Hi bushg,
in your case the birds could not have been put back for the reasons below so dont worry about it .. you did the right thing
if the nest is easily accesible then they can be placed back in the nest, depending on species of bird some are more temprementle than others, so may reject a very young chick and throw it from the nest if it has been touched by humans, however if the parents are off the nest it is usually quite safe to add the little feller back at any stage of development, however if it fell from a gutter the nest would not be easily accesible and taking it to a wild life rehabilitator would have been the best thing to do ..
if the chick has no feathers then it has either been thrown from the nest this could be for one of many reasons ie illness (parents can detect this you are right

) or it could have been thrown because the parents want to start breeding again or it could have been thrown by another bird trying to steal the nest site..
or it could have accidentally fell, if the parents claws are to sharp sometimes they can catch them when leaving the nest and accidentally pull them out of it ,
however if the bird has a lof of feathers even though it is small the chances are that it has fledged and the parents will be feeding it from above, sometimes the chicks are caught up by cats but this is nature... the parents will keep an eye and try to drive off smaller threats ie cats and magpies , but they wont come down when people are their, they will try to hide the chick but as with all animals and people they dont like doing as they are told and do tend to venture out into the world, but do try and hide again in nearbye cover at signs of danger...
if you watch a baby bird that has feathers you will see that every now and again it will make calls out to the parents to let them know where it is so they can feed it
however if a bird is injured it is usually best to either leave it be for nature to take course it will either heal or it will die

unfortunately not a lot can be done but with more serious injury like broken bones as you described above the bird can be took to a wildlife rehabilitation center, their are quite a few of them around and they will do all they can to help and rear the bird...
hand rearing is not as simple as what it sounds and even after 20 years of doing it we all still have accidents or make mistakes... some ending in fatality, some are to weak to make it through the fight and stress of bieng somewhere strange....
if you feed a baby bird the wrogn way it can kill it in seconds or even linger a death on with illness for days by causing pneumonia or impaction.....
this is why it is always important to leave the job to somebody who knows how to do it properly and is trained in that specific area.. as much as we try and help by trying to feed them often this can end up with death
as mentioned above some will have to be tube fed and others dont have to be, it differs on age and species of bird,
if you cant get a baby bird to gape (open its beak wide and beg) the chances are that by trying to force it eat will kill it as they will not have the swallowing reflex that shuts off the tube to the lung ... if the bird gapes well it is possible to feed them either with worms or by soaked dogmeat... the worms fed from hand the dogmeat fed by tweezer or spoon...or even a syringe ..
a parent will reject a baby bird that is ill or very very weak, this can happen at a young age as soon after birth or it could be noticed late on, either way they will stop feeding the bird and/or throw it from the nest and let nature take its course, as the bird doesnt know what medication it it cant help and some or most of the illnesses can be very life threatening, contagious or cause severe deformitys that would make it unsuitable for adulthood..
i dont know if this confused anybody as ive lost track of what i wrote lol but if you want me to try explain further on any part let me know and i will do
katie