One of the most frequent questions received at the refuge is "how many waterfowl are in the area"? We are usually able to provide that information with some up to date survey sheets. Quite often the next question is "how do you know or how do you do that."? Most of the assessment of waterfowl populations is accomplished by estimating numbers from an overflying aircraft. This task appears quite mysterious to many people, but in fact it is a straightforward technique that is easily learned, but takes time to perfect.
In order to a good job of aerial waterfowl census, there are some basic requirements that have to be met by the people and equipment involved. The aircraft and the pilot in command are just as important as the selection of the person or persons doing the actual counting. Lets first talk a little about the airplane.
To be ideal for wildlife census work, the airplane should have good visibility to the front and sides. High wing aircraft are eminently more useful since the wing doesnt block the view of the ground. Perhaps you have flown in a commercial airliner and your seat was right over the wing. Much of your view of the ground was obscured, wasnt it. Its the same way with light aircraft. The plane you chose must be capable of safely flying at low speeds (less than 80 mph) so that there is time to look at the flocks of birds and formulate an estimate. There are, of course, a multitude of federal regulations that must be met by the aircraft so that it may be used for this work to insure airworthiness and safety.
The pilot in command is the most important person in the aircraft. Without a competent pilot, there is no chance of success with the census mission and, in fact, a pilot with inadequate skills can truly compromise the safety of the mission. A good census pilot has to be comfortable with low level flying since most of the work is done at elevations below 200 feet above the ground. He also has to do this at low speeds. There is an old pilot saying that goes "low and slow, look out below." Mix this with lots of bird traffic, powerlines, radio towers, and other obstructions and you can really appreciate what a demanding and difficult job this is. I have been very fortunate to have had a number of highly experienced and very talented pilots do this work for me during the 20 years I have been doing this work. Last, but hopefully not least in the mix, is the observer. This, of course, is the person who will actually do the estimates of waterfowl numbers. There are few attributes that make for a successful observer. Obviously, they have to be able to tell one species from another. This is a little different because you are seeing everything from above. Wing speculum characteristics are very useful in differentiating ducks. Geese are usually distinguished from overall color, characteristics of the flock, and occasionally the coloration of the tail fan. To be a successful observer, you have to be comfortable with flying in small aircraft. A good job of counting is not possible by an observer who is overcome by fear or air sickness! Of course, you have to be able to concentrate on the counting and not be sightseeing when in the bird use areas.
So, now that we have a nice day for flying, a good airplane, good pilot, and an observer who is comfortable with flying and can differentiate waterfowl species, just how do we go about coming up with those numbers? Even though everyone calls the result a waterfowl count, it is really an estimation of population numbers. Normally, the aircraft is flown back and forth across the area to be censused in transects that are separated by about 1/2 mile. During migration, the sizes of the flocks are too large to actually count and we must make a quick estimate (educated guess) of their numbers as they past by the plane. This is often confounded when the flock is made up of a number of species. My technique in that case is to make one pass by such a flock and estimate total numbers and then make a second pass to determine the percentages of the various species. All the data taken in the air are usually dictated into a small voice recorder and then later back in the office transcribed and assembled into tables.
As you have deduced by now, a lot of the validity of the census data depends on the ability of the observer to estimate numbers. This is like a lot of other skills in that if you dont use it, you lose it. To keep in practice, sometimes I take pictures of birds that I am estimating and actually count the total birds as a check. I am usually within 10-20% although sometimes high and other times low. I believe that in general the errors pretty well cancel themselves out in the course of a survey of large areas. There are other training aids such as computer programs that randomly generate various visual images of bird aggregations in different habitat circumstances, pictures of known large numbers of objects such as rice grains on a dark background, and actual pictures of waterfowl concentrations that can be used to develop and maintain estimating skills (see below).
You should be able to see by now that although it is a quite straightforward process, there are a lot of critical elements that have to come together just right to result in a successful waterfowl census effort. I have spent more than two thousand hours in the skies over the Klamath Basin estimating bird numbers and I still find it one of the most exhilarating and rewarding jobs I do. So, when you pick up one of the refuge waterfowl census reports remember that it probably represents about five or more hours of time in the air as well as an equal amount of time preparing the report. It is a lot of work, but it is one of our best measures of how well our habitats are performing.