THE BIRDS OF THE LEA VALLEY RESERVOIRS. 187
The five species of Tern are all passage-migrants, but none
occurs annually. The occasional visits of the Black Tern are
confined to the autumn, and although the Common Tern has
been noted in spring, it has been seen more often in autumn.
The Sandwich Tern has been observed on three occasions in
autumn. The Arctic and Little Terns have been identified
once and twice respectively, and on each occasion during autumn.
Fifteen Arctics were seen, this being the largest number of any
species of Tern.
The Black-headed Gull is by far the most numerous of the
five species of the group. During winter it may be seen in
flocks, containing some hundreds. The Common Gull is a
winter visitor, whose numbers vary very much with the weather
conditions, but it is not often that the numbers reach fifty.
The Herring Gull appears irregularly and in small numbers.
The Lesser Black-back has been recorded on four occasions
and the Kittiwake once only. The chief feature of the gulls,
however, is the huge roosting parties which appear at sunset
at the King George Reservoir during winter. One of these
gatherings, seen by the present writer, certainly contained six
thousand birds, but the flocks at times are said to be very much
larger. The predominating species is the Black-headed Gull,
but the Common and Herring Gulls are also strongly represented.
Most of these gulls approach the reservoir from the south,
apparently coming from the Thames.
The two Rails are both of unusual occurrence. The Landrail
has been observed in three different years and it is not improbable
that it bred in 1927. The Water-rail, for which there is not
much suitable cover, has occurred on three occasions during
winter.
To conclude this rapid survey of the birds of the reservoirs,
we deal with two well-known species. The Moorhen is a common
resident, nesting freely. The Coot also breeds, but in limited
numbers. During the winter it becomes more abundant, and
the numbers then run into some hundreds. During the course
of my remarks I have endeavoured to convey to you an
impression of the bird-life without giving full details of the
status of the species, which are given in the systematic list.
We now have a general view of the birds of the reservoirs,
so that we are in the position to make comparisons. Along the