548 Journal of Agricultural Research voi. xii. no. 9 



In discussing the monthly egg distribution Pearl and Surface (7, p. 

 89-90) state: 



Considering the form of the polygon somewhat more in detail, we note that the 

 line starts from a low point in November and rises rather rapidly and in almost a 

 straight line to January. The slope of the line from January to February is down- 

 ward. In other words, there is an indentation in the ascending limb of the egg-pro- 

 duction polygon in the month of February. This is a very characteristic featiire of 

 the distribution of egg production, not only observed with the birds here under 

 discussion but also in published records from other sources. 



A study of Table i shows that this is generally true for every year covered by the 

 investigation. While the February mean production is not necessarily lower than the 

 January, though this is true in many cases, there is a perceptible slowing of the rate 

 of increase in egg production which has obtained up to that time. The most prob- 

 able interpretation of this appears to us to be that the February indentation in the 

 egg-production curve represents a rest or reaction after the winter laying and in 

 anticipation of the heavy March and April production. It marks the completion of a 

 laying cycle on the part of those birds which have been laying dturing the winter 

 months. 



The mean egg production for February, however, seems to rest on a 

 basis of 28 days. If reduced to a basis of 31 days, the mean production 

 is 12.03 eggs, a value, however, that produces a pronounced indentation 

 in the upward slope of the polygon. Pearl believes that this change in 

 the slope of the polygon, disregarding entirely any actual drop in pro- 

 duction, is indicative of a winter cycle. However, a change in the 

 slope of such a polygon would occur if a flock of birds began to lay in 

 some given month, gradually increased in production for a definite 

 period of some length until they reached a maixmum and maintained 

 this maximum for a period of several months. Pearl, moreover, is 

 inclined to believe that the change in mean temperature that occurs in 

 March bears no causal relation to the increase in egg production that 

 is observed at this season. Now, Avhile it is clear that there are other 

 factors than the change in temperature that increases production at 

 this season,, it is not at all clear that temperature can be entirely elimin- 

 ated as a factor. In the imaginary case just mentioned it seems pos- 

 sible that the maximum production under one set of environmental 

 conditions might be different from that observed under another set. 

 Hence, it seems entirely probable that the maximum production 

 possible in January and February would not be as high as in March, 

 and that the indentation noted in the curve of production merely means 

 that the maximum production possible for midwinter conditions has 

 been reached. 



In several other papers, notably those of Pearl (5, 6), the winter cycle 

 is again discussed. In the former paper he states (5, p. 173-174) : 



(2) The upper limit of the winter period at March i is arbitrary, and only approxi- 

 mately coincides with the biological reality. Actually with most birds the spring 

 or reproductive cycle of production (cf. 37) begins in the latter part of February. 

 In handling the material it has been found necessary (for reasons which will be 



