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What is frequency?

"Frequency" is the percentage of checklists that report a species within a specified date range and region. This is the most common way of displaying the eBird data and provides a good idea of relative occurrence and seasonal timing. Frequency is used is on the eBird bar charts and the key to the bar widths can be seen here

These bar charts are for Central Park, New York. They show migration timing and give a good idea of relative occurrence. For instance, the first Common Yellowthroats appear around the 2nd week of April, along with the earliest Yellow Warblers which increase in the third week of April. In Fall, Yellow Warbler is a relatively early migrant with peak frequency the first week of August, while Blackpoll Warbler is a later migrant with the peak in the last week of September and first week of October.

By clicking on the species name on the bar chart, you can view frequency on a graph where the y axis is frequency and the x axis is the date range. This provides additional detail that cannot be seen from the bar charts. To gain the best understanding of occurrence, be sure to explore the other tabs on this page where you can also view abundance, birds per hour, average count, high count and totals.

How can frequency be misleading?

Do not confuse frequency with abundance or the "number of records" for a species. Frequency is the simple percentage of checklists that report a species, and there are times when it can be misleading, particularly when a rare species was seen by many individuals. Widely observed rarities can be seen by hundreds of people and reported on thousands of checklists, all from a single site. This is an especially large issue in underbirded areas that attract a lot of birders to see one very special rarity. However, as more checklists accumulate over time, frequency values will drop if the species does not occur again. 
These bar charts are for Arapahoe County on the southeast side of the Denver metro area of Colorado, USA. They provide a good example of the ways that rarities can create misleading frequency values. The Ross's Gull was a single individual bird seen for several days in November 2010. It caused great excitement and many birders converged on Cherry Creek Reservoir to find that individual bird. The result is that over 12% of checklists from the third week of November report this species, even though there was only a single bird present one year.

Mew Gull is a rare but regular species at Cherry Creek Reservoir, and also often "chased" by many birders. The resulting bar chart may imply the species is more common, if you forget that these bar charts are simply the percentage of checklists that report that species. For that reason, we recommend clicking on the species name to explore frequency and other measures of status.

Looking at the frequency graph shows that Mew Gull is quite scarce and reported on barely more than 3% of checklists even in November. 

Looking at other tabs, gives an even better idea of the status of Mew Gull in Arapahoe County. Here we see the average count for Mew Gull in Arapahoe County. Average count is the average number of birds reported to eBird when the species is seen. In the case of Mew Gull in Arapahoe county, most of the time, only a single individual was seen. In late November it appears that there were two birds, but even then people usually only saw one bird. If you were to click on high count, you would see the high count is two for this time period. In addition to the tabs, you can also click on the map to see individual checklists that recorded that species. Using these additional metrics helps to understand frequency and interpret it correctly.

Bar charts and frequency displays in eBird are a good way to get an initial impression of status, but should be combined with other output in eBird including maps.

eBird Targets also makes use of frequency and the same caveats apply. In these cases, the frequency is expressed as a percentage with 5 decimal places, which helps to distinguish between subtle differences in areas with lots of data. When you get toward the bottom of the list it can be misleading to view this as what species are really most likely. For example, for eBird Project Leader Chris Wood's November Targets for Arapahoe county, two very rare species appear in the Top 5 most likely additions. Both Prothonotary Warbler and Ross's Gull were single individuals that remained for long periods of time and were successfully twitched by many birders (Chris wasn't among them). Over time these frequency values will drop.




Complete checklists

With every checklist entry, eBird asks if you are reporting a complete checklist of the species you saw or heard. This is important for frequency because frequency values can only be computed from complete checklists: if you report only Mew Gull on your checklist from Cherry Creek Reservoir, and mark it a complete checklist, the is as though the thousands of Ring-billed Gulls that the Mew Gull was flocking with were not even there. It is very important to understand this question and use it correctly. To mark a checklist complete you must first make an effort to detect all species present, even if you are only surveying for a short time (1-2 minutes is OK as long as you have had time to look and listen for species in the area). Second, you must confirm that you are submitting everything that you were able to identify in the area. If you are submitting only highlights, this would not be considered a complete checklist.
Read more about the importance of complete checklists. Please make an effort to report complete checklists whenever possible.

eBird output still acknowledges records that were submitted on "not complete" checklists. On the bar charts, this may appear as a very thin line (see Cape May Warbler in mid-December in the first example above). This shows that the species was present, but it is either very very rare or only reported on incomplete checklists. Similarly, on eBird grid maps, the palest color indicates that the species has been recorded, but is either very infrequent or has only been reported on incomplete lists. Finally, eBird Targets may occasionally show a value of 0.00000. This is not unexpected, and these are species that have only been reported on incomplete checklists; anything reported on at least one complete checklist will have a non-zero frequency value.




 
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