This has come up before during a webinar on parrots and hormones presented by Dr. Stephanie Lamb.
Here is what Dr. Lamb replied:
Yes, I have seen birds that routinely gain weight during the breeding season and then lose it afterwards. This is something that should be happening in a normal cycle if she were in the wild so it’s not abnormal at all for it to be happening in captivity. The reason it happens is because in the spring time when hormones are high the reproductive tract increases in size substantially. The extra weight is actually just an enlarged oviduct and ovary. The same thing happens with males and they will gain weight because the testicles increase in size.
Thank you for asking Lafeber,