294 
BLACK-HEADED GULL. 
nearly when this is completed, they assemble at their 
stations,* remain during the night, and commence 
all the operations of incubation. On approaching 
their retreat they become very clamorous, stooping 
at the stranger, whether man or animal ; all settling 
for a few minutes, and again simultaneously rising 
to renew the attack ; in a large colony, the noise of 
their cackling or laughing voices will nearly drown 
any other sound. At this season they frequent fal- 
lows, the banks of a river when flooded, and in or- 
dinary state will often assemble on the shingle to 
dress and feed on the aquatic insects, some parts 
being much more resorted to than others. Thoy 
will also watch the appearance of the Pryganidee , or 
river-flies, which rise in troops at stated times ; and 
while the trout make war on them as they rise and 
leave their eloment, the gulls pursue them on the 
wing, skimming the surface of the water, and fol- 
lowing the insect almost like swallows. When the 
insect flight is over, the birds again settle ; but their 
perceptions tell when it is again about to rise, and 
before an observer can detect them, they are again 
active, skimming and hovering over the stream. 
When the young are able to travel they resort to 
the shores, and in autumn they may be found in 
flocks, the young easily distinguished by their diffe- 
rent plumage, the black bar at the end of the tail, 
and their far weaker cry. The Black-headod Gull 
* Mr. Yarrell states that a gull belonging to the Zool. Soc. 
changed from white to dark brown in five days, the colour 
changing without any moult or accession of feathers. 
