94 Proceedings of the Biological Society of Washington. 



INSTINCTIVE FEEDING HABITS OF YOUNG HERONS. 



During 1916 I noticed an interesting phenomenon exhibited by certain 

 young herons during feeding. Mr. Alfred Worm of Manila, Philippine 

 Islands, had, in his possession, two young herons, only a few weeks old, 

 belonging to the species Pyrrherodias manillensis ( Meyen ) . These he was 

 accustomed to feed on small, dried fish which were thrown to them on 

 the floor of the verandah where they were kept. I was present on one 

 occasion when the birds were being fed and noticed that they seemed to 

 be unable to secure their food without considerable effort. The first 

 thrust for each fish in turn invariably fell short of its mark, and it was 

 only after several attempts that the object was secured. The birds seemed 

 to profit by experience enough to vary their attempts until the morsel 

 was obtained, yet as each new tidbit was thrown to them it was, like the 

 others, first missed and then, after several attempts, captured. 



The persistence of this first under-reaching suggested a possible clue 

 and an experiment was made to test out the theory. A basin of water 

 was obtained and placed on the floor of the verandah and into the water 

 was thrown one of the dried fish. True to expectations one of the herons 

 secured this at the first attempt. The experiment was repeated a number 

 of times, and always the fish was picked up with one thrust of the bird's 

 beak. Another fish, thrown on the bare floor, was missed as before. 



The explanation of the matter seems to lie in the refraction of light 

 rays passing from a denser to a less dense medium, as in this case, from 

 water into air. By this refraction, the fish submerged under water would, 

 to the herons looking down diagonally from above, appear to be some- 

 what more distant than they were in reality, and to reach them by a 

 direct stroke, it would be necessary for the birds to aim their thrust at a 

 point nearer than the apparent object. When the same ' correction ' 

 would be applied in the case of food not submerged, the result would, of 

 course, be a miss. 



Since perhaps the majority of food secured by herons is taken from 

 under a few inches of water, it is quite conceivable that a habit which 

 would lead them to make a correction for refraction would result in an 

 increase in the food supply obtained and would tend to become instinc- 

 tive. That the habit was instinctive in the two birds under consideration 

 seems unquestionable since these were taken from the nest before they 

 can have had any actual experience or instruction in the art of capturing 

 submerged prey. 



Whether or not birds of this group in a wild state and born with this 

 instinct would eventually learn to discriminate between non-submerged 

 and submerged objects and would acquire the ability to strike unerringly 

 and with equal facility at desirable prey in either situation, is a question 

 which I am at present unable to answer. It would be interesting to know 

 whether or not the same habit is inherent in all young herons. It may 

 prove to be not so much a habit as a phenomenon due to some special 

 modification of the herons' organs of vision. — John T. Zimmer. 



