By Popular Demand: Bird Range Maps


A few of you, in response to WC’s post on Say’s Phoebes in the White Mountains, asked how it is that range maps get made.

Range of the newly created species Pacific Wren. Record from Barrow, Alaska could be this species or (Eastern) Winter Wren (confirmation needed). This species should be watched for east of the mapped range (especially western Nebraska, Kansas, Oklahoma, central Texas, but also farther east). Map based on the Sibley Guides, copyright David Sibley.

Range of the newly created species Pacific Wren. Record from Barrow, Alaska could be this species or (Eastern) Winter Wren (confirmation needed). This species should be watched for east of the mapped range (especially western Nebraska, Kansas, Oklahoma, central Texas, but also farther east). Map based on the Sibley Guides, copyright David Sibley.

Here’s a fairly typical range map. Purple is year-round, orange is summer, blue is winter and yellow is migration.

Let’s go back to first principles. First, birds are wild creatures. They go where they want. WC has personally seen and photographed a Eurasian Collared-Dover on Maclaren River Bridge, probably a thousand miles out of its accepted range. WC has seen and photographed a Eurasian Bulfinch on the end of Chena Ridge, mysteriously a thousand miles from its Eurasian habitat. Birds go where they want and bird guides are guides, useful but subject to the whims and vagaries of bird behavior.

Second, bird ranges are changing. They’ve probably always been changing – there are now Eurasian Starlings in Anchorage, for Pete’s Sake – but climate change is forcing range extensions and withdrawals. Alpine species are retreating up slope; timing chick feeding to prey populations is forcing species north. It’s not a static picture. And range maps, which are based on historic data, are always running behind.

Third, the approach to range mapping is changing in the Internet Age. In the past, a range map might be built on state bird checklists and museum collections (which document where the specimen was taken as well as what it is). The checklists themselves are maintained by state birder committees, who receive reports of rare birds and decide whether the documentation is adequate to add the bird to a state checklist.

Today, most North American bird guide authors rely on eBird. Founded by Cornell Lab of Ornithology and National Audubon Society. eBird is a crowd-sourced checklist of bird sightings in North America.1 Quite literally, millions of bird sightings are posted to eBird each month. The result is a vast trove of where species have been seen. As just one example of what eBird can do, here’s an animated GIF of Winter and Pacific Wren populations by month across the Lower 48.

Pacific and Winter Wren Ranges, as shown by eBird reports from North America

Pacific and Winter Wren Ranges, as shown by eBird reports from North America

You can see that the eBird data, displayed for a calendar year, graphically demonstrates the two separate populations and the approximate ranges. You can infer that this is a whole lot easier than visiting the ornithological collections in 48 states and hand-summarizing the data.

David Sibley, author of WC’s personal favorite bird guide, The Sibley Guide to Birds, Second Edition, also seeks and obtains public input from his website.

And that’s how range maps are made: from ornithological collections, from state records and, increasingly, from sites like eBird, which in a real sense are revolutionizing the art.


  1. WC cautions that if you are even slightly interested in birds or birding eBird can been a tremendous time sink. You have been warned. 
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