Thanks so much. I think you may be right and it was an overheating issue. I have adjusted the lamp and opened the window to ventilate the room a little.
Thanks for the advice!
We bought 4 3-day old chicks on saturday as a surprise for our kids. We have them in a tub inside with a covered heat lamp, shavings, water and food feeders. They have been happy until now, scratching, eating, drinking. Suddenly one took ill this morning and within an hour, died. Then another...