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Understanding the eBird Bar Charts

We are pleased to announce improvements to the eBird frequency histograms--or as they are more popularly known--the eBird bar charts. These charts represent how bird occurrence changes over the course of the year and are pulled directly from eBird data. Each month is broken down into four periods (each approximately 7 days). Then we calculate the percentage of checklists reporting a species, but only those that report all species. The wider green bars show the periods when a species is least likely to be missed, while the narrower green bars show when species is present (or sometimes present), but infrequently detected. It is possible to explore data by state, county, or Bird Conservation Region (BCR), a birding hot spot, a personal location...even your backyard! You can do this for all years, a range of years, or a single year! We also give pointers on how to PRINT the bar chart, a common request.

What Has Changed?

Those of you that have been using the eBird bar charts for years may immediately notice the three big differences:

  1. More widths: While we previously had five different line widths, we have now expanded that to nine different line widths.
  2. The gray shaded areas: Previously, we indicated periods with less than five checklists with a narrow gray bar. Now we indicate periods with NO checklists with a thick area of gray shading.
  3. The super-narrow: A very fine (pencil thin) line width is now being used for the extremely rare birds. This is the line width we use for species reported less than 0.3% of the time but this is also the width that is used for species that are reported on checklists that do not report all species. For such reports, we cannot calculate frequency at all (since we do not know what other species were or were not there).

How the Bar Charts are Calculated

The bar charts display species frequency (not abundance). Frequency is a measure of the percentage of checklists that record a given species. Observations that do not report all species are excluded and in these cases any such observations are marked only as present (the narrowest green bar)--this is one of many reasons why we stress that you should report all species whenever possible.

Frequency calculations are made for each of four periods within a month simply by dividing the number of checklists with a given species by the total number of checklists for that period. When you click the Download Histogram Data at the bottom of any bar chart you can see the exact frequencies that have been calculated as well as the sample sizes for each period.

Since most months do not divide equally into 7-day periods, any remaining days are added to the last period. Thus, while the first, second, and third periods are all seven days, the final period ranges from seven to ten days, depending on if the month has 28, 29, 30, or 31 days. The fact that the final period is consistently longer does not seem to bias the results strongly, but please do keep this in mind as you explore data using this tool.

How to Interpret the Bar Charts

The below example shows data from the western tip of St. Lawrence Island at Gambell, Alaska, for all years. Paul Lehman's intense coverage there and regular eBirding has made these bar charts pretty accurate during fall migration, at least: we have 182 fall checklists (ranging from 63 in late August to just 3 in early October), but only 42 from spring. (If you've been to Gambell and can add some complete checklists, please do!)

The numbers on the image are discussed in detail below:

  1. The gray shading here indicates that there is no data for this period. The user should not infer presence or absence of a species, since no data exist for this period. More birders need to go to Gambell in the off seasons!
  2. The blank space for American Wigeon in spring indicates that the species is unknown during this period. We have data from this period but do not have any reports of American Wigeon, so we can infer that it does not occur at this season, or at least is so rare that we don't have any records in eBird. In fact the latter is the case--American Wigeon has been seen at Gambell a number of times in the spring. (If you have seen some please enter them in eBird!)
  3. The very narrow green bar indicates that Cackling Goose was recorded during the first week of June, but that it is extremely rare (on less than 0.3% of checklists) or was reported on a checklist that did not report all species. Since the sample size for this bar chart is too small to calculate frequencies of 0.3%, we know that this was due to a checklist that did not report all species.
  4.  The moderate green bar for American Wigeon in fall indicates that the species has been recorded here at a frequency of 1-5% (actual frequency was 4.5%, seen on 1/22 checklists). This is the only fall record in eBird--Gambell is one of the few places in North America where Eurasian Wigeon is often more likely than American!
  5. The very thick bars for Northern Pintail in late August and early September indicate that it occurs at the highest frequency during this period, found on about 65% of all checklists, so these earned our thickest bar.
Note that you can always find out the sample sizes and the exact frequencies by clicking the Download Histogram Data link at the bottom of any bar chart. This downloads the raw frequencies, along with the sample sizes, that are used to generate the bar widths. Note that in the downloaded data a species reported in a given period on a "not complete" checklist is shown with a value of 0.0015, which ensures that it gets the thin line width on the bar chart.

About the Frequencies

The frequency values were selected to best show bird abundance at various scales.

Width Min Max
9 60% 100%
8 40% < 60%
7 30% < 40%
6 20% < 30%
5 10% < 20%
4 5% < 10%
3 1% < 5%
2 0.3% < 1%
1 0 < 0.3%

In the above chart, note that the maximum values generally are not included in the range, while the minimum values generally are.These calculations are generally updated each night.

Be a Champion eBirder

The bar charts look best when we have lots of data. In fact, to accurately show all bar widths, we need at least 301 (!) checklists per period! Although this volume of data is often only attainable at the state or county level, some champion eBirders have put in so many visits to their local parks that those bar charts have evolved to be almost accurate as state-level lists. Check out the following parks all of which have very active eBirders (or teams of eBirders) that visit them regularly:

  1. Cane Creek Park, TN (831 checklists)
  2. Myers Point, NY (Okay, we admit that this is right next to Cornell where there are tons of eBirders. It's also Chris Wood's local patch) 
  3. Millennium Park, MA  (Marshall Iliff's local patch)

Unfortunately, even some heavily birded locations still do not have enough data for eBird to draw a bar chart that starts to capture some of the interesting changes in abundance across the year. For example, Huntington Central Park in coastal southern California is an urban oasis with a long track record as one of the best migrant and vagrant traps in Orange County; its 100 eBird checklists (at the time of writing) include no more than 8 for any week, which produces a bar chart not nearly as full as those linked above.

Remember, it is always OK to keep several lists for any one visit to your local park. If you consider keeping two or more complete lists, or develop a standard route of multiple point counts on each visit, your local park's bar chart will become more accurate more quickly.

This is awesome, but how do I PRINT these?

Now that the bar charts are new and improved and each of you is going to put in significant effort to improve the one for you local patch, your yard and your county, you'll probably want to print them. Unfortunately, printing isn't as easy as we would like.

To generate these bar charts we create something that a lot of browsers call background images. The default for many browsers is to not to print these. If you try to print our bar charts, you won't see the green bars.

Exactly how you correct this depends upon your browser. Generally, you want  to look for something called "Print Background Images" and make sure that is turned on. This is typically within the page set up for printing. Here are some specific instructions for the most popular browsers:

  • Windows-Firefox 3.0: Under File … Page Setup on the “Format & Options” tab, check the “Print Background Colors and Images” checkbox.
  • Windows-Internet Explorer 7.0: Under Tools, select "Internet Options". On the “Advanced” tab, about 2/3 of the way down the scroll window, check the “Print background colors and images” checkbox under the heading for “Printing”. Printing under Internet Explorer will print a blank page following every print job sent to your printer. This is an IE issue only and cannot be remedied with any browser setting. If you tire of collecting blank sheets of paper to run back through your printer, we suggest moving to the Windows version of Firefox 3.0, at least for printing. Installing a second browser on your PC will not affect other applications with which you use Internet Explorer, but you may like using Firefox so much more than IE that you decide to dump it!
  • Mac-Firefox 3.0: Check the boxes for “Print Background Colors” and “Print Background Images”. These are located in the Firefox options on the print window. This is the last window to appear before your pages are sent to the printer.
  • Mac-Safari. With the bar chart page open, choose File > Print.
    Choose Safari from the print dialog's pull-down menu that defaults to "Copies & Pages." Select the checkbox labeled "Print backgrounds." In other version of Safari, you will need to hit the little down arrow that appears to the right of the name of your printer. This will open up a window that allows you to "print backgrounds".
  • Other browsers. If you do not see your browser listed here, do an internet search for the name of the browser and "print background images" or "print backgrounds". Or contact the technical support for your browser and say that you want to print background images.
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