Knowing your chicken and its behavior will help you spot problems early on. You should really handle your chicken every week and check it over for injury and mites. Disclaimer: I am no expert, the following is what I personally do, but I accept no responsibility if you follow it. Additional information and articles on chicken health can be found in the poultrykeeper Poultry Diseases and Other Health Problems section.
Chickens have one exit point called the vent. This link show how eggs and droppings are separated within the chicken so that eggs are sterile when layed. The Digestive System of a Chicken breaks down the food that chickens eat both chemically and mechanically by using grit to grind down their food. Nutrients are absorbed within the digestive system before waste is expelled from the vent. This link shows the flight feathers in a chicken. There are primary and secondary flight feathers, the former which will need to be trimmed on one wing to unbalance the chicken in flight and reduce the possibility of escape. Chickens with full flight feathers can fly up to 5 or 6 feet, the height increasing as the weight of chicken decreases down to bantams. The crown/crest and wattles become redder as the chicken matures, if they are pale it could be down to illness. The legs of a chicken turn from yellow to white as she matures and can be an indicator, along with crest colour of when she will lay an egg. The body becomes less slim and fills out, and the hips widen so the legs may seem slightly further apart. The chicken starts to squat shortly before laying.
Once a year most breeds will go though a heavy moult, where all their feathers drop out and the new ones start to form. During this process does look some what unhealthy but it is all part of the natural process of being a chicken, and is not something to worry about. The moult typically happens during August/October, but if it happens a bit sooner or later don't worry. Not all breeds go though a complete moult (i.e. the loss of all feathers), some (mostly hybrids) tend to have a mini-moult where only a limited number of feathers drop out and are grown, this process is much quicker than the full moult. While moulting chickens don't lay eggs, the moult typically signals the end of the laying period for that year and the chicken will not start laying until early the next year. The moult behaviour varies in different breeds so what's described here will not fit all chickens.
Very small red mites, or northern fowl mites can live in crevices in the chicken house and come out at night to feed on your chicken. Mites can be found in the feathers, or sometimes on the legs manifesting in scaly legs. Red Mite live in the house, not on the bird - usually they are only found at night underneath wooden roosting bars, if you wipe the underneath of the bars with a white tissue and see streaks this indicates blood from mites. This Red Mite page gives you lots of ideas on how to get rid of red mites. They can be treated with powder on the chicken and roosting area, and Microshield poultry shield is a cleaner which eradicates mites and can be used as a preventative measure. I use diatom pressed onto the ends of the roosting bars which is natural and works by breaking parasites open on a cellular level. This blog post on How To Get Rid of Red Mites explains how to clean the house out and use diatom.
If you are unsure as to what to look for, there are some good close up photos on this Red Mite page.
Lice if present can be found under the feathers crawling on the skin. Poultry shield as with mites can help prevent parasites. Lice can be treated with powder on the chicken and roosting area. We have used lice/mite powder, diatom and deosect which we made into a spray, just follow the directions. We found the spray easiest but treatment is very unpopular with the chickens. You can treat at intervals to prevent lice, or check regularly for organgy brown creatures crawling around if you separate the feathers. Lice can live around the eyes, around the vent, under the wings in particular. You need to treat very thoroughly on the skin at the base of the feathers. There is a 14 days wait between treatments of deosect. Trinny had scraggly tail feathers when she had lice, so scraggly/missing feathers is a good indicator. Chickens can catch lice from wild birds. A dust bath can really help birds to naturally rid themselves of mites and lice and dusting powders like Diatom can be added to the dust baths.
Worming chickens every six months prevents a large number of potential worms species infesting your beloved chicken. With gapeworm the chicken opens their beak wide as gasping for breath. I play cautious and worm early when I have seen this, although hard to say if actually gapeworm or stretching their beak. Flubenvet powder can be mixed in with their food every six months, or even 3 months for 7 consecutive days. It can also be mixed into treats and fed to individual chickens, or mixed with a little water and syringed down the throat to make sure the chickens get the dose. Verm X is a herbal solution given for a few days as pellets mixed with the food, or in the water, but is much more expensive. With Verm X and flubenvet, eggs can be eaten through treatment. Panacur is given to cats/dogs and has a withdrawal period, but is not technically licensed for chickens in the UK. It may be prescribed by a vet, 4ml of the 2.5% suspension is syringed down the throat. Note if you syringe incorrectly you can get the airpipe and effectively drown the chicken. However, if you add about 120mms of thin flexible plastic tubing to the end of the syringe, this allows you to pass this down the throat and bypass the entry to the airpipe - with practice this can be a speedy way to get fluid based medicine etc down bird. Useful way to ensure bird does not dehydrate when appetite has gone. We've found you can add drops to the beak (also to the side of the beak, absorption by capillary action seems to trigger swallowing) and the chicken drinks it, or you can add to a favourite food. If necessary, prod bread pellet etc. past back of tongue to ensure swallowing.Chickens should not stay on the same area of ground for longer than a year (spread of disease) unless there is covering on the floor that can be changed e.g. wood chips, which are better than bark chips which go mouldy. Some people use aubiose, a horse bedding. Whist worms are killed during treatment, the eggs are not and many eggs will be picked up directly from the ground and indirectly from snails, slugs and earth worms so it is important to practice good husbandry techniques as well as treating for the prescribed length of time. Different worms hatch at different times (between about 2 and 5 weeks) so it is important to repeat treatment before the worms are old enough to lay more eggs if you suspect a heavy infestation. Here is some more information on worming chickens.
Garlic, apple cider vinegar (or ACV) and diatom are also suggested to help with worms/internal parasites. You could also get a worm count to see if worming is successful, if there may be a worms problem, or to see if treatment is needed if you would like to avoid worming. A sample can be sent by post to a poultry lab - for example the cost of a Worm egg count (worm eggs and coccidia) in a faeces sample is currently (Oct 09) £12.26 for small private producers and small backyard flocks at Minster Veterinary Practice. Here is a list of poultry vets in the UK that can test samples as well as veterinary laboratories that can do this.
The loss of a nail or a minor scrape may happen e.g. if introducing new hens and the pecking order is disturbed an antiseptic e.g. teatree oil can be applied. Wound powder, (or even flour!) can be used to stem bleeding. We found blood from the vent which the vet advised was probably from straining to lay a soft egg. remove blood from injured chickens as it can encourage pecking/cannibalism from other chickens. Anti-peck spray may be needed.
The crop is part of the digestive system and forms a bump on the chickens chest when the chicken has been eating food all day, if blocked the lump will be present in the morning. Hay or long grass is a classic trigger for this. Massaging it/dropping oil down the chicken's throat/visiting a vet may be needed. You can feed bread soaked in cooking oil as it is easier than putting down the chicken's throat. We took Trinny to the vet and they fed her water to soften it. They said using a tube to empty the crop could cause diahorrhea, which is recommended by some people, an operation may be needed if the crop does not go down. Trinny had some stretchy material tied around her front to put pressure on the crop. Here is a picture of pendulous crop which occurs if the impacted crop is not cleared.
Feels like a balloon full of water. This can be caused by the chicken just having had a big drink, or the crop not emptying. It may be from a distended crop where the crop is stretched irreversibly. The chicken may also have weak muscles and be prone to crop problems.
Sometimes an egg may not be laid and remain in the chicken. This is called egg binding. The chicken may hunch up, walk funny, perhaps look like they are straining. I'm sure Katy's eyes watered a bit too. Soft eggs can be harder to lay as they don't just slide out. You can hold the chicken over steam (not too close) to warm the muscles and help pass the egg. It should be for about 20 minutes and repeated if it doesn't work, but I've not lasted that long. I find you need to get a kettle of water ready, get the chicken in position over a bowl, wrap with a towel, add boiling water, find a point that is warm but not too hot (as judged by my hand holding chicken). Lower as the water cools, and replace if it cools too much. An egg bound hen will deteriorate quite quickly so it is important to act fast.
The chicken may also take a few weeks or months to start laying from the point of lay stage. Short daylight hours in winter, illness (commonly worms or lice/mites), and moulting can stop egg production. Young chickens may also lay smaller eggs than normal. Chicken's eggs increase in size over the year after the Winter break.
Chicken can often get soft eggs where there is a lack of calcium in the diet, it is not being absorbed, or they have the incorrect diet. Shellstim (commercial supplements) can be added to the water, but not indefinitely as it affects absorption. Eggshells can be baked in the oven, cooled, crushed and mixed with the food. Apple cider vinegar] is also thought to increase calcium absorption. Vets can also give a calcium injection, presumably if the above don't work. We've found shellstim to work after a few days. Vitamin D3 (Cholecalciferol) is used in order for a chicken to metabolise calcium and phosphorus as well as to form egg shells. If Vitamin D is not present in the diet then this can also cause soft egg shells. If they are fed the correct formulated layers feed, they should be getting sufficient Vitamin D. If scraps only are fed, they are unlikely to be able to produce sufficicient Vitamin D in their bodies.
Caring for a sick chicken
If your chicken is ill you may need to separate her from the flock and keep her warm in a cardboard box/suitable pet cage/holder inside your house. If she is separated you can monitor how much she is eating/drinking and what her stools are like which will be useful if you have to take her to the vet. We sometimes syringe feed water with some dissolved sugar, especially if they have the runs as it seems to pep them up a bit and helps with dehydration.
When to call the vet
You should take your chicken to a vet (a cat carrier / basket is ideal) if you suspect there is a serious problem. Chickens can go downhill very quickly, they have a habit of hiding illness until they are suddenly very ill. If you suspect they have a problem, (but are not in a serious condition), do not ignore it. The first thing to do is to find out as much information as you can about the symptoms - the internet is a great place to start - there are many articles in the keeping chickens section of the poultrykeeper site with articles about some of the common illnesses chickens can have. The poultry forum is a good poultry forum to ask questions, there are many experienced poultry keepers visiting this forum and there are FAQ sections for chickens health, waterfowl health and so on.
Ultimately, you are responsible for your birds...
Without wanting to put you off keeping chickens... in the UK you have a legal obligation under the Animal Welfare Act 2006 which states that ...
"...you commit an offence if you don't take steps as are reasonable in all the circumstances to ensure the needs of an animal for which you are responsible are met to the extent required by good practice."
So please don't hesitate to call a poultry vet if your birds are ill and you are not experienced enough to know how to treat them.
Here is a list of poultry vets in the UK. Not all vet will deal with chickens.