Bird was born in Guildford, Surrey in the United Kingdom into a "solidly professional middle-class" family, as the third of four children of Heather (née Gage) and Graham Bird, both of whom are economics professors at the University of Surrey. Bird's three siblings are Alan, currently a Deputy Head at Brighton College; Anne, a doctor in New York City in the United States; and Tom, a medical student at the University of East Anglia.