BROAD FEATURES OF ORNITHOLOGY OF ESSEX. 265
than it is today. It was recorded in 1832 that there was a
gullery on Pewit Island off Bradwell, from which ten to twelve
thousand eggs were taken annually. According to Parsons
they nested in great numbers on the New England saltings,
and Christy relates that they nested abundantly on Foulness.
There are, or were recently, six ferneries in the county, of
which the Common Tern claims two and the Little Tern four.
All these colonies are small, fifty-one nests—Common Tern—
being the largest number counted in any one colony. In both
cases the Common Tern has used the short turf of the saltings
for nesting purposes, whereas the Little Tern has always used
the shingle of the beach on which to make the scrape which
serves to hold its eggs.
Of the various problems with which we are confronted in
the study of birds migration may be the most complicated. It
bristles with difficulties, but we are making progress with their
solution. It is probable that there is a certain amount of move-
ment taking place throughout the year, nevertheless at certain
periods there are migrations of an outstanding nature. Let us
consider the comings and goings of the birds through a year.
We start with the winter residents installed and the first pro-
nounced migration is caused by their departure, which may
commence as early as February and is more or less completed
by April. Before this movement is finished another is in
progress, namely the arrival of the summer-residents, the birds,
such as the Swallow, which come to rear their young. This may
commence from the middle of March and be completed early
in May. During the period of these alterations another form of
migration is in progress. This is the flight of the passage-
migrants, which are coming from an area south of Essex, where
they have been wintering, to another to the north, where they
will breed. It is probable that many more species than those
which are placed in the list of passage-migrants participate in this
spring passage, as, for example, individuals of the species which
are summer-residents in Essex. These migrations, which
probably follow mainly a northerly route, are not long completed
before a return flight to winter quarters is observable. This
starts with the appearance of occasional passage-migrants in
the middle of July, and about the beginning of August the summer-