FEEDING-HABITS OF FISH AND BIRDS. 53 



Ephemeridse could be arranged in an order of palatability, both for birds 

 and fish, headed by Ephemera danica, as probably the most palatable, and 

 Heptagenia sulfuria, as the least. 



The habits of tlie Swift are of particular interest because of its great 

 agility, and the great pace at which it hunts would undoubtedly enable it to 

 take any flying insect — for instance, when feeding on the ''May-fly," the 

 impact of bird and fly can be distinctly heard at a distance of ten to fifteen 

 yards. I have seen them take white butterflies on two occasions. 



Obs. — " 7. 5. 1913, K. Kennefc, swifts feeding on '• May fly,' many Pierines cross- 

 ing the river : the bird caught one with the greatest ease and at once rejected it, 

 the insect fell on the water and was not recovered. 15. G. 1910, 'swift 7 took a 

 small garden white at a siugle swoop, then passed behind a willow tree, out 

 of view." 



These observations are recorded because they show that the birds could 

 capture these butterflies did they care to. As a proof that much less agile 

 birds are capable of capturing rapidly flying insects, it may be mentioned 

 that on three occasions Sparrows have been observed amongst new-mown hay 

 successfully chasing Tri/phcena pronuba, Hiibn. (Yellow (Jnder-wing Moth) ; 

 on two occasions small flocks of these birds were observed feeding in this 

 manner for several hours. Wagtails on several occasions have been observed 

 systematically hunting Musca domestica, Linn, (the House-fly), and Scatophaga 

 stercoraria, Latr. (the Dung-fly). 



The Significance of these Observations with respect to Warning 



Coloration. 



It aj^pears that whether or no an animal will prey upon another depends 

 upon a number of factors, of which the following are the most important: — 



(1) The animal's general hunger : a hungry trout will take all that conies 

 within its reach, sometimes even tadpoles; a less hungry fish will only take 

 the more palatable foods, whilst an almost satisfied fish will only take the 

 most palatable. 



(2) The animal's special hunger : trout, and less certainly birds, prefer a 

 mixed diet, if an animal has lately become satiated with a particular food, 

 then this food is temporarily lowered in the scale of palatability. 



(3) The readiness with which a food can be gathered : animals difficult to 

 capture will be neglected at the expense of those easy to take. 



(J-j The prevalence of a food-supply : trout, and less certainly birds, appear 

 to concentrate their energies upon hunting one prey at ;i time ; for this 

 reason they will attack a prevalent prey in prefeience to a scarce one (it has 

 been often observed that birds will feed upon insects when they are unusually 

 abundant and which they have never before been seen to attack). 



