A good example that measurements are not of necessity in- 

 dices of bulk is given by a comparison between the black 

 vulture and the turkey buzzard, for the latter's measure- 

 ments are greater than the former's, yet its bulk is less (100). 

 This point is mentioned as illustrating again how con- 

 fusing would be bulk or measurements if considered as in- 

 timately related to, or strongly influencing, the length of 

 incubation. 



It seems to the writer that bodily bulk must have been 

 meant when the expression "size of bird" was used; bodily 

 bulk and weight are closely related, but both vary more or 

 less according to the bird's age, the sex, the season of the 

 year and the abundance of food, etc., etc. 



The sequence of the steps of development in bird em- 

 bryos being practically the same in all birds, it would seem 

 reasonable to believe that the larger the bulk to be grown, 

 the longer it should take to complete its evolution. How- 

 ever, it is not alone the diifering bulk to be produced wdiich 

 may help to bring about varying incubation periods, but 

 also the fact that many of these steps of development in 

 different species are greatly abbreviated or jumped almost 

 completely ; hence, one can say that it is the speed and 

 duration of the different steps in embryonic development 

 which produces differences in the lengths of incubation. 



It is quite patent that, in a general way, there is a rela- 

 tjon between the size (or bulk) of a bird and the duration 

 of its incubation period, yet there are so many striking ex- 

 ceptions that one, at best, must hold it to be only a loose 

 and rather indefinite correlation, and probably not a rela- 

 tion of cause and effect, but rather an example of two effects 

 influenced by a single underlying cause, a suggestion to be 

 more fully elaborated later on. 



Using the words "size of bird" in the rather indefinite 

 way found in most writings, one finds some interesting con- 

 ditions in its relation to the length of incubation, having 

 in mind the prevailing notion that the larger is the bird, 

 the longer is its period of incubation. 



Notice the difference in the sizes (or bulks) of the fol- 

 lowing pairs of species, each pair being recorded as having 

 similar incubation periods: great-tailed grackle and tree 

 creeper, chipping sparrow and evening grosbeak, golden 

 eagle and puffin, ostrich and kiwi. If bulk or size alone 

 counted with these species as the controlling factor deter- 

 mining the length of incubation, there should be noticeable 

 differences in the incubation periods of these birds, rather 

 than a definite similarity, as is the case. On the other hand, 

 it is not uncommon to find a decidedly smaller bird having 

 a longer incubation than is found with a decidedly larger 



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