What many folks call the bird "count" is actually a statistical index. I forget how many routes are run in the state, I'm thinking it is 60 something. Each route is about 30 miles long selected by the area biologist to survey a cross section of the habitat in that given county. It is run 4 times, 2 in the first half of the 6 week period and 2 in the second half. The results end up as birds per mile and the results are run through statistical equations that compare one year's results to another. It won't tell you how many birds are out there, but does tell you whether the population has increased or decreased and whether the size of the increase is statistically significant or not. Any number of factors can influence the index. Weather, dew, cropping changes, ditch mowing and the list goes on. The more routes you have and the more times you run them increase the precision of the index. If major weather events sit on those 6 weeks and reduce the number of routes run, the index is weaker. A significant effort is made to get all routes run 4 times.
Conversely, some wildlife area biologists are running actual counts. In these operations, biologists and volunteers got to listening stations marked on the wildlife area in the dark and listen for morning covey calls, marking the location of those coveys on an aerial photo. After the calling period has ended, they go out and try to flush each of those coveys and count the number of birds in each covey. More statistics and you can end up with a fair estimate of your fall population.