The Behavior of Xoddy and Sooly Terns. 213 



rence, especially late in the afternoon, when the frigate-birds are returning, 

 to see hundreds of such fights. The noddy is always careful to attack the 

 frigate-birds by sudden thrusts (usually made from below), dodging quickly 

 to avoid their fearful and powerful beaks. 



The curlew is also an occasional visitant on the island. The appearance 

 of this uncouth bird brings forth fierce attacks from the noddies at this period 

 of their stay. Figure 23, plate 10, shows an attack upon one of these birds. 



The parent birds alternately feed the young. The length of the intervals 

 between feedings varies from 2 to 4 hours. The appearance of the young 

 does not alter the feeding relations of the parents, which were found to 

 hold in the table shown above. The interesting additional factor is that now 

 when the birds go out to feed they must bring back a supply for the young 

 bird. The question immediately arises: Does the parent actually catch more 

 fish after the young is hatched, or does it catch the normal supply and share 

 that with the young? If it catches an additional supply, what is the stimulus 

 which leads to a more abundant filling of the crop? If an additional supply 

 of fish is not caught, we should expect ( i ) either that the birds would feed 

 oftener or (2) that they would become very much emaciated while they are 

 caring for the young. Since my observations show that the birds do be- 

 come emaciated during tlie rearing of the young, and that they apparently 

 do not go out to feed any oftener, I am inclined to think that no additional 

 supply of fish is (instinctively) provided for the young. The parents simply 

 disgorge when the appropriate stimulus is at hand, /. r.. the sight and contact 

 of the young. Since this process would tend to make the parent birds verv 

 hungry (or rather enormously to decrease intra-organic pressure) thev prob- 

 ably actually catch more fish on any one trip than they would otherwise.^ 

 But this larger catch may be due to an emptier stomach and not to any 

 instinctive tendency to provide food for the voung. 



The apparatus for disgorgement is easily thrown into activity in these 

 birds very much more so than is the case with the sooties. My appearance 

 at a group of nests in which young were present would call forth many such 

 responses. As has already been mentioned, the noddy stays close by to 

 defend its young. The bird usually sits on the nest and scolds in its peculiar, 

 rasping way. Apparently this vocal effort can be better accomplished after 

 the crop has been relieved of its contents : the bird can also probablv fight 

 better. Outside of these two possible reasons. I can offer no additional 

 suggestion as to the biological value of this function when it is exercised 

 in this connection. 



At this point, as well as at all others, one finds extreme monotony, fixed- 

 ness, and lack of variability in the responses. 



' Since the birds nearly always are supplied with enough fish to disgorge at dis- 

 turbance I doubt whether "hunger" in the usual sense of the word could under ordi- 

 nary conditions be attributed to them. Increase or decrease in the hitra-organic 

 pressure could function in the same way. 



