The world of Dragon Cave is governed by some of the same ecological laws that apply to our world. If there are too many dragons of a specific breed, then competition for resources will be fierce, and dragons of that breed will be less likely to produce offspring.
On the other hand, if a breed is underpopulated, then their population will grow rapidly without natural checks and balances. These breeds will be more successful at producing offspring.
What does this mean for you? Well, this natural behavior is something you will need to keep in mind when hunting for eggs and trying to have your dragons breed. If you’re trying to find an egg from an overpopulated breed, your search will be more difficult than when trying to find eggs of a less plentiful breed.