CONDUCTING BREEDING INVESTIGATIONS 593 



Since so much has been said about the Maine Station investigations 

 of fecundity in fowls, perhaps it would be of some interest to know how 

 records are obtained there. The type of trap-nest in use is shown in 

 Fig. 229. Details of construction need not be taken up here, except to 

 remark that durability of materials is a prime requisite for continuous 

 service. The absence of any springs or other involved contrivances has 

 made it possible to use this type of trap-nest in extensive breeding in- 

 vestigations involving a flock of about 2000 hens. Ten such nests are 

 used in a pen of fifty birds, and an attendant visits the pens at intervals 

 of one hour or more, depending upon the rate of egg laying. Obviously 

 a method such as this is expensive even when reduced to the simplest 

 terms, and it is, therefore, applicable only to the selection and production 

 of breeding stock. It is difficult, however, to conceive of any other 

 accurate criterion which might be adopted. 



It should be noted that statistical requirements do not demand that 

 complete records be obtained, for the existence of modifiability and other 

 kinds of individual variability make it impossible in any event to get 

 anything but an approximate record. Accordingly in recording the data 

 of production of dairy cows, for example, it is not necessary to weigh and 

 test the milk every day for the whole period of lactation, but two or 

 three 7-day periods at stated times with respect to the beginning 

 of lactation will give a sufficiently accurate estimate for all practical 

 purposes. Similarly in poultry breeding, Pearl has found that produc- 

 tion during the winter period is a sufficiently accurate and distinctive 

 index of the egg-laying capacity of a hen. 



Further the danger from unjust comparisons should always be empha- 

 sized. A comparison between egg production of hens in the second 

 laying season and pullets would favor the pullets, for pullets ordinarily 

 lay more eggs during the first season than they do as hens in the second 

 season. Moreover different parts of a given season are not equivalent. 

 A pullet lays more eggs in a given length of time during the spring cycle 

 beginning about March 1, than she does during the winter cycle. A cow, 

 likewise, produces more milk during the early part of her lactation period 

 than she does later on, and she reaches her maximum capacity at 5 

 or 6 years of age. With respect to these points we have reproduced 

 in Table LXVI, the comparative indices which Pearl has calculated and 

 which provide a method of comparing the productions of cows of different 

 ages at different stages in the lactation period. As an additional variable 

 in this case we should include the time at which a cow freshens, whether 

 in spring, summer, or fall, as having a definite influence on herd produc- 

 tion of milk and butter fat. We could go on recounting without end 

 such factors which must be considered in making accurate comparisons. 

 The point, however, is sufficiently obvious, namely, that even objective 



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