380 The Wilson Bulletin — No. 90 



&gg laid : this naturally results in irregularities in hatching and 

 maturing of nestlings ; far more frequent in the Altrices for 

 the obvious reason that belated embryos of Prcecociah usual- 

 ly perish in the shell after the parents desert the nest with 

 the first hatched young. 



The period of incubation seems to depend almost altogether 

 upon mere size or bulk ; conditions being equal, the larg^er the 

 egg generally the longer the period of incubation. This -rule 

 would seem tO' be good, with some exceptions ; but more truly 

 applicable to the various members of the several groups, in 

 some instances possibly not higher than families. 



The assertion that the length of incubation depends upon 

 the state of perfection in which the young issues from the 

 shell, i.e. chicks able to leave the nest almost immediately af- 

 ter hatching (prsecocial) require a much longer time than 

 those hatched in a helpless state (altricial), appears unreli- 

 able when applied generally, even though the former averages" 

 larger eggs. The absence of data on comparative sizes, 

 weights or other means of equalization render it somewhat 

 difficult to prove, but for some of the many probable 'excep- 

 tions compare in size and period of incubation the Petrels 

 with the smaller Terns, the Tropic Birds with the Gulls, the 

 larger Raptor es with the Anaiidcc, the Woodpecker with the 

 Coot, the smaller Raptores with the smaller GaHincc. the 

 Kingfisher with the Bob-white, or even the Chat with the 

 Spotted Sandpiper. The Noddy, departing from the typical 

 Sfeniiiice in its habit of nesting in bushes and there rearing its 

 young for upwards of two months (therefore not typical 

 p-rjecocial) is said to incubate for 35-36 days, which seems 

 a much longer period than that of any other of its praecocial 

 family. 



Birds nesting in snug rock crevices, tree cavities, or in 

 covered nests of any descriptions capable of keeping out the 

 weather and conserving the heat from the birds' bo^y, doubt- 

 less are at no great disadvantage over the loss of the direct 

 rays of the sun ; but species nesting deep in earthy burrows 

 (Puffins, Petrels and Kingfishers) or in draughty flues 



