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. They 

 will also watch the appearance of the Pryganidoe, or 

 river-flies, which rise in troops at stated times ; and 

 while the trout make war on them as they rise and 

 leave their element, 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 di fife- 

 rent plumage, the black bar at the end of the tail, 

 and their far weaker cry. The Black-headed 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. 



