Darla and buttercup both sing, it's quiet most of the time like they're singing to themselves and it's cute little tunes. Definitely not flock calling in any way. Sometimes it sounds like they're muttering to themselves as they walk around and Sometimes they'll get really into it and sing a little tune louder and flap their wings or hang upside down. They only ever do it one at a time, never singing together. When I told a vet nurse about it she said it's really weird for girls to do that and so did pet shop employees (not that i put much faith in pet shop employee opinions to be honest). Just wondering if other peoples females do this also?