Leviticus 1:14

These birds were appointed for the relief of the poor who could not bring better. And these birds are preferred before others, partly because they were easily gotten, and partly because they are fit representations of Christ's chastity, and meekness, and gentleness, for which these birds are remarkable. The pigeons must be young, because then they are best; but the turtle-doves are better when they are more grown up, and therefore they are not confined to that age.