I've noticed lately Cosmo has been sitting sort of hunched over, but whenever I come near to take a picture, she straightens up. She is pretty quiet also, she has been ever since I got her. Is she sick?
Some budgies will not adopt a straight up position when they are relaxed and resting on their perch. The amount of singing will also depend on the budgie's personality, if Cosmo has always been on the quite side she is likely more shy and reserved. Also recently acquired budgies may not be so keen on showing their singing skills so soon and will only do so after being perfectly at ease and acclimated to their new home.
If Cosmo has been eating/drinking well, you don't notice a steep decline on her energy levels throughout the day, she's not constantly fluffed up and her poops look normal, then all should be well with her.
RIP sweet Tito (Summer 2008 - January 17th 2013).
You are missed and never will be forgotten.