ESSEX BIRD-MIGRATION. 127
achieved. The most definite type of information has been
derived from the "Recovery of Marked Birds" and "Movements
of Ringed Birds from Abroad to the British Islands and from
the British Islands Abroad," covering a period from 1910 to
1935 inclusive. These contributions appeared in the pages of
British Birds Magazine. Dr. Eagle Clarke's "Studies in Bird-
Migration," published in 1912, furnishes some of the most striking
information of the movements of birds to and from and in the
proximity of the Essex coast.
Those birds, which as species are completely migratory,
present fewer difficulties than those which are partially migratory,
i.e., species which are represented, frequently strongly so,
throughout the year, but of which at certain periods individuals
appear and disappear to and from the county. Of the total
number of 281 birds, which I accepted in 1929 as representing
the avifauna of Essex, 149 are irregular visitors, and, although
in many cases migration has been indirectly responsible for their
appearance, they may be excluded from the present consideration.
Of the remaining 132 birds 63 are completely migratory. Of
the latter figure 26 are seen only in the breeding season, 26
annually pass the winter in Essex and n are seen only on their
way to and from their more northerly nesting grounds. It
is the remaining 69 species which are described as residents,
which present the difficulties, for, although these species are
represented in the county throughout the year, yet some of the
individuals representing some of the species are migratory,
leaving the country at some period or other. Let us now
consider the course of migration in the county through the year.
I am of the opinion that there is a certain amount, at times very
limited, throughout the year, and irregular migrations, often
very large, are caused by altering weather conditions. We start
the year with the winter-residents installed and the first
regular alteration is caused by their departure, which may
commence as early as February and is in the main completed by
April. The birds simply disappear; how they go we do not know.
Before the emigration of the winter-residents is completed
another important migration is in progress, namely, the arrival
of the summer-residents, the birds which come to rear their
young in the county. This immigration may commence from
the middle of March and is completed early in May. While these