2 HANDBOOK OF BRITISH INLAND BIRDS 
mind most clearly and thoroughly, as the very 
foundation-stone of any understanding of bird life, 
that no sharp line can be drawn between migratory 
and non-migratory birds, for many, indeed most, 
of our so-called " resident birds " are really migra- 
tory in their habits, like the birds of passage." 
Missel Thrushes and Robins, for example, move 
southward in great numbers for the cold months 
of scarcity, and northward again for the summer 
months of plenty, just as Cuckoos and Swallows do. 
Birds' movements are like the swing of a great 
pendulum. When the pendulum swings south in 
autumn, it sweeps all the Cuckoos, Swallows, and 
many other species far beyond our shores alto- 
gether, only to bring them back in spring, and 
these we call birds of passage. Missel Thrushes 
and Robins, on the other hand, are to be seen in 
England all the year round, because as fast as some 
of them leave us for the south, other flocks come 
pouring in from colder lands to the north ; and 
thus, though the pendulum-swing does carry them 
south, it does not carry them far enough south to 
empty England of them altogether during the 
winter season. Thus it is correct to speak of the 
Robin and Missel Thrush as resident species^ be- 
cause representatives of these species are always 
with us ; but it is far from correct to regard all 
individual Missel Thrushes and Robins as resident 
