. If she doesn't perk up, or seems lonely, even with your other bird there, adding a new friend may be a consideration, but it may be best to give her some time first.

Poor Greenie, though bonded birds do often show signs of grief after losing a mate, not all do. While you are not seeing signs of grief in Greenie, it can be extremely difficult to determine whether a bird might be interested in another cage mate. In the wild, birds often do pair up and many of them live in large flocks so it makes sense that many house birds would want a friend, however, in the wild birds (even bonded mating pairs) do not live in such close quarters as they do in cages in our homes. Because of this, I always tell my bird owners that if they are considering adding another bird to their family, prepare for what you might do if the two do not end up getting along (you may need space for two cages, as well as twice the number of cages, bowls, toys, etc.). Even if two birds will ultimately get along I recommend owners introduce their birds on neutral territory (do not stick the new bird immediately into your other bird's cage). Spend a few days with the two birds getting to know each other outside of a cage, try different scenarios (feeding, toys, bathing, etc.) and always be on hand to make sure if they do not get along, you can be available to keep everyone safe. If things go well you can introduce them into a more neutral cage, such as the one the new bird has been in for only a few days, for short periods of supervised time, separating them at night. After a few days, if this goes well you can try them in the cage you want them to be in together long term. The key is slow a introduction, patience, and lots of praise and rewards for good behavior. Good luck, I hope this helps!

Birds, just like people, can mourn the loss of a friend, however diarrhea and changes in eating/drinking habits over a prolonged period of time are not normal. If your other budgie died suddenly from an illness, and Rocco now has loose stool, it's always best to have a vet examine him and perform a fecal on a stool sample to make sure there isn't an underlying illness causing his symptoms.

Noticed you posted twice so I'm reposting my answer her to make sure you get it. It's possible Kicolina might be wishing for a mate. If she seems down, adding in a toy that she can snuggle against such as a soft nest or preening toy, or even adding in one of the little "fake" parakeets that sing may help her feel a bit more comfortable. Interacting together and offering her some favorite treats may also help. If she continues to act depressed, or shows any signs of illness, a vet checkup is always a good idea to rule out any health issues.

Give Buddy some time! It takes awhile for birds to get used to new surroundings. Make sure he has many perches, toys, and fresh food and water available, and try to leave him alone for a few days to adjust to his new environment.