While looking through checklists on eBird, you may have noticed that some have the "Nocturnal" designation. What exactly does that mean? Whenever you submit a checklist, there are specific GPS coordinates associated with the location of that checklist. Our system automatically looks up the times of sunrise and sunset at that location, and identifies a checklist as nocturnal or not. A checklist is flagged as nocturnal based on the start time – whether the checklist starts earlier than 40 minutes before sunrise, or later than 20 minutes after sunset.
When submitting a checklist, if you enter a time that falls during the nocturnal period, a warning will appear that it is Nocturnal. This is to try to prevent any unintentional AM/PM errors or accidentally typing 03:00 instead of 13:00 in 24-hour time.
Because of the definition above, some counts that begin more than 40 minutes before sunrise or more than 20 minutes after sunset will still be in a period of high bird activity with substantial daylight. Obviously, the length of twilight periods (either civil twilight or astronomical twilight) vary with latitude and by time of year. The above method does not perfectly categorize checklists, and should be viewed primarily as an AM/PM double-check (not used for data analysis). If you started a list before sunrise or after sunset and feel the Nocturnal designation is incorrect, please just ignore it.
If you are intentionally submitting a nocturnal count, and were focused on counting flight calls from migrating birds at night, see this article. Please be sure that if any type of observation continues past midnight that you start a new count, so that birds are recorded on the correct calendar day.
If you are submitting a checklist that is Nocturnal in part, but extends into the daytime, we recommend that you keep separate lists for the Nocturnal and daytime periods.
**Note: The service that we use to generate these flags on occasion does not take into account Daylight Savings time. As a result some lists may appear as "Nocturnal" that do not match the conditions described above, and some may not show as "Nocturnal" that do match the conditions for a Nocturnal checklist. If you find this happening with your checklists, please email us.