You might think this is a piebald sparrow, or partly leucistic as it is also called.
This white colour has a different reason, not suitable food during the nestling time. No substance to give the feathers structure and colour.
You can also see how the tips are well formed and then go more fuzzy and colourless. Points to a nest period feed by the parents and then found by a well meaning human.
The structure that keeps the feather together to hold it together so it can move air, is basically non existing.
And I had sparrows who still had colourful feathers, but bars breaking up the structure, those bars can be seen especially good in the tailfeathers, which means they will break in the matter of a few weeks where these weak bars run horizontal through the feathers.
Often these birds also have no water resistance to their feathers, so a rain will seriously drench them and .,…they die because they are unable to fly, but also because they will cool out.
Healthy feathers are shiny, coloured well and without any bars or *frizz*
The bird will look fat and fluffy.
Even the better looking body feathers are sparcer than they should be, giving the bird a rather scruffy look after a few days hopping around.
It will take at least 6 weeks for the feathers to fall out and grow anew.
But with these birds, often there is organ damage from the wrong food and this sparrow is a good size smaller than he should be.
He needs a reptile heat lamp to keep warm because the dunes are already breaking off and have been already sparce to begin with.
Sparrows, like other songbirds, are feed insects as hatchlings and only get feed predigested plant material shortly before they leave the nest.
Only a few finch species are feeding their nestlings more plant material after the first few days.(And pigeons are like the only vegetarians)
Cat food, dog food, oats, nuts and seeds are not food for baby song birds.
Pure, dried insects, ground down into powder, made wet with a bit of water and mixed with cooked egg yolk so the feed balls hold together, are something rehabilitators use when no fresh insects are around.
Swifts are even more sensible and can stop proper feather growth even after one wrong meal.
So yeah, if you are a rehabilitator still using the kibble method, please stop.
I know that is still done in some places, and people do not see the birds die after a few days or weeks when released, so they think this method is fine.
But really, really looking at the feathers and the organs does show, it is not fine.