Does anyone else have a cocktiel who behaves like this?
Sure do! about 4 of them and over the years about 35 of them they are self appointed guardians to another tiel and the cared for tiel by them is almost always either a senior or disabled or special needs tiel
Sven is a protective guardian and caretaker to Squeakers. I've lived with this natural and instinctive tiel flock dynamic for 22 years now. It's very natural and to be expected. Give poor Squeakers a break from Sven by spending one on one time with Sven away from Squeakers. This is how I would apply D&D, dilution and diversion therapy to this situation which works. I'd work with Squeakers to create a less intense bond. Senior/disabled cockatiels have been my joy in caring for, for over 20 years now. I've watched how they truely care about each other and care for each other. When I've had this issue of overly clingy which in my experience is almost always either 2 boys or a boy and a girl I just start D&D and in time the caretaker isn't as clingy and the offended or only tolerant senior or disabled tiel being cared for isn't bothered and actually likes the care taking once it's dampened down a bit. I've never had issues with 2 senior girls with one being a caretaker and the other not liking it or only tolerating the care taking.
Bringing seeds might happen someday or preening areas on him which he either cannot reach due to his bad back probably will happen. I've had caretakers actually dip up water and splash on the beak in an attempt to give water despite the water cup only two steps over from the very senior or disabled tiel! and with the senior or disabled tiel drinking many times a day.
The care some tiels give to another tiel is heart warming and I think they really do care about each other. I've watched enough native tiels being filmed by hidden camera in their native Aust. and even in the wild they do caretaking meaning the 'old folk' tiels all care for each other, bringing food, preening each other all over and all the rest. I figure if they do it in the wild I won't interfere in my living room.
D&D may work and I'd give it a try. Tiels are very underrated in the complexities of their behavior in captivity and are actually very sensitive, intelligent and complex just like their larger cockatoo cousins.
Sven may also be so very, very clingy to Squeakers due to being housed with the budgies. Sven may see Squeakers needing more care and time as he may see the budgies as a potential threat to Sven. If you are inclined, you may try a holiday of placing S&S in a large cage right beside the budgie aviary and see if Sven's very clingy caretaker behavior lessons to more normal which Squeakers will tolerate well and even like. Based on my experience it will but of course each tiel and each home is different. I'd do a say, decent size 20'' or 24'' square cage for just S&S as if they are not interested in flight that's large enough to me and place their cage so the top of the cage and the top of the budgie aviary are equal. This will let Sven see that the budgies are not above them in height which is very important for Sven to relax and not be so clingy on caring for Squeakers.
I'm not an expert at all but in my years of housing budges and tiels and I have always had senior or special needs tiels, the caretaker tiel is less clingy and not bothersome to the cared for tiel if they are housed just the two of them when there are other parrots in the home and especially if they are housed with other non tiel parrots.
You see the budges FLY! and these two don't so Sven could be seeing this as not just a caretaking issue but also one of protection from potential bother from all these fast flying budgies living in the same house. My experience has showed me that tiels protect tiels when housed with budgies by simply being near each other and doing for each other and when they are not housed with budgies (or finches) the caretaker tiel relaxes and isn't as clingy to the 'needy' tiel. Tiels are so peaceful (most of the time) that some house them with budgies or finches or doves. But tiels are sometimes inwardly frightened and protective of each other by being caged with higher activity birds such as budgies (geriataric budgies exempt) or finches. Again, in my experience in flock dynamics of interspecies being housed tiels protect each other by being very clingy but I'm not any expert and each home is different.
If Sven ever starts to regurgitate to Squeakers he will need to be watched closely as once in a while is ok but daily is bad for Sven's health. It can happen but is more likely to happen in caretaker hens but can happen in caretaker males. Also it can help, if you are consistent as in daily, to hand Squeakers a plain Cheerio or other sugar free treat a couple times a day for Sven to see there is someone else that cares for Squeakers. I've completely eliminated over clingly caretaking behaviors by housing both together alone without other birds and simply handing the cared for tiel a plain cheerio in the dish once or twice a day which they gobble down or any food s/he likes. I've seen the look on the caretaker tiels face like 'okay..... this is good' and they are not as clingy. Sven must know Spueakers is safe from other younger, faster birds from potentially bothering him, know you are a reliable and consistent feeder by seeing you place in his food cup near him each day goodies which Squeakers eats, and having their own cage with the same cage top height as the budgie cage. It's also been my experience that once any tiel has become a caretaker they will never stop so I work with instinct and not against it and since they do this in the wild I don't try to break it up.
Just my .02