50 THE ECOLOGY OF ANIMALS 



Various methods of taking animal censuses have 

 been evolved. They consist of complete counts or 

 estimates of all the individuals in a given area. Where 

 the animals are very small or very numerous or the 

 area very large, complete sample counts are made at 

 representative stations and used to calculate the 

 total numbers on the larger area. The method of 

 direct counts is used for some mammals, birds, and 

 reptiles, and the method of sample counts usually 

 for all smaller animals. Sampling has been applied 

 with great success and accuracy to animal popula- 

 tions in the soil, in freshwater and marine plankton, 

 also in bottom faunas of fresh water and the sea, 

 and for the estimation of parasite densities. It also 

 has to be used in a good deal of work on vertebrates, 

 such as voles or small birds, or in the case of sea- 

 bird colonies. Each of these branches of study is 

 evolving suitable techniques which have so far been 

 most highly developed in aquatic work and in soil 

 investigations. Bird censuses have recently received 

 much attention from naturalists and reference may 

 be made to Nicholson (1932) who summarizes the 

 methods that can be used. In England several 

 large-scale bird censuses have been carried out by 

 organizing the assistance of naturahsts aU over the 

 country. Thus the British heron census (Nicholson, 

 1929) proved that there were about 8,000 breeding 

 herons in England and Wales in the year 1928. A 

 census of the great crested grebe (Harrisson and 

 Hollom, 1932) showed that there were about 2,650 

 grebes in England and Wales. Nicholson (1932) 

 has published maps summarizing the distribution of 

 the density of these two aquatic birds in different 

 parts of England. By such censuses it is possible 

 to construct maps showing the contouring of density 

 of numbers. The maps can then be correlated with 

 various features in the environment which in turn 

 suggest reasons for distribution and varying numbers. 

 When such censuses are taken at periodic intervals 



