318 BULLETIN 16 7, UNITED STATES NATIONAL MUSEUM 



Eggs. — Normally two, but sometimes three in number, while very 

 old birds may produce only one. They are usually white, not in- 

 frequently nest stained, but genuine markings, if they occur at all, 

 must be of very rare occurrence. Some supposed instances are cer- 

 tainly due to faulty identification where other large raptorial birds 

 are breeding. There is little or no gloss; some eggs are covered 

 with shallow grooves ; others show distinct but shallow pores. In size 

 there is much variation. Schiller (1931) records the measurements of 

 29 eggs of H. a. groenlandicus as averaging 75.3 by 58.3 millimeters. 

 Maxima, 82.3 by 63.8; minima, 67.4 by 55.3 and 70 by 55 millimeters. 

 Twenty-one eggs measured by the writer together with 11 by Dr. 

 Rey of the same race average rather larger, 77.54 by 58.09 : Maxima, 

 84.4 by 60.7 and 76.4 by 61.7; minima, 72.4 by 56.7 and 75.6 by 53.4 

 millimeters. 



Eggs from Scotland are also large : Average of 52 (measured by 

 the writer), 75.73 by 58.68 millimeters. Central European eggs are 

 smaller: 21 Danish eggs measure 74.5 by 56.9, according to Schi0ler 

 (1931), while 12 eggs from Asia Minor average only 71.8 by 56.2 

 millimeters (writer's measurements). Dr. Rey gives the average 

 weight of German eggs as 13.325 grams. 



The eggs are laid at intervals of a day or two, and incubation 

 apparently begins with the first egg laid, for there is generally a 

 decided difference in size between the young. The greater part of 

 the incubation is done by the female, but apparently the male takes 

 some part. Siewert (1928) flushed only the hen from the eggs and 

 many females have been shot from the nest. Saxby (1874), how- 

 ever, believed that the male was sometimes on the eggs, and in con- 

 finement he has been seen to take his turn. The period is still some- 

 what uncertain; Faber (1826) gave it as 35 days, but Krause (1926) 

 says 40 to 42 and Hortling (1929) about 45 days. 



Young. — The newly hatched young are covered with creamy or 

 grayish buff down at first, and as they differ in size considerably, 

 the smaller is generally bullied and not infrequently dies. In nests 

 on open ledges or on the ground, the smaller bird can back away 

 more readily when attacked and stands a better chance of survival 

 than in a tree nest. The food is chiefly brought to the nest by the 

 male and distributed to the young by the female. The growth of 

 the young is a long process; Heinroth has given the weights 

 of nestlings at different periods showing the rate of increase from 

 the early stages to 42 days old — from 86 to 4,090 grams! The stay 

 in the nest is at least six to seven weeks, so that the whole period 

 from the laying of the first Qgg to the flight of the young lasts cer- 

 tainly not less than three months, and naturally only one brood is 

 reared in the season. 



