Other people have touched on this, but it is important to understand the difference between cleaning and disinfecting. Different products and techniques can be used to achieve different degrees of cleanliness. Natural wood is a rather difficult surface to properly clean. Not all approaches are equal.
First, let's define our terms so there is less confusion:
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Cleaning removes germs, dirt, and impurities from surfaces or objects. Cleaning works by using soap (or detergent) and water to physically remove germs from surfaces. This process does not necessarily kill germs, but by removing them, it lowers their numbers and the risk of spreading infection.
Disinfecting kills germs on surfaces or objects. Disinfecting works by using chemicals or other methods (steam/uv light/etc) to kill germs on surfaces or objects. This process does not necessarily clean dirty surfaces or remove germs, but by killing germs on a surface after cleaning, it can further lower the risk of spreading infection.
Sanitizing lowers the number of germs on surfaces or objects to a safe level, as judged by public health standards or requirements. This process
works by either cleaning or disinfecting surfaces or objects to lower the risk of spreading infection.
Sterilization describes a process that
destroys or eliminates all forms of microbial life and is carried out in health-care facilities by physical or chemical methods."
Source:
IntroductionDisinfection & Sterilization Guidelines | Guidelines Library | Infection Control | CDC
When you gather branches or pieces of wood from the wild to use as natural perches, you are concerned about a few things - mold/mildew, disease-causing bacteria from wild birds, insect life, and harmful chemicals, like herbicides/insecticides. Removing the bark and cleaning with soap and water will help remove surface-level contaminants, but it isn't going to sterilize or disinfect the wood. To get the wood really clean you will need to properly disinfect OR sterilize the wood AFTER clean away surface debris. When mixed to an appropriate concentration, bleach is a good disinfectant on clean nonporous surfaces. F10 is another proven disinfectant that can be used as an alternative to bleach. In contrast, even undiluted white vinegar is not an effective disinfectant against many harmful bacteria. Likewise, strong alcohol does not kill all germs you will want dead. It is not a good alternative to using bleach.
If you want to disinfect something, you must use an appropriate disinfectant and be sure to mix it to the proper concentration AND allow the product to remain on the surface long enough to disinfect. When reading instructions, this is called "dwell time" or "contact time". If you just spray bleach on a surface and wipe away immediately, it doesn't have enough time to properly disinfect the surface. Some germs will survive. Be sure to read and follow all directions when using a disinfectant.
Unfortunately, chemical disinfectants are NOT that good at cleaning natural wood perches because wood branches are frequently rough and porous. This creates tiny cracks and crevices where germs will hide to escape destruction because the chemical agent can't reach them. One way people try to get around this is by soaking in bleach for several hours or overnight to allow the solution to penatrate deeper into the wood. Another option is baking. Baking wood is a form of heat sterilization. If it is done correctly, it should kills ALL bacteria, insects, and mold spores, even if they are hiding inside the wood or under the bark, but it will not necessarily remove surface contaminants, like pesticide sprays, so be sure to select wood from chemical-free areas, when possible, and wash the wood before baking.
Several people also mentioned leaving wood out in the sun to dry. This will kill some mildew/mold or bacteria due to the combination of heat and UV light in strong sunlight. Just be aware that this is not the same as baking since it only affects the surface directly exposed to the light and it will not kill most insects.
In my opinion, the best method for properly sanatizing wood is to clean with soap/water and a wire brush to remove debris and surface contamination, then bake the wood to kill everything else. This should give you a piece of wood that is free from harmful organisms. If you can't or don't want to bake your wood, soaking in bleach is a safe alternative, just prepare the solution away from your birds, following proper directions, and allow the wood to dry completely before putting it into the cage. Once dry and free of bleach odor, the wood is safe to use around birds.