Tube feeding a chicken is about the safest most fool-proof things a chicken keeper can do. There is pretty nearly nothing that can go wrong. Even a child can do it, and this past spring, two kids on this forum successfully saved the lives of several hens in their flock by tubing this life-saving solution over three days.
You've got your feeding kit. Good. Cut the bottom of the tubing off at around nine or ten inches to make it easier to manage. Hold it up to the hen with the bottom of the tube at the bottom of the crop. Make a mark at the spot close to her beak. This will make it easy to judge when the tube is far enough into the crop.
You will prepare a solution of one-half cup warm water with one teaspoon Epsom salts. This will be tubed into the hens' crop in its entirety. You will do it two times a day for three consecutive days. Do it when the crop is at its emptiest. In fact, massage any crop contents so they leave the crop so there's plenty of room for all of the solution.
Now, what most people fear most - the part where you insert the tube. But this is very easy as the chicken has a direct channel going from the right side of the throat right into their crop, by-passing the airway so there is zero chance of aspiration.
After wrapping my chicken securely in a towel to confine wings, I hold my chicken on a work bench with my weak arm. With that hand I pry open the beak holding it open, and with my strong hand, I insert the tube in her right side of the beak, going slightly under the right side of the tongue. This channels the tube right into the esophagus which goes directly into the crop. You can see this in the photo below, only it will be the tube and not the syringe.
The first time if you hit the wrong hole your chicken will start to cough. It's okay, no harm, just back out and try again. Once the tube is in, the chicken will be calm and comfortable. Then you can begin feeding the solution. It feels pleasant to the chicken, and most are very cooperative throughout the process.
If the chicken struggles and bucks all of a sudden, it's not because of anything you did. They get bored and tired of the process like a toddler would. Pause until the chicken settles down again and finish. The whole process takes no more than five minutes. Each time you do it, it gets easier and quicker.
In between flushes, let her have her regular feed and plenty of fresh water. It may seem that this might stress a chicken, but most will tolerate it well, and even appear to be energized by it.
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