366 BULLETIN 17 6, UNITED STATES NATIONAL MUSEUM 



another hummer alight in the nest and rearrange some of the material, after- 

 wards sitting there for some time until the presumably rightful owner presently 

 darted at the intruder and drove her away. The nest was composed largely 

 of small achenes bearing soft pappus. Other items noticed were fibers, minute 

 leaves, feathers and a short piece of string, the whole bound securely to the 

 branch with cobwebs. 



Eggs. — Costa's hummingbird lays almost invariably two eggs. Sid- 

 ney B. Peyton has a set of three eggs, taken in Ventura County, 

 Calif.; a third egg was laid to complete a set of two, now in the 

 Thayer collection, after one ^^^ had fallen from the nest. I have 

 no record of a complete set of one. The eggs are like other humming- 

 birds' eggs, dead white, without gloss, and elliptical-oval with an 

 occasional tendency toward elliptical-ovate. The measurements of 

 51 eggs average 12.4 by 8.2 millimeters; the eggs showing the four 

 extremes measure 14.0 by 7.8, 12.7 by 9.4, 11.4 by 7.8, and 11.6 by 7.6 

 millimeters. 



Young. — There seems to be considerable difference of opinion as 

 to the period of incubation, which is performed by the female alone. 

 Mr. Woods (1927b) writes: 



An interval of about two days separates the laying of the eggs. Incubation, 

 in every case that I have observed, has begun with the laying of the first egg, 

 and the young are usually hatched out a day or more apart. If we are to 

 accept the widely differing periods reported — from nine to eighteen days — the 

 time of incubation must be regarded as extremely variable. On account of ab- 

 sences I have not succeeded in collecting as full information on this subject 

 as could be desired, but data of varying accuracy obtained during five successive 

 years indicate that the normal incubation period for Costa's Hummingbird in 

 the San Gabriel Valley is about sixteen days, lengthening in certain instances 

 to as much as eighteen days, but never falling below fifteen days. * * * 



The growing period of the young is even more markedly prolonged than is 

 the incubation. Some eight broods for which the time was determined with 

 fair accuracy remained in the nest from twenty to twenty-three days after 

 hatching, with all but two approximating the higher figure. * * * 



Even more pronounced than in most other altricial birds is the contrast be- 

 tween the newly hatched Hummingbird and its parents. The minute grub-like 

 creature is black above and brownish below, with the body entirely bare except 

 for a row of yellowish filaments along each side of the median line of the back. 

 The bill is yellow and triangular, its length being but slightly greater than its 

 width at the base. The eye sockets project beyond the base of the bill. Until 

 about the sixth day, when the pin-feathers begin to appear, the most notable 

 change, aside from the increase in size, is the gradual lengthening and darkening 

 of the bill. The first part of the young Hummingbirds' lives is spent stretched 

 out on the bottom of the nest, but after a time they become longer than the 

 interior of the nest, so that they are gradually forced to raise their heads 

 against its sides until at one stage of their growth their bills are pointing 

 directly upward. After this their development is more rapid, and when they 

 begin habitually to hold up their heads and assume an alert appearance, they 

 are nearly ready to fiy. The last few days before leaving the nest, the young 

 birds frequently exercise their wings, sometimes perching on the edge of the 

 nest for freer action. Finally a time is reached when, contrary to their former 



