Species sighting charts
The species sighting charts appear on all butterfly species that have sightings in the system; they include all Wisconsin sightings that have been reported since 2002.
How to read the charts
Each bar in the chart represents a week in the current calendar year; the bar week represents the first full week of March, starting on a Sunday. A label and hash mark appear below first full week of each month, and the bars alternate color by month.
As an example, let’s take this chart for the Olympia Marble. It shows that the first Marble sighting was the second full week of April; in 2008, that’s April 13–19, but from the height of the bars, I see that last two weeks of April and the first two weeks of May are when I should be looking for them — probably earlier in the southern parts of the state, later as you move north. After the first week of June, I probably won’t see one; if I do, I’ll be sure to submit a sighting to the website!
Chart data
The sighting data in the charts is only five years of data, and even that data is heavily weighted on the years following 2005. There are certainly distortions based on weather patterns and other events in a certain year; the hope is that as more years’ data accumulates, we will get a better picture of the flight times of each species.
There are also distortions between species in the total number of sighings seen per species. Reporters certainly tend to report more interesting species more often. Looking at the Giant Swallowtail sighting chart, for example: its two 26-sighting weeks misrepresent its overall abundance relative to the Cabbage White, which has only a 35-sighting week (figures as of this writing). The lesson is that comparisons of total sightings between species are probably not very useful.