If you have established the pattern of answers beginning with a "B", then you will know that 10a begins with the letter B. There is a set, repeating pattern.
If you have an idea of the name of the bird you are looking for, that will tell you what letter 10b starts with. Each new bird starts after a line of C's and also follow a set pattern.
If you use the Chambers website given earlier by smouse, all the answers are among the lists. If you have a Chambers Dictionary, you can check the meanings.