76 HOMING AND RELATED ACTIVITIES OF BIRDS. 



RECOGNITION OF THE YOUNG. 



Closely related to the problem of orientation and recognition of the nest 

 in the terns is that of the recognition of the young, and in this respect the two 

 species show much greater differences than in nest recognition. The noddies 

 do not distinguish their own young from other young noddies and I have been 

 unable to discover any difference in their active behavior toward either strange 

 noddy or sooty young. 



Experiment 12. 



I placed a sooty chick, 24 hours old, in the nest of a noddy, taking out the noddy egg which 

 was just hatching. The noddy came back to the nest and covered the young sooty immedi- 

 ately. I watched the nest at frequent intervals throughout the day. The noddy brooded 

 the young sooty constantly, but was not seen to feed it. On the second and third days the 

 noddies continued on the nest, but were not seen to feed the sooty, which on the third day 

 was considerably smaller than other chicks of the same age. On the fourth day, when the 

 noddies were driven from the nest, the chick was found dead, apparently from starvation. 

 Another sooty chick, 4 days old, was placed in the nest. The noddies continued to brood it 

 for 3 days. On the fourth day the chick was dead and the noddies had abandoned the nest. 



The noddies accept sooty chicks in exchange for their own young of any age, 

 as was determined by a number of such substitutions. In no case, however, 

 was a noddy seen to feed a young sooty. Apparently the peculiar behavior of 

 the young noddy is required to call out from the adult the instinctive act of 

 regurgitation and feeding. 



Experiment 1.3. 



Two 2-day old noddy chicks were placed in a nest which already contained a 3-day chick. 

 One of the younger chicks was white, the other black. The older chick was black. The 

 adult noddy returned and covered the three chicks immediately. The nest was observed 

 regularly for 1.5 days, when the work was interrupted by heavy storms. The three young 

 chicks were fed by the single pair of adults and throve as well as chicks which were alone 

 in the other nests. On the eleventh day one of the chicks disappeared. The others were 

 still in the nest when observations were interrupted. 



During the first days after the hatching of the egg the noddies do not dis- 

 tinguish their own young from those of other birds and react to them only 

 because of their presence in the nest. On the fourteenth day, or earlier, the 

 young noddies leave the nest and lie hidden in the bushes during the day. 

 In the evening they are fed, returning to the nest to meet their parents. Wat- 

 son states that at this time the noddies distinguish tlieir own young. I have 

 no clear evidence upon this subject, but it .seems probable that, as in the 

 case of the sooties, the recognition is rather due to a dovetailing of the habits 

 of parents and young than to a visual or auditory recognition. 



Unlike the noddies, the sooties give prompt reaction both to the appear- 

 ance of the chicks and to the number of chicks in the nest. They will not 

 usually accept young noddies under any circumstances. In one instance a 

 sooty which had been given a noddy egg hatched it and brooded the chick for 

 a few hours in the morning, but in the evening both adult and chick were gone. 

 Usually where a young noddy is placed in a sooty nest the old bird throws it 

 out and drives it far away into the bushes. The reaction is to the visual stim- 

 ulus,* and is purely in.stinctive, as is shown by such results as the following: 



Experiment 14. 



(a) I interchanged a sooty egg, just ready to hatch, and a 24-hour chick. The parent of 

 the egg accepted the chick immediately. The other hesitated in covering the egg, looked 

 about, walked to and fro for several minutes, and finally settled down upon the nest. 



♦Figures of young noddies and sooties are given by Watson, opus cil. 



