For rare/notable sightings, eBird requires additional information. This is a part of the eBird data quality process, and an essential part in making sure that eBird sightings are useful for other birders, researchers, and conservationists.
If you report something notable or rare with eBird Mobile, you will see a 'confirm' checkbox next to these species. This is a visual indicator that your are recording a rare/unusual species, or unusually high counts of common species. eBird requires that these observations have supporting information in the 'Species Comments' field to help ensure that the sighting has enough documentation to be included in the eBird database.
To add details to an observation using eBird Mobile, tap on the rare bird in your list, which will open up the comments field and let you add some details. Guidelines for how to describe a rare bird can be found here. While a full description is often difficult to add in the field, especially using a mobile keyboard, it is often very helpful to include how similar species were eliminated, add notes about relevant field marks, or indicate if you were able to get a photo to support your observation. Once you've added some details to the species comments, click OK and go back to your list. Be sure to check the confirmation checkbox before submitting your list.