VOL. XIV.] SOME NOTES ON DIVING DUCKS. 107 



depth, will disclose a variation between the longest and 

 shortest dive of as much as fifteen seconds, or even more. 

 The period of submersion is not, therefore, always a reliable 

 guide to the depth of water ; and it is only by taking an 

 a\'erage of several consecutive dives that we can hope to 

 make deductions of any value. 



Most species, I find, have their favourite depth and stick 

 to it, and in this case a knowledge of their habits will assist us. 



Thus, if we see a Buffel-head diving in, to us, unknown 

 waters, we may — irrespective of the length of its dives — ■ 

 reasonably conclude that the depth does not exceed two 

 fathoms, because this species seldom or never dives at a 

 greater depth. But in the case of deep-sea feeders, such as 

 the Surf-Scoter— a duck that has, incidentally, occurred 

 on several occasions in Great Britain, and which dives in 

 any depth from one to five fathoms— a single dive of, sa}', 

 forty-five seconds would be quite an unreliable guide ; for 

 I have known this bird to remain submerged for that period 

 both in one and a half and three fathoms. On the other 

 hand, four or five consecutive dives of that duration would 

 rather point to the greater depth. 



There is, I think, one very obvious explanation of the 

 \-arying periods of different dives. In addition to molluscs 

 and marine plants, fish also play a large part in the diet of 

 nearly all marine ducks. But in order to catch these they 

 do not necessarily have to dive to the bottom, and I strongly 

 suspect that the period of the dive depends, in many cases, 

 not so much on the depth of water, as on the piscatory skill 

 of the feathered angler. All the species mentioned in these 

 notes occasionally catch quite large fish, and it is not unreason- 

 able to suppose that an unusually refractive victim, or even 

 an extra tough mollusc, might considerabh' delay the captor's 

 return to the surface. 



As a general rule, however, there can be no doubt that 

 depth of water, after the first fathom, does play a large part 

 in determining the period of submersion ; and this is, I think, 

 borne out in the table of dives (p. io8), taken from my 

 records. The time is in seconds, the depth in fathoms, and 

 I have, of course, made allowance for the state of the tide. 



It would seem that the period of the dive not only 

 increases with the depth of water, but that it also tends to 

 l)ecome more regular. 



As an example of the variation in the periods of individual 

 d'.ves and sets of dives, I cannot do better than give a few 

 further extracts from my records. The first three sets may 



