The eBird Top100 (e.g., World, Iceland, ABA Area, or 2013 in Hidalgo County, Texas) is quite simply a display of list totals directly from an eBirder's "My eBird" pages. Top100 is updated nightly (see the "last updated" time at the top right) so your totals may not be current immediately after entering a new bird.
Please note that it is possible to view the Top100 for both species totals and checklist totals. We especially encourage competition over the checklist numbers, since eBird really benefits from large amounts of data. If you are at the top of the Top100 for complete checklist submissions, please accept our congratulations and thanks! It is important to understand that the checklist totals are for complete checklists--ones for which you answered "yes" to "are you reporting all species". **This is one of the most important questions in eBird, so please make sure you understand it.** Your "My eBird" pages will display your total number of checklists, but the Top100 is meant to incentivize reporting complete checklists (hence the disparity between Top100 and your "My eBird" pages).
It is also very important to understand that this is meant to be a fun and personal style of output, and is thus quite different from other data visualizations in eBird. The list totals here are not vetted by our data quality system and it is therefore possible that some people's lists will include typographical errors, incorrect taxonomic species selections, and even misidentifications.
Although every record submitted to eBird is of value and goes through a data quality system to identify potential errors, we use this for the scientific output (e.g., maps, bar charts, arrivals tables etc.) only. eBird does not do any "policing" of individual's lists, so the totals seen here are simply the precise totals from an individual's account.
It is also important to stress that the Top100 is intended only for single-person or single-group accounts, where all of the people birding were together (e.g., a single team traveling together for a big day), or for single observers.
The percentages shown are calculated from the overall list of validated sightings in eBird. On rare occasions, it is possible to have a list of greater than 100% if your list has some birds that are still awaiting validation.
Please enjoy the Top100 and watch your ranking improve as you submit more data and make eBird more a part of your daily birding routine!