xviii THE BRITISH BIRD BOOK 



which are divided into local races, British forms alone receive special attention, 

 the distribution of the other forms being roughly indicated. The zoo-geographical 

 regions (Palsearctic, etc.) referred to are shown on the map, p. xix, which is that 

 of Sclater. His system is here adopted, not because it is necessarily the best, but 

 because it is for the time being sanctioned by custom. 



Migration. In respect to the migrations, it should be noted that the term 

 resident is applied to the species. A species may, for instance, be present in this 

 country throughout the year, and yet, owing to emigration and immigration, be 

 composed to a greater or less extent of different individuals at different times. In 

 other words, a species may be resident but not stationary. A bird of passage is 

 one that visits our shores only on its way to its destination elsewhere. The other 

 terms explain themselves. 



Nest and Eggs. The statement under Nest does not mean that a nest is 

 necessarily composed of all the materials mentioned, but that all or some of them 

 may enter into its composition. The periods of incubation must be regarded as 

 approximate only, for the sources of information frequently omit to state how the 

 period was reckoned, whether from the last egg laid or another. 



Food. These are unavoidably incomplete, owing to lack of reliable 

 information. 



Song Period. See previous note on Food. 



Authorship. At the head of every set of Notes will be found the names of 

 the compilers, the share of each of the latter being shown by the initials placed 

 in square brackets at the end of the Note for which he is responsible. 



