These maps are most appropriately thought of as depicting the geographic distribution of migrants at counting or capture sites within North America. Interpretation is unfortunately limited in some regions due to lack of sites, especially during spring migration. Comparisons among species and between spring and fall for the same species are also limited both because of the distribution of numbers the scaling of proportions. It should be noted that individual sites are only very coarsely comparable. Such information will be useful in judging proposed sampling frameworks for migration monitoring and are pretty nifty in their own right.
Sites represent different collection techniques.
Species have different probabilities of being detected or caught among sites. Proportions were used and were not standardized by effort. Each map was standardized to the largest proportional count for that species. A background X indicates a counting technique was used at that site. Few sites collected information on the same schedule of dates and times.