Weight Authority 



Red-backed Junco $ July 70 78 



Cassin's Sparrow $ July .70 78 



Western Song Sparrow $ Feb. 28 88 175 



" Tree " 9 avg. 2, Feb. 28 65 175 



$ avg. 3, Feb. 28.... .71 175 



Spurred Towhee 9 June 1.50 78 



Black-headed Grosbeak $ June 1.30 78 



Lark Bunting $ June 1.50 78 



It will be necessary here to consider but a few compari- 

 sons of different species and their weights and lengths of 

 incubation; the ostrich, kiwi and emperor penguin have 

 identical incubation periods, yet their weights are two hun- 

 dred fifty, four, and ninety pounds, respectively; the do- 

 mestic goose and the sparrow hawk have nearly similar in- 

 cubation lengths, yet the first weighs ten pounds, while the 

 second but five (more or less) ounces; the ruby -throated 

 hummingbird and Cassin's vireo incubate their eggs almost 

 exactly the same length of time, yet one weighs one-tenth 

 of an ounce and the other six-tenths of an ounce. These 

 examples form comparisons between species of different 

 families, where one can expect such lack of parallelism be- 

 tween weights and incubation periods. With species within 

 the confines of a single family, however, equally sharp lack 

 of correlation of weights and incubation lengths occur; the 

 bobwhite and the grey partridge have similar incubation 

 periods, yet their weights are as five and one-half is to 

 twelve, and there are several species in Fringilline birds 

 which have identical incubation lengths, but differ markedly 

 in weight. 



While the available data on bird weights is, deplorably 

 insignificant, when the species involved are compared to 

 the total number of known birds, yet it would seem reason- 

 able to expect more indications of a relation of weight to 

 incubation length, if the weight fixed this length, than one 

 finds in the data at hand. I feel that whatever relationship 

 appears in these curves of weight is not one of cause and 

 effect, but, as has been said before, a correlation of two effects 

 to a third factor as an underlying single cause. The singu- 

 lar and suggestive fact in this phase of our question is that 

 with man's domesticated birds with which one can see a 

 variation of. at times, several hundred per cent, in weight, 

 there should be no corresponding change in incubation. 



It is highly desirable to have more recorded weights of 

 birds, especially of the breeding female, since biologic char- 

 acters of birds will be more and more called upon to aid in 

 the future in solving many riddles in avian physiology. 

 Until a much larger mass of data along these particular 



25 



