please do NOT feed the baby. As humans, food is comfort for us, so when we find something hurt or suffering, we always want to feed it to make it feel better. In this case, it is a very bad idea. Herons are federally protected, and while you may fall under the good samaritan law, you only have 72 hours to get that baby to a permitted rehabber. If you do not, you are in violation of federal law.
A rehabber knows what they are doing, and they can help that baby far better than you can (and most importantly, they posess the necissary permits to do so). One thing that rehabbers will tell you is one of the worst things you can do is feed that baby. Most of the time, what people choose to feed can actually do more harm than good. Especially when you don't know what kind of shape that baby is in internally. Most injureds, especially babies, are severely dehydrated, and feeding them anything could stress their system and kill them. The very best thing you can do is get that baby on a heating pad, or under a heat lamp, see if it will take some pedialite at the very most, and get on the phone, and find someone that knows what to do.
This is a picture of a baby squirrel that someone wanted to "make better" before calling us. It was fed impropper (undigestable) foods, and nearly died. PLEASE get this little one to a rehabber asap. The sooner he gets to someone that can help him, the better. Please trust me.
The whole point of a wildlife rehabber IS to make the animals better...you wouldn't want your accountant performing surgery on you, please, don't try to do the rehabber's job for them...I know you are trying to help, and that is absolutely commendable, but in this case, it can most certainly do more harm than good. I am not trying to be unkind, but right now, a life hangs in the ballance, and you may make that animal's road to recovery much harder by holding on to this baby rather than putting it in the hands of a good rehabber.