Now this neck stretching and her comb changes are major worries here. This is because it is a sign of respiratory distress and compromised oxygenation in this species. I am glad to see that we can rule out airway irritants (since there was no species listed in your original post), but this means we have a number of other issues to be worried about. As well, many of these carry risk to any other birds in contact with her. Specifically, these signs fit with issues like Infectious Coryza, Acute Fowl Cholera (Pasteurella multocida), Influenza, ILT IRT, infectious bronchitis, Chlamydiosis, and mycoplasma (Mycoplasma gallisepticum). Furthermore, since she has neck extension we also have to be worried about swelling, discharge, possibly infectious material or worms blocking up the throat. In that case, we'd need to be thinking seriously about localized throat based disease due to Trichomoniasis (canker), Fowl pox (wet form causes canker lesions in the throat) and Syngamus trachea (gape worm) infestations. And of course as the only bird affected, foreign bodies or tumor partial obstruction in the throat.
Now if she is this depressed, then we do have to again worry about advanced disease but if she is breathing at a normal rate while so subdued then hopefully we can get this under control before this progresses any further. To start, you need to get her away from any other birds to reduce risk of spread. If you are comfortable doing so, you may consider having a peek down her throat to rule out you gapeworms. You may be lucky and see them or one of the other causative agents (ie the plaques of Fowl Pox or discharge of canker). If you can't see anything, since you can only look down a chicken throat so far, then you can try the "Q-tip test". To do this, you need to place the bird in your lap, gently open her beak, and swab a Q-tip down her throat (twirl it as you do this). Twirl as you bring it back up, and if she's got gapeworm, you'll see thin, red strings on the q-tip. This way you will know if this is the cause. But if you end up with a cheesy discharge then canker or pox would be higher on the differential list. And if there is any other discharge, the bacterial causes would be suspect. And once you have identified what is present, you will be in a better position to know if you are treating them appropriately.
Otherwise, further measures to pinpoint the causative agent and increase your treatment success, you do want to consider involving your vet. They can listen to her lungs, examine her airway, remove any caught items, and help you determine the disease local and therefore rule out some of these agents. Furthermore together you can collect some respiratory secretions from this bird to be cultured. This will tell you what agents are present and what treatment will actually clear them . As well, you might consider having a fecal exam (even though the diarrhoea might just be a secondary issue) performed as well to tell you if parasites are playing a role (directly or via compromising the immune system).
Furthermore, once you have samples for culture (which will tell you what drugs any pathogenic bacteria present are vulnerable to), you can consider a broad spectrum treatment to try and tackle as many of the bacterial causes as possible. There are a range of options that would include erythromycin, oxytetracycline, or macrolides. Other options may be tilmicosin, tylosin, or spiramycin.
Otherwise, supportive care is that key facet that we need to make sure you are addressing. This means we need to give her ease each of food/water and tempt her to eat and drink. We also want to keep her somewhere warm and dimly lit to reduce stress and support her thermoregulation. If you think she sounds congested in the upper airway and trying to clear it, then consider a bit of steam treatment here. You can achieve this by putting her in a carrier in the bathroom while you run a hot shower. Or if you have a nebulizer/humidifier you can set up a wee steam tent (by putting her into a carrier and covering it and the humidifier with a bed sheet). This can just help reduce some of those airway clogging secretions and help her to breathe easier.
Overall, there are a range of agents that can be to blame for the signs you are seeing with wee Freya. Therefore, isolation was the first step but now you need to initiate the above supportive care, monitor her breathing, and take some of the above diagnostic steps to determine which is to blame for her signs. If you narrow down the differentials but cannot identify a cause (ie if its one of those more subtle ones or is lurking in the lungs) or she is struggling with that neck extension and cyanosis of her comb, then it would be ideal to follow up with her vet as soon as we can. That way we can give her the best chance her but also determine the root cause so we can keep the others safe.
Just in case you do need an avian vet and do not have one already, you can check where you can find one at near you at AAV (http://www.aav.org/search/), Avian web(http://www.beautyofbirds.com/recommendedvets.htm) or Birdsnway(http://www.birdsnways.com/birds/vets.htm).
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