8 BULLETIN 612, U. S. DEPARTMENT OF AGRICULTURE. 



these lower extremities become affected almost at once, and the 

 bird sits on the full length of the tarsus instead of standing on the 

 toes. These birds are unable to escape when approached, but strug- 

 gle with beating wings to rise to a standing position. In a short 

 time they sink on the breast in the mud and then are entirely helpless. 

 In ducks and most other species, except the shore-birds just men- 

 tioned, the legs become affected more gradually. Individual ducks 

 are able to swim for some time after they are unable to support the 

 body in a standing position. Slowly the muscles become more 

 helpless until, capable of slight movement only, the feet remain in 

 normal position under the abdomen. Sick birds seem to sense this 

 increasing helplessness, and, when able, endeavor to work their way 

 out of the water on mud bars, or seek shelter, if available, in rushes 

 or other aquatic growth. 



In the next stage of the sickness the neck muscles weaken and 

 the head is supported with difficulty (PI. II, fig. 1). Birds remain 

 quiet unless frightened, and when disturbed the head, with slight 

 control, sways drunkenly from side to side. Finally the bird lies, 

 unable to move, with the head prone on one side (PI. Ill, fig. 1). 

 At this point, if the individual is in water, death comes by drowning, 

 but on land the bird may live for some time longer. Many, however, 

 avert drowning by throwing the head on the back, where it lies in 

 an unnatural position. 



Affected individuals early show considerable difficulty in breathing, 

 and as the trouble progresses this is more acute. Inspiration be- 

 comes increasingly difficult and may be spasmodic, while the number 

 of respirations per minute is lessened. In a great majority of cases 

 death comes from cessation of breathing due to paralysis of the 

 muscles controlling this function. In some birds the muscles slowly 

 become passive, until finally only those in the anterior thoracic 

 region respond. The intake and outflow of air is slight, and on 

 casual observation such birds might be considered dead. 



In severe cases the heart also is affected. Normally the pulse in 

 a wild duck at rest and not unduly excited averages 120 beats per 

 minute. With any struggling or attempt at flight it jumps at once 

 to 180 or 200. In some severe cases of the duck sickness the heart 

 action was weak and irregular. One mallard lived in the laboratory 

 for two days with the heart beating only 30 times each minute and 

 the respirations reduced to 8 for the same period. The body tem- 

 perature (rectal) in sick birds remains about normal unless the heart 

 is affected. With a decrease in the circulation the temperature 

 falls slowly from the normal register of 104° F. to 109.8° F. (mean, 

 106.7°) to less than 100° F. 



One important external symptom, easily overlooked, remains to be 

 mentioned. Besides upper and lower eyelids birds possess a third 



