Table 1. Elastic-net multiple regression model testing the association between HA sequence diversity and alternative hypothesized mechanisms.

Hypothesis	Test Variable	Coefficient
Host immunity	Amino acid substitution rate	−0.995
Host diversity	Host Shannon Index	0.111
Geographic structure	Fixation index, (FST)	−0.285
Mutation rate	Nucleotide mutation rate	0.276
Transmission ecology	Environmental durability	0.687

The associations tested (see Text S1, section S4 for additional analyses) are between subtype-specific average and (I) strength of selection (amino acid subsitution rate), (II) Shannon Index of bird species identified in genetic sequences, (III) geographic population structure (F_ST), (IV) nucleotide mutation rate, and (VI) environmental durability (Rt). All variables have been standardized (converted to a mean of 0 and a variance of 1) to have directly comparable coefficients.