The AKN histograms (or bar charts) draw directly on AKN data to represent how bird occurrence changes over the course of the year. Each month is broken down into four periods (each approximately 7 days), with frequency independently calculated for each. The wider green bars show the periods when a species is least likely to be missed on a given bird survey, while the narrower green bars show the time where the species is present (or sometimes present), but rare. It is possible to explore data by state, county, or Bird Conservation Region (BCR), across all years or in a given date span, and to look at data from a single dataset or across multiple datasets.

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An important caution: Please note that in order to properly understand what the histograms portray for a given dataset, it may be important to refer to the metadata document. For example, some projects may change protocol over the course of the year, and this may have a major effect on how the bird data are displayed. 

How to interpret the histograms: The below example shows data from Mountain Birdwatch data for all years for the state of Vermont. Mountain Birdwatch, a project of Vermont Center for Ecostudies, conducts summer point count and playback surveys on mountaintops in the Northeast. Numbered symbols are explained 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 during this period, since no data exist for this period. In this example, this is because Mountain Birdwatch is a summer survey conducted from mid-May to mid-July.

2. The very narrow green bar indicates that Blackpoll Warbler was recorded during the second and third weeks of July, but that the report lacked sufficient information to calculate frequency. In this case, that is because it was recorded on counts which did not report all species. Since we cannot infer absence in these examples, these bars should not be interpreted to indicate that the species was frequently observed or infrequently observed. The Mountain Birdwatch metadata explains why these mid-July counts do not provide frequency information: protocols change after the first week of July, shifting from point counts reporting all species to targeted playback surveys.

3. The very thin green bar indicates that Black-throated Blue Warbler was reported in the first three weeks of June. In this case, this is because Mountain Birdwatch conducts targeted playback for Bicknell's Thrush on the later surveys, and these surveys do not report all species observed. This example shows why examining metadata is essential--one should not infer that Bicknell's Thrush is rare or less detectable at this season, since the change in bar width is the result of a protocol change. In fact, for the narrowest bar width, no inferences should be made except that the species is sometimes present in this period.

4. The thick bars for Yellow-rumped Warbler indicate the frequency at which the species was recorded between June and early July. In this case, data from the first week of June recorded the species on ~15% of checklists, during the rest of June it was recorded on ~25% of checklists, and in the first week of July the species was recorded on ~35% of checklists. In this case, the spike in frequency in early July may be due in part to a smaller sample size, since the species was recorded on 25 out of 35 surveys (71%), as opposed to in late June when it was recorded on 27 of 167 surveys (16%). Always remember to refer to metadata to understand the scope of the survey and changes in protocol that may affect the bar charts.

5. The blank space for Black-throated Blue Warbler at the end of June and July indicates that we have data for the period but that this species was not reported in this period. For any period with inflated bar widths it is possible to infer that the species may be absent during that period. Please note that in this case it is not possible to infer absence of this species, or any species, in the second and third weeks of July, since the very narrow green bar (#2 above) indicates that surveys during these periods did not report all species. Note however that sometimes these very narrow bars may be interspersed among larger bars, in which case absence can be inferred.

How the histograms are calculated: The bar charts display species frequency (not abundance). Frequency is a measure of the percentage of sampling events (checklists) that record a given species. Observations that do not report all species are excluded and in these cases any observations are marked only as present (the narrowest green bar).

Frequency calculations are made for each of four periods within a month. 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 eleven 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.

The taxonomy and nomenclature conform with the AOU Check-List and its most recent supplements; read more about the AKN taxonomy.

About the frequencies: The frequency values were selected to best show bird abundance at various scales.

In the above chart, note that the maximum values generally are not included in the range, while the minimum values generally are. For width one, the minimum and maximum frequencies are zero since we cannot calculate a frequency for lists without all species reported.

These calculations are made quarterly and are not necessarily a "live" representation of what is in the AKN database. The bar charts were last updated in September 2008.