My week-old baby chicks are sometimes trembling?

I have three baby chicks that are about a week old. Two are Mille Fleur (spelling?) and if I remember correctly the other is a production red? Anyway, I'm worried because when i hold them, they feel a bit like they're trembling, off and on every few seconds. I know it's not their breathing. I'm wondering if they're not warm enough? I have a 40-watt bulb about a foot above their cage at all times. Help please? I'm happy to provide additional info.