60 Gross, The Yellow-hilled Tropic-bird. [j^ 



in. Life History. 



In the study of the Hfe history of the Tropic-bird I was somewhat 

 handicapped, since the life of the young spent in the nest extends 

 over a peroid longer than the time at my disposal during either 

 summer in Bermuda. 



However, there was an abundance of material near the Bio- 

 logical Station, which enabled me to secure measurements, descrip- 

 tions and photographs of every phase of their development. Daily 

 observations and measurements were made of five birds of various 

 ages, two of which are at either end of a complete series. By a 

 careful comparison of the measurements (see Table, pp. 61-62) 

 and descriptions of these two birds, it was possible to match them 

 at the 33rd day of development. In making these comparisons 

 the measurements of the tail, wing, bill, and foot were given greater 

 weight, since these measurements seem to be more constant for 

 different individuals of the same age. Furthermore, it is extremely 

 difficult to measure accurately the extent and length of an active 

 living bird, which never fails to offer great resistance to such a 

 procedure. If the comparisons are correct, the length of time 

 spent by the young Tropic-bird in the nest extends over a period 

 of 62 days, or about two months. The time required for incubation, 

 previously noted, is about 4 weeks, making the complete period 

 about 3 months. The adult birds remain in the islands about 

 7 months, which affords them ample time to rear two broods, but 

 not more, during any one summer in Bermuda. The following 

 records of three of our land birds are interesting in comparison with 

 those of the Tropic-bird. 



The measurement of extent in the freshly hatched bird is less 

 than the length, but this relation is reversed at the end of the 6th 

 day, and by the time the young bird is ready to leave the nest the 

 extent becomes three times the length minus the tail or more than 



1 W. L. Finley, American Birds, pp. 245-246. 



2 A. R. Sherman, Wilson Bull., 1910, pp. 135-171. 



