First of all, that thing is adorable. However, you either need to find where it came from or be prepared to take care of it. If not, go to your local wildlife rescue and they can take care of it. The mother rejecting the baby because of scent in birds is an urban legend; it will accept it back if you found out where it came from. Baby birds like that need to be kept warm; you should take it inside for now if you can. Find a box, get something soft and place it in there somewhere warm. It should snuggle up. This one appears to be old enough to at least hop, so it has a lower chance of catching pneumonia.
Secondly a little one like that can't eat those sunflower seeds, it's too hard for them. This one doesn't appear to be just a nestling as it has feathers and what not. Some species eat mainly fruit and others insects. You can also
crack open sunflower seeds, and crush them for the bird. Try to identify the species ideally before feeding it. Make sure you leave a little container of water for it somewhere; but if it can't drink soak it's food or it'll get dehydrated. Never force water down their throats, most who do this actually drown the bird. I'm not sure what species this is; but don't feed it anything too hard or too manmade/processed. Look around for any birds like it, they might accept it and it can go back into the wild or if night doesn't fall leave it on the ground if there is no danger, the parents will feed it on the ground. Bring it inside for the night, it will get too cold.
You can try and identify the bird
here. Once you've identified your bird, protein eating birds can be fed a mixture of baby bird food blended with meal or earth worms. Fruit eaters can be fed crushed fresh native berries like blueberries, blackberries,and raspberries. Most pet stores carry baby bird formula mix. If it won't eat gently spoon feed it, do not force open it's mouth or force it down it's throat.
On another note that poor thing looks kind of damp and cold, better get it warm.
Hope this helps!
I don't know much about caring for baby birds, just make sure you do some real research and maybe call some wildlife organization.
