This question came to my mind recently. Out of curiosity I decided to compare eBird's Bird Observation Maps to the range maps found in my four field guides: Sibley, Peterson, Nat-Geo, and Birds of Western North America: a Photographic Guide. I also referred to the range map on Cornell's All About Birds online Bird Guide. I found some interesting results. Take a look with me at a couple of species:
For the Rusty Blackbird all of the range maps were pretty good when comparing them to eBird. Sibley's had the nice feature of showing a "rare" range that covered the areas where Rusties have been seen. Many of the field guide range maps only show winter and breeding locations which appeared accurate, but not as useful as Sibley's or eBird's.
The next species I looked at was the Ruby-crowned Kinglet. All of the field guides, but Sibley's, show the Ruby-crowned Kinglet being absent from the northern midwest and Great Lakes areas...well, perhaps not "absent" per se, but not shown because they only occur there during migration. A look at eBird's map of Ruby-crowned sightings indicates a pretty strong presence of Ruby-crowned Kinglets in those states, so I wonder if it is during migration only. eBird's map of the Ruby-crowned Kinglet's range is limited by the number of observations submitted and therefore shows very little presence in central Nevada and through the Dakotas. The field guide range maps vary in whether this kinglet is a winter or year-round resident of Nevada, but they do show its presence covering the entire state. As far as the Dakota's and eastern Montana go, the field guides all indicate less Ruby-crown presence, so perhaps eBird's map is pretty accurate for that region.
Looking at the Long-billed Curlew, eBird shows a much larger distribution than the maps in the field guides, again with the exception of Sibley's. Surprisingly, Sibley's map almost mirrors eBird's while the others seem to really focus on traditional breeding and winter ranges.
To review: I have really started to rely on eBird's maps, over my field guide maps, to learn where species are likely to be found. The strength of the eBird map of sightings is that it shows where birds have actually been "seen", not just where they are "probable" or "possible". eBird is constantly being updated, so the maps will always be up to date. The weakness of eBird's maps is that they are 100% dependent on us submitting our sightings. Regions of North America that don't get birded very often may not show the bird being in those locations when in reality they do exist there. As more people start using eBird and we start using it better, and we make efforts to bird in under-birded areas, eBird's maps will just get better and better. Based on my three species case studies, of all the field guides, Sibley's has the most accurate or at least the most useful maps as compared to eBird's maps of actual sightings...at least for the purposes that I use range maps.
All this brings me to my next question....How do you use the range maps in your birding field guides? Knowing how birders use their range maps will certainly help field guide compilers in the future.
I have used mine for predominately two reasons. 1) When I am going to a new place I like to thumb through my field guide and write down a list of all the birds that are shown in that area during the season I am going there.* 2) When confirming a new species I have just seen to make sure it was within the "right" range.
I would argue that range maps in field guides are primarily used by birders to know if it is reasonable that a bird is found in a certain area. While perhaps factually interesting, range maps only showing winter and breeding distribution do little service to the average birder's purpose at looking at them. It will be interesting to watch the range map revolution resulting from eBird over the coming years.
* I have since learned that most refuges have a pretty good checklist already, or at least the state does. This old method I used was a waste of time...except that I got familiar with my field guide and it filled me with wonder of all the potential birds to see.