In Scotland, if your parents were unmarried at the time of your birth, but could have legally married at that time, if they marry later on, it makes you legitimate. A new birth certificate can be issued with the father's name added and the date of the parents' subsequent marriage.
If Donald's father, Hugh Campbell had married Annie Cameron later on, she would be described on Donald's marriage certificate as "Annie Airlie, formerly Campbell, M.S. Cameron" or words to that effect. If she is called Annie Cameron without "M.S.", then she didn't marry Donald's father.