FIG 4.

Influenza A virus hemagglutinin (HA) subtype diversity from migratory waterfowl sampled during (A) summer (June to August), (B) fall (September to November), (C) winter (December to February), and (D) spring (March to May) between 1976 and 2015. Each pie segment represents the proportion of the respective HA subtype (color-coded) isolated in the respective location and season. Size of circles are relative to the number of samples collected at a location. An annual trend is evident of progressively increased HA subtype diversity during southbound migration, peaking during winter.