Some observers are confused by the meaning of the question: "Are you submitting a complete checklist of the birds you were able to identify?". We want to find out whether you are reporting all the birds you were able to identify to the best of your ability. In other words, answer "Yes" to this question when you submit more than just the highlights of your birding event, and try to note every species present. We realize that all birds are not identifiable and user abilities vary. You should always answer "Yes" to this question unless you are purposefully excluding some species (e.g., European Starlings) from your checklist. You do not need to count all the individuals you observe to answer "Yes" to this question. A complete checklist indicates all species observed, and while estimates of numbers are valuable, they are not necessary to submit a complete checklist. We do, however, strongly recommend estimating numbers for each species on your list. Your best estimate always tells us more than an "X", which could be 1 or it could be 1,000,000!
When eBird users submit complete checklists of birds it allows us to do a variety of interesting things. We can show where a species is found and where it hasn't been reported. We can calculate the chances that a species will be found at a given time of year in certain locations. We can also begin to understand detectability, in other words, how easy or hard it is for birders to find a certain species. For instance, in North America, it's much easier to find singing Grasshopper Sparrows in spring than it is to find them when silent on the wintering grounds.
This Long-tailed Tit map above shows the power of reporting all species. The gray shaded areas show where birders have submitted complete checklists, but have not detected Long-tailed Tit. These are different from the areas that are not covered by any color; these represent where we do not have enough complete checklists submitted to determine whether Long-tailed Tits are found there. From this map we can see that Long-tailed Tits are found across much of southwestern Europe. We can also see they are not found on Mallorca. But we can't determine with confidence how far down Italy they go, or even if they're present in Northwestern Spain, because we don't have enough complete checklists from those regions.
Submitting complete checklists of birds allows eBird to realize its full potential and creates better lists for its users. Everyone likes highlights and the rare and unusual birds they see on their outings, but don't forget the common birds. By submitting data for all species you're collecting important information about all species, allowing us to make informed decisions about the next generation of rarities or species of special concern. Please report all species on every checklist you submit!