52 THE ECOLOGY OF ANIMALS 



habitat of the heron there are again local groupings 

 or aggregations in density, brought about by chance 

 crowding for abundant food supply or by social 

 habits leading to communal roosting and nesting. 

 It is convenient to use a third term — Highest Density 

 — for these groupings, which are of importance eco- 

 logically since they play an important part in the 

 spread of parasites and disease and in other ways. 

 It often happens that these three ways of expressing 

 density are convenient for different purposes. Thus 

 the Lowest Density for partridges in an area of farming 

 country is of great interest to the sportsman who 

 wishes to know how many birds are available : the 

 Economic Density gives the best measure of fluctua- 

 tions in numbers from year to year ; while the 

 Highest Density (e.g. the numbers in a covey roost- 

 ing on a given patch of ground) is of importance in 

 studying disease. 



The definition of densities in these terms has given 

 us a composite picture of the distribution of indi- 

 viduals in an animal population. We see that every 

 species of animal tends to be distributed over any 

 large area of varied country in two degrees or scales 

 of density : the concentration mto suitable general 

 habitats, and the further local concentration into 

 various forms of aggregation or of social groupings. 

 The term aggregation is used in a general sense to 

 cover local grouping of individuals. It may or may 

 not be associated with actual social instincts or 

 habits. Thus aggregations may be brought about by 

 insects swarming to a suitable source of food, as 

 bees and flies to fruit blossom, or by seeking common 

 shelter, or surface for attachment, as with freshwater 

 shrimps in weeds and under stones, or with barnacles 

 on rocks. They may, on the other hand, be associated 

 with regular instmcts and habits, as when ants and 

 bees form colonies, or rooks and herons nest together 

 in groups. There is a very large and scattered litera- 

 ture on the subject of animal aggregations, and this 



