The spirit cobra's special move ophidian incubation will transform a single egg into a cockatrice egg, at the cost of 3 special move points. Oph. incubation scrolls and spirit cobra pouches can be difficult to buy on the Grand Exchange, so you may have to make your own using crimson charms and snake hide. If you are around 63 Summoning this may be a good use of your charms anyway. Eggs are generally easy to buy, but they have a buying limit of 1,000 every four hours.