188 THE ESSEX NATURALIST.
course of the Thames Valley there is also a series of reservoirs 
of varying size, stretching from Hammersmith to Staines, and
if we compare the birds of this group with those of the Lea
Valley series we may find points of difference and interest. The
chief difference in construction is that the basins in the Thames
Valley possess no islands. It is probably mainly due to the
islets that the Lea Valley Reservoirs are of greater interest
during the breeding season. There are, for example, neither
Herons, Great Crested Grebes nor Coots nesting in the Thames
Valley. Apart from the breeding season, however, the birds
of the Thames Valley group are more numerous and of greater
variety. Well represented as are such common birds like the
Mallard and Tufted Duck in the Lea Valley, they are still more
numerous in the Thames Valley. The disparity between some
species is very marked. In the Thames area the numbers of
Teal and Wigeon exceed in each instance two hundred, aud-
it was estimated that in December, 1933, there were about
seven hundred Wigeon on Staines Reservoir. Even if this
figure was over estimated there were certainly many more than
the average number. The Goldeneye, Goosander and Smew
still further emphasise the difference. It is not unusual to
find fifty Goldeneye on Staines Reservoir; the presence of
sixty Goosanders at this locality would cause no surprise;
and Smew up to thirty might be seen at Hampton. In
December, 1933, no fewer than 174 Goosanders were counted
at Staines Reservoir and 51 Smew were counted at Molesey
in January, 1934. The Thames claims such birds as the Common
Scoter and Black-necked Grebe as annual visitors, but the
Lea has for these species two and one records respectively.
Numerous as the Great Crested Grebe is at the time of emigration
in the Lea Valley it is still more numerous along the Thames.
Terns are of much more frequent occurrence along the Thames,
the Black and Common being seen annually, while they are
only occasionally observed along the Lea. The next group to
which I shall refer in this respect is perhaps the most notable
example. The gatherings of gulls, which every evening about
sunset migrate up the Thames to roost at the reservoirs and
elsewhere in the Valley, are very remarkable, as their numbers
must run into several thousands, probably many thousands.
Until recently this movement was peculiar to the Thames,