voL.viii] A STUDY OF BIRD (ECOLOGY. 18^ 



other sorts of flighting ; the flocking and other flights 

 of birds in the early morning or late afternoon are not 

 usually regarded as a part of migration, but it seems 

 to me probable that their causes or motives are similar 

 and in part the same. At any rate, no practical study 

 of the movements of birds is likely to be satisfactory 

 where such a distinction is permitted ; as a rule one 

 cannot possibly tell whether a bird seen flying over is 

 or is not " migrating " in the stricter sense. Recent 

 investigations, notably the " marking " schemes, have 

 proved the great intricacy of bird movement ; they also 

 tend to show that a proper study of bird-movement 

 must be restricted neither to the appearance and dis- 

 appearance of certain particular species, nor to the 

 passage of birds fortuitously known to be migrants, 

 nor to flights tending north in spring and south in 

 autumn. Every fact must be taken into account ; 

 many apparent changes in the number of species may 

 be due only to inaccuracy of observation, but they 

 must not be omitted on this accovmt ; many of the 

 birds to be seen flying over in the morning and evening 

 may appear to be going in an impossible direction for 

 true " migration," and it may be that they are only 

 flying to or from roosts. But we cannot judge of these 

 things at the moment. No arbitrarily hmited obser- 

 vations can have any real value. On the other hand, 

 if everything in the nature of a movement is noted and 

 the effect of the environment on such movements 

 studied, we may in the course of time understand the 

 phenomena of migration and at the same time the 

 cognate phenomena of flocking. 



For instance, very early in the summer, sometimes 

 even during May, or at any rate June, family parties 

 of Tits begin to appear all about the country, and even 

 in our gardens. Many ornithologists indulge in nesting- 

 boxes, which are perhaps inhabited by Blue-Tits as 

 often as by any birds. No doubt the old birds are 

 often watched feeding their young, but how many 



