GULLS 109 



for, special attention. It is shown, magnified 

 fifteen times, on the lower half of the plate. 

 This globular object, the size of a small millet 

 seed 1 , is a hard, bony vesicle which floats in 

 water. The presence of this vesicle inside a 

 gull is certain evidence that the bird has fed on 

 a member of the herring family during the day 

 — in this case a sprat. 



My attention was first drawn to these bony 

 vesicles in the following manner : My procedure 

 when I examine the contents of a gull is to 

 remove the entire alimentary tract, then open 

 the gullet and gizzard, and pick out the larger 

 objects ; the remainder of the tract is then cut 

 open, and the whole washed out in a basin of 

 water, filtered, and examined, if necessary, 

 under the microscope. While I was examining 

 the contents of kittiwakes which had been 

 following up shoals of sprats off the East Coast, 

 I found these hollow globules in large numbers. 

 Next I boiled the skulls of sprats, and rubbed 

 the softened bones between the finger and 

 thumb, and obtained a pair of these globules 

 with each skull. This is exactly what happens 

 in the gizzard of the gull ; the bones are softened 

 by the gastric juices, and in the gizzard the softer 



