PREFACE xi 
of papers to be found in the Proceedings and Trans- 
actions of learned societies. Wherever it has been 
possible, I have verified what I have learnt from 
books by the methods explained in the concluding 
chapter. Discussions with mathematicians, in par- 
ticular with Mr. T. J. Bowlker and Mr. R. C. Gilson, 
to whom my most cordial thanks are due, have 
greatly helped me in-.dealing with the difficult 
problems of flight. Mr. A. H. Macpherson has 
very kindly put at my service the results of some 
of his observations on the song and on the migration 
of birds. I have also to thank Mr. F. E. Beddard, 
without whose encouragement I should not have 
begun this work. In important cases I have 
mentioned in footnotes the source to which I am 
indebted for facts or theories, and at the end of each 
chapter I have given a list of some of the best books 
and papers on the subject treated of. Those of my 
readers who wish to proceed to a more special study 
of any of the different branches of ornithology will, 
I hope, find these references an assistance. 
On many parts of the subject [ have delivered 
lectures to the Hailevbury Natural Science Society. 
In this book I have followed much the same lines, in 
the hope that it may prove useful to lovers of birds 
here and elsewhere. Technical terms, except such as 
are unavoidable, I have dispensed with, and, as far as 
possible, I have explained difficulties, never, inten- 
tionally, wrapped them in the obscurity of big words. 
There is only one species of ornithologists with whom 
