CONCORD AND DISCORD IN BIRDS. 
351 
the other. The causes productive of jealousies, are 
rivalry among the males of a species in the time of 
their amours; among the individuals of both sexes 
indiscriminately in some species, especially during 
summer when each pair observes certain limits for 
its range after provender; among birds of the same 
and of opposite species on account of aggressions ; 
and among birds of one and of opposite kinds during 
times of scarcity. The causes of union and com¬ 
bination of interests are, first, the better resistance 
of danger, observed chiefly among those of a species, 
but also frequently among incongruous kinds; 
secondly, a sort of friendship or desire of rendering- 
service, seen between individuals of one species, 
and more rarely between different kinds ; the better 
discernment of food by combined powers and senses; 
the easier attainment of a given object, as that of 
arriving at a destination by the shortest route in 
the case of migrators; and lastly and principally, a 
desire of society, as men assemble to concert for 
their common good for a variety of reasons. 
The generality of birds separate in pairs at the 
time of the alliance of sexes, when each couple 
(save in some rare instances) observes a certain 
range without interfering with the domains of others 
of their kind, yet some species congregate at this 
period, making a conjoint nursery at some one spot, 
as the Rook, (which however is gregarious at all 
times ,) and very many of the aquatic fowls. 
Jealousies however, and bickerings commonly 
disturb these harmonious combinations. It is 
strange also that some instances occur, of pairs, in 
species usually collective at this season, retiring to 
some secluded spot apart from their kind, as seen 
in the Cornish Chough and many other birds. 
Besides the circumstance of rivalry among the 
males of a species, keeping such birds apart, the 
distribution and occurrence of food is ordinarily 
