transects and counting all the individuals within the strip. An alternative approach is to use the
line-transect methodology discussed above to estimate abundance (Burnham & Anderson 1984).
Method
The details of the methods of aerial surveys are covered in Norton-Griffiths (1978) and Krebs
(1999). The aircraft used must allow the observers unlimited downward vision, and there must be
precise control of the aircraft’s altitude to permit a strip of defined width to be counted. Streamers
on the wing struts of the aircraft can be used to delineate the width of the strip to be counted.
Advantages and disadvantages
There is no other method for covering very large areas rapidly for mammals that are relatively
sparse on the ground or that range over very large spatial areas. Much experience in doing aerial
counts has been accumulated, and the statistical analysis is well defined. The major disadvantage is
the cost of aircraft usage, together with a very slight danger of aircraft accidents while surveying in
remote regions. Aerial surveys can be difficult to use in areas with high relief, although helicopters
can be useful in these situations if blocks defined by topography are counted instead of strips.
Large groups of animals can be photographed if they cannot be counted quickly. There is great
variation among observers in their ability to count from aircraft, and this must be standardised as
much as possible. Observers grow tired after a few hours of counting, and counting becomes less
precise the faster one has to count.
Biases
Most observers in aerial censuses tend to undercount, and this negative bias typically ranges from
zero when there are few animals to a very high fraction when many animals have to be counted
quickly (Marsh & Sinclair 1989). Additional biases with aerial surveys can occur when strict
control of the aircraft’s altitude is not maintained, so that the actual strip width becomes too wide or
Individual recognition 359
too narrow. Observer training can reduce the differences among individuals in the ability to detect
animals, but even trained observers differ dramatically in their counts (LeResche&Rausch 1974).
Individual recognition
Many large mammals have individual markings that lend themselves to being visually recognised.
Killer whales exhibit individual colour variation and in addition they can be distinguished from
one another by the shape and size of their dorsal fins. Wild dogs in Africa have coat-colour
variation, so individuals can be identified from photographs. The use of individual recognition