Hello folks,
I've been with Zico and Maradona for 29 days today, we'll soon complete a month together. However I've been starting to doubt Maradona's gender for a few reasons. I'd like to check with more experienced budgie-owners if I'm correct or just tripping. I'd like to note that the pet shop owner told me they were about 1 to 1,5 month old when I got them and that they were surely both males. So here goes.
1- When I got them, they both had very light bluish-lilac cere. However as time passed, Zico's cere got way more blue, while Maradona's remained a light blue, almost white cere.
2- Zico vocalizes a lot, every morning when I put their cage on the window he immediately starts chirping and singing and squawking. Maradona does it too, but only a few minutes later and way less. Also his (her?) chirps have a very different tone from Zico's.
3- Zico seems to be clearly bigger than Maradona.
4- Maradona recently decided to take possession of the nesting box, and doesn't even let Zico peek inside when he (she?) is in there. Could this be some female nesting instinct or something?

Here's a photo of them two that I took yesterday night when I began wondering.

Hope anyone can help me figure out this mystery. My sister even suggested renaming Maradona as Madonna, if he is really a she lmao.
Thanks everyone.