58 Gross, The Yellow-billed Tropic-bird. [f^ 



The period of incubation was not accurately ascertained, since 

 the exact dates of the laying of the eggs under observation was not 

 determined. One egg, which seemed fresh when found, required 

 28 days of incubation for the young to emerge. Other eggs under 

 daily observation required from 15-25 days, but evidently these 

 had been incubated some days when first observed. 



The shell membranes of the eggs are resistant and leathery, a 

 condition well adapted to withstand the sharp points of the stones 

 and the hard barren surfaces on which the single egg is usually 

 deposited. The young require considerable time to rupture this 

 membrane even after the egg is pipped and fragments of the cal- 

 careous shell are In-oken away (Plate VI, fig. 9). In one, perhaps 

 unusual, case the egg was pipped and the "peep" of the contained 

 embryo was heard 42 hours before it had completely emerged. 



The adult birds became accustomed to my frequent visits to their 

 nests and allowed me to stroke them or to remove the egg without 

 any sign of resistance. At the hatching of the egg, however, there 

 was a profound change in the behavior of the parent birds. 



The presence of the little one seemed to incite in them a ferocious 

 antagonism against any intruder, and it was only with the greatest 

 care and precaution that the little one could be safely removed for 

 study. After some time had elapsed the parent birds seemed to 

 adjust themselves to the new conditions and were approached 

 with less or no opposition. 



Nesting birds, although not accustomed to regular visits by 

 any one, would allow an observer to operate a camera within a few 

 feet of them, if care were taken to make no quick or unexpected 

 movements. If the intruder approached nearer, the bird bristled 

 up its feathers, spread its wings, and responded with a quick thrust 

 of its sharp beak. After repeated annoyance the parent bird will 

 leave the nest, an act, however, which it seems very averse to doing. 

 This reluctance may be due to its helplessness while on its feet. 

 The birds never walk upright, but the body is shoved along in a 

 cumbersome manner by their diminutive legs. The wings are 

 often brought into service for supporting and balancing the com- 

 paratively heavy body, which is scarcely raised above the surface 

 on which the bird is moving (Plate IV, fig. 3). On first leaving 

 the nest the adult bird leaps from the ledge and nimbly catches 



