How can I convince my dad to let me get some degus?

Well, I've wanted some degus for ageeees (for those of you who don't know, degus are a type of rodent) and I've done all my research about them, and have decided they're the perfect pet for me! The only problem is, my dad objects. He offered to buy me a degu last year but I said no because I knew nothing about them. Well, my birthdays coming up and there's this adorable pair of degus at our local shelter, so I asked him whether I could get them. He now says I already have too many pets! (I have a guinea pig and 2 chickens which are well cared for and loved, I'm not getting degus because I am 'bored of my animals' which I'm not by the way, I just would like some degus :)) I know it's his house and his rules but when he was younger he used to keep racing pigeons, rabbits, mice, rats, guinea pigs, cats, dogs, and a few chinchillas. I would be paying for everything, so it's not the cost that matters. I'm just confused as to why he doesn't want them, he admitted himself that he loved degus when he offered to buy me some! Please help me come up with ways to convince him! Oh, and no nasty comments please , or stuff like 'it's his descision' and stuff, I know it's his descision, he can say no if he wants :P I just want to try to convince him :) answers greatly appreciated thankyou :D