We define two tau functions, τ and τ^, on moduli spaces of spectral covers of GL(n) Hitchin systems. Analyzing the properties of τ, we express the rational divisor class of the universal Hitchin's discriminant in terms of standard generators of the rational Picard group of the moduli spaces of spectral covers with variable base. The function τ^is used to compute the divisor of canonical 1-forms with multiple zeros.