Sacha writes

I've seen several people remark over the years that they have yet to
find any choked to death nestlings on the ground or anywhere else.
Nonetheless, we do put peanuts in such feeders and still continue to
put out seed and the birds go at them like nobody's business. I don't
know if it's the adults taking stuff to feed their young, or if they're
just topping themselves up when they get a spare moment. Certainly
they give the impression of being rather harassed!

The BTO reports that garden feeding supports greater densities of small
birds such as tits, but that their breeding success is lower than in the
countryside, which they ascribe to the nestlings being fed on insects.
So that even though the parents are feeding at feeders for themselves,
they are still seeking insects and caterpillars for their young, and
these are often not at high densities in the urban garden.

So it would seem that the feeders are good at maintaining health in the
adults, but are a last resort for feeding young. The RSPB is making the
point that it is difficult for us to understand when there are food
shortages, and therefore when the adults will resort to feeding their
young from feeders, and therefore, to be on the safe side, don't feed
anything that might cause a problem if fed to the young (even though it
probably won't be fed to them)

I must admit, I hadn't seen the thing about not putting out fat in
spring and summer, and rather assumed that it would be as important in
spring and summer (breeding and moulting) as it would be in winter.
--
Kay