FEEDING-HABITS OF FISH AND BIRDS. 51 



The only other animal which trout appear to avoid is the tadpole. In 

 lakes and in still pools of most trout-streams tadpoles are found, but I have 

 never seen trout feeding upon them, nor have they been recovered from the 

 alimentary canal. On the river Exe, during 1911, a small pool was observed 

 in which were four trout and tadpoles estimated to the number of two 

 hundred : no diminution of the number was detected during four days. 

 Similar observations were made in a hatch-hole on the river Lambourne at 

 Shelford in 1910, during ten days, with similar result ; the possibility of 

 tadpoles dropping down from above was excluded. Nevertheless, on a few 

 occasions anglers have observed trout to be feeding on tadpoles in waters 

 where there was a scarcity of food. The colour of the tadpole deep black, 

 its gregarious habits, its making no attempt at concealment, indicate that the 

 animal is not much open to attack. It is also noteworthy that the vivid 

 yellow of the "yellow May dun" (//. sulfuria) makes this insect a con- 

 spicuous object on the water ; it is certainly more easily seen than any other 

 of the smaller British Ephemeridse. 



These observations indicate that trout do recognise a difference in palata- 

 bility between different foods ; also that the relatively unpalatable foods are 

 occasionally devoured. No small animal which lives either in, or upon the 

 surface of, fresh water has been observed to be entirely immune from the 

 attacks of trout. 



The feeding-habits of other fish, in respect of floating insects, is of con- 

 siderable interest : Thijmallus vulgaris, Nilss. (the Grayling), takes floating 

 insects as freely as trout : Leuciscus leuciscus (the Dace) also feeds freely on 

 floating insects ; Leuciscus cephalus, Fl.em. (the Chub), in mam- streams, 

 only feeds on floating insects during the " grannom " (Bracliycentrus sub- 

 nubilus) and " May-fly " seasons, or when an especially large hatch of some 

 other insect occurs, in other streams it rises almost as freely as the dace ; 

 Leuciscus rutilus, Flem. (the Roach), is very seldom observed to be feeding 

 on floating insects, but during the "May-fly" (Ephemera danica) season it 

 not infrequently takes the sub-imago. 



As will be seen, a similar sequence is found among the birds : some feed 

 only upon the " May-fly " (E. danica), others take other water-insects as 

 well, whilst others again feed freely upon almost every riverside species. 



Before considering the significance of these conclusions with regard to 

 warning coloration, some observations on the feeding-habits of birds will be 

 described. 



Cypselus apus, III. (Swift), I/irundo rustica, Linn. (Swallow), Chelidou 

 urbica, Boie (Martin), and Cotile riparia, Boie (Sand-Martin), feed freely on 

 the sub-imago of the Ephemeridre, either taking the insects whilst resting 

 on the water or after they have risen into the air. It is rare to see a hatch of 

 these insects without these birds hunting them, so much so that by watching 



