Fig. 1.

The number of volunteer observers contributing data into the eBird database in April of each calendar year. Plotted is the number of unique observer ID values, regardless of whether these observers were associated with a unique observation event or with groups of observers collectively making observations (i.e. eBird “shared checklists”). Note that the ranges of years plotted on the x-axes differ between the graphs, because the active promotion of eBird to local bird watchers began later on the Iberian Peninsula than in North America. Also note that left and right y-axis scales on each graph are not identical.