VOL. xm.J NOTES. 315 



the long wings, and as it approached nearer and turned 

 sHghtly over, I saw the white upper tail-coverts and the 

 whitish head. W. Walmesley White. 



PERIODS OF DIVES IN RELATION TO DEPTH 

 OF WATER. 



Mr. J. Alastair Anderson's note on the diving of the 

 Long-tailed Duck {antea, p. 298) supports what I believe is a 

 general rule for the periods of dives made by diving birds. 

 The rule is a simple one, namely, twenty seconds for the first 

 fathom and ten seconds for every fathom thereafter. The 

 conditions are that the birds are actually going to the 

 bottom, that they are not disturbed or alarmed in any way, 

 and that the times of not less than three consecutive dives 

 are used in reaching an average. The depth recorded by 

 Mr. Anderson is 35 feet, or 5f fathoms, and the times 65 to 

 70 seconds, which agrees very closely with the rule. I have 

 discovered an important exception to the rule. When a bird 

 is travelling along the bottom and crosses a reef at right 

 angles to its course, the period of the dive does not correspond 

 to the general depth of the bottom, but to the general depth 

 plus the height of the reef. Thus, a bird is diving in three 

 fathoms of water and crosses a reef one fathom in height. 

 The probable time of the dive will not be 40 seconds (accord- 

 ing to the rule), but 40 seconds pins 10 seconds=:5:) seconds, 

 the reef having an effect on the period of the dive, as if it 

 increased the depth to an extent equivalent to its height 

 above the bottom. 



Great practical difficulties attend the recording at the same 

 time of the periods of the dives and the depth of the water. 

 Since I began to collect records, I have been forced to the 

 conclusion that the only practicable way of obtaining the 

 depth, without disturbing the birds, is the indirect one of 

 determining the position of the bird on the water in relation 

 to fixed points. The method is not free from liability to error, 

 and on sea-water the chances of making mistakes are increased 

 by the rise and fall of the tide. Hence, it would appear that 

 the problem is one for as many observers as possible, working 

 independently and maintaining a critical attitude towards 

 their results. Since 1910 I have collected the times of over 

 three thousand dives made by nineteen species of Auks, 

 Grebes, Divers, Cormorants, and diving Ducks. Such is the 

 regularity of these birds when they are working in a given 

 depth of water, that I am convinced there is always a constant 

 and narrow relation between the period of the dive and the 



