Adjusting this value only becomes important when using large values of “Maximum location count” (approximately 700 or larger). Increasing “Request length slack” reduces the maximum attainable location count.
The maximum length of an eBird bar chart query is limited by the total length of the HTTP request header in characters. The Polygon Tool automatically calculates this as much as it can, but there is a part of this it cannot calculate, as it lacks access: the total length of your ebird.org cookies. To work around this, a value as large as anyone is likely to need (457) is the default.
If you clear your ebird.org cookies and let eBird generate them freshly, you can replace this number with 81 (the length of “Cookie: EBIRD_SESSIONID=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX; ebirdprefsviewed=true” followed by a CRLF).
If you are using the Polygon Tool in a Private or Incognito browser window, you can replace this number with 0.
The actual rules are a bit more complex than what is described above. You can add a little to the number just to be sure it'll work.
If you want to use the “Download Histogram Data” function from the bar chart page, and/or click a species name on the barchart page to view Line Graphs for that species, then you will need to add 44 to the value of this number.
For “Download Histogram Data”, the length of “barchartData” minus the length of “barchart” plus the length of “&bmo=1&emo=12&byr=1900&eyr=2018&fmt=tsv” is 43.
For species Line Graphs, the length of “&bmo=1&emo=12&byr=1900&eyr=2018&spp=XXXXXXXX” is 44.
Thus to be able to freely use either of the above two functions, add only the larger value of 44.
With a large “Maximum location count”, you will currently not be able to use “Change Date” or “Change Location”, as these lengthen the URL quite considerably, including a lengthening of 2 characters per location.
Adding Date Range functionality to this eBird Polygon Tool page itself is planned.
In the meantime, you can add “&bmo=1&emo=12&byr=1900&eyr=2018” to the end of the URL manually, editing the values of “bmo” (beginning month), “emo” (ending month), “byr” (beginning year), and “eyr” (ending year) as desired. Of course, make sure you increased the value of “Request length slack” enough to make room for this (a value of 32 will be sufficient for any combination of months and years; if you already added 44 as described above, no further adding will be necessary).
The error you will see if the value of “Request length slack” is too small is:
Your browser sent a request that this server could not understand.
Move to the area in which you want to create the polygon, zooming as close as you can while still keeping the entire area in view.
In the menu, do Add → Polygon, or click the button.
(Optional) Go to the "Style, Color" tab and change "Area" to "Outlined" to make it easier to see what you're doing while creating the polygon.
If necessary, move the "Google Earth - New Polygon" window to the side, so that it isn't blocking your view of the map. (This will subsequently be called the "New Polygon" window.) The new window position will be remembered, so this only has to be done once.
Click points on the map to create the vertices of your polygon. It's recommended that you do not click and drag, as this creates an excessive number of vertices which can only subsequently be erased one by one.
You can move already-placed vertices by dragging them with the mouse to reshape the polygon.
Clicking to create new vertices will create them after the last vertex created or dragged, whichever happened most recently.
To erase the last vertex created or dragged, click the right mouse button. You may do this as many times in a row as needed to erase previous vertices.
The mouse cannot be used to move or zoom the current map view while creating a polygon, but you can use the arrow keys and + and - keys to do so (the keyboard focus must be on the map instead of the "New Polygon" window).
Click OK in the "New Polygon" window.
(Optional) To edit an already-created polygon, right-click its entry in "Places" and click "Properties" in the context menu.
Either Save the polygon as a .kml or .kmz file and upload it using the first form above, or Copy the polygon and Paste it into the second form above, and then click the corresponding "Explore eBird Hotspots inside polygon" button.
Select "Polygon" from the list box (it defaults to "Polyline")
(Optional) To make it easier to see what you're doing while creating the polygon, use "Style Options" to change "fillOpacity" to a smaller value, click "Click here to save style changes", then click the X.
Create the polygon.
To move the map view, either click and drag the map, or use the four-arrow control on the upper left.
To zoom in or out, use either the mouse wheel, or the + and - control on the left.
Click points on the map to create the vertices of your polygon.
Use the "Delete Last Point", "Edit lines", "Clear map", or "Next shape" buttons as necessary. Note that if you use "Edit lines", you must change back from "Polyline" to "Polygon" (this is a bug).
Click the "Select and copy text" button (or use whichever method you like to Select All within the area with "Live code presentation in textarea" printed above it).
Press Ctrl+C (or do whatever copies to the clipboard on your system).
Go back to this page and Paste into the second form above, and then click the corresponding "Explore eBird Hotspots inside polygon" button.
Important: It is recommended that you use this tool in a Private Window / incognito window, or without being logged onto ebird.org, because being signed in on ebird.org can interfere with viewing a Bar Chart that includes non-hotspot locations, causing the chart to be severely lacking in data (and sometimes even blank).
Note: Integration of this tool with a Google Maps API polygon-drawing interface is planned. This will be a third method of entering the polygon (the .kml/.kmz upload and copy-paste methods will be preserved).
The current algorithm does not currently properly handle polygons that cross the antimeridian, and polygons' sides are assumed to be straight in an equirectangular projection (rather than being rhumb line segments to be consistent with Google Maps, or geodesics to be consistent with Google Earth). Neither of these things should have any effect on practical usage of this polygon tool.