The problem with 'X'!
Simply put, when you record an 'X' on an eBird checklist it means that the bird was present in unknown numbers. In other words, an 'X' is at least 1, but could be 1,000,000 or more! The reality is that your best estimate of numbers will always convey more information about the birds than an 'X'. This is especially important when you have a list that mixes counts with an 'X' since it is hard to use that checklist for abundance analysis if even one species is not counted, since the relative abundance of all species on the list cannot be understood.
Why record numbers?
All data submitted to eBird are valuable for certain types of basic analysis. But in order for your eBird submissions to be used for the highest level of scientific analysis, it is important that you begin to provide estimates of numbers for each species you encounter on your birding events. By doing this you add significant value to your submissions. A complete checklist with numbers for each species allows us to begin to study relative abundance, which in turn will allow us to analyze population trends in birds at a variety of spatial scales--from your backyard all the way up to the continental scale!
What if I'm not sure about my estimates?
Many birders do not record numbers because they are unsure of their ability to count birds accurately. At eBird all that we ask is that you make your best conservative estimate of the numbers of each species you've detected. You don't need to be exact. The fact is that every count made by birders is an estimate of some kind; we rarely if ever make exact counts of all the possible birds present in a given birding situation. As such, analysts looking at numbers are keenly aware of the limitations of assuming these are exact counts, and adjust their models accordingly.
Estimating numbers in difficult situations
In order to help you feel more confident about providing numbers on your eBird checklists we've put together an article that describes general practices about how to count birds (Bird Counting 101), and one that provides guidance for counting birds in difficult situations (Bird Counting 201). We hope you enjoy these articles, and don't hesitate to contact us with questions about how to count birds.
Situations where 'X' is appropriate
When eBird was designed, the idea of allowing the inclusion of 'X' was meant to facilitate the entry of historic data where numbers for birds were lacking. If you have old checklists of birds from your past birding events that only have 'X's, we still want those, and using 'X' is appropriate in this case. Do not try to estimate numbers for past checklists, rather start from this point forward and submit complete checklists of birds with numbers for each species. By doing that you're adding tremendous value to your data, and to the eBird dataset as a whole.
Let's go count some birds!