The trouble with 'X' - Why you should always estimate numbers!

June 3, 2009

White Tern c. Brent Stephenson

Over the years at eBird we have always been flexible about allowing users to report either numbers for each species, or to provide an 'X' to indicate presence. In our latest round of analysis, however, we found that checklists where users estimated numbers for ALL species encountered were of significantly higher conservation value when compared with those that contain 'Xs', or even a single 'X' mixed in with numbers. In this piece we'll talk about why recording numbers is important, as well as provide some guidance as to how to estimate numbers in difficult situations. Read more about why making your best estimate of numbers always trumps the dreaded 'X'!

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'.

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, and 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!

Team eBird