Does a double-egg yoke make two chicks?

Patricia called in to ask "if you have an egg with two yolks in it, will you get two chicks if it's fertilised?" Animal expert Sophie Mowles from ARU gave the answer...

Sophie Mowles - No unfortunately not. Well actually very happily for the chicks. So what's happening there? You usually get double yolk eggs from chickens, when they are new to laying and it's a mistake that they've made so they've overprovisioned an egg. They're often in very small eggs as well they're coming from pullets. So when the chicken makes the egg they're creating this yolk that's going to be all of this money it's full of fat and resources that's going to allow the developing embryo to grow. Now it's a mistake that they've made because they're not used to that process yet, and they've overinvested in the egg, but when the egg is working its way down her tract, it will be fertilized by one sperm from the rooster. So you would not get two embryos.

Chris Smith - So they're not two 'fertilizerable' sort of precursors in there in the same way, as a human egg could actually split and make two or you could have two human eggs and get two sperms and therefore have two non identical twins in that case you can't do the same with a chicken egg, you can't split the embryo into two and end up with two developing chicken eggs in an egg.

Sophie Mowles - I'm fairly sure you can't. I've never known this to happen in birds. However, in reptiles these sort of mistakes do seem to happen and this is why you seem to have a lot of very strange photos occurring of double headed snakes, two headed turtles, it seems to happen more often than would be predicted by chance. I mean, it is a chance event and so it seems to be that you can create twins in reptiles but not in birds