The emergency information described below is for immediate baby bird care only whilst you contact your local wildlife rescue for further help and advice.
Unfortunately, it's becoming increasingly common for members of the public who have found a baby bird to decide to 'have a go' themselves, without the faintest idea what they're doing. Additionally, they will search the Internet and find some cobbled-together nutritionally unsound diet, which leads very quickly to growth problems or death, so please NEVER try to rear a baby bird yourself.
Once you have found a suitable container to house your baby bird in, line it with either a towel or absorbent kitchen paper. Darken the container to help reduce any stress, and place it in a quiet area away from any children or pets. If the bird is either a hatchling, or a nestling, it will be necessary for you to make it a cosy nest. This can be done by either using a plastic pot lined with a soft cloth, or a towel twisted into a nest shape, please DO NOT use cotton wool.
Warmth is absolutely vital, particularly if the baby bird is a hatchling or nestling. Supportive warmth can be provided by using either a hot water bottle wrapped in a towel, or a plastic drinks bottle filled with very hot water, again wrapped in a towel. The wrapped hot water bottle can be placed under your housing container, and the drinks bottle can be placed inside your housing container, alongside the nest. It's VERY IMPORTANT to check that the baby bird is NOT becoming overheated, or is placed directly on a heat source, as this will result in either burning to the delicate skin or death.
Please DO NOT attempt to feed, or give fluids, to your baby bird unless you have been advised to do so by an experienced wildlife carer. A cold baby bird will not gape for food and, if the incorrect diet is fed, this can quickly result in the death of the bird. NEVER feed a baby bird bread and milk! There is NO species of bird that feeds its young on milk, and bread can very easily block a baby bird's digestive system. If fluids are given incorrectly this can easily end up in the bird's lungs resulting in its death.
Please do the right thing by the baby bird you have just found by taking it to your nearest wildlife rescue. They are very experienced in rearing and rehabilitating baby birds, so your bird will have the best chance of being released back into the wild where it belongs.