Why do I lose my whistle register as the day goes on?
I recently discovered how to access my whistle register, which brought my range up from 3 octaves to nearly 4 (yay!). Usually my whistle is strongest in the morning, presumably because my vocal muscles are loose and relaxed, but as I practice singing throughout the day, eventually I lose the ability to hit the whistle notes. I'm doing my best to stay relaxed and practice without any strain, vocalizing with an open throat, etc., but it seems like without fail, eventually as the day goes on the whistle goes bye bye. Is this common when you first learn how to get to it? And will it eventually get strong enough to stick around permanently? Any tips on how to strengthen it?