Exposing chicks that still have mostly down and no feathers to the direct sun causes them to absorb too much radiant heat. Just as they have no insulation against cold, that also goes for heat. Until they get a good amount of feather coverage, they need protection from direct heat as well as cold.
Clue: if you stick a thermometer out in the direct sun, what happens? It zooms way high even though the air temp may be much lower! That's what happens to a baby chick with no feathers!
It's also why you need to place the heat lamp at the far end of your brooder, so the chicks can move away from it if they absorb too much heat. But they can't get away from the huge heat lamp in the sky unless they have some shade handy!
Clue: if you stick a thermometer out in the direct sun, what happens? It zooms way high even though the air temp may be much lower! That's what happens to a baby chick with no feathers!
It's also why you need to place the heat lamp at the far end of your brooder, so the chicks can move away from it if they absorb too much heat. But they can't get away from the huge heat lamp in the sky unless they have some shade handy!