of determination of clutch length, but not the interwal be-~ 
tween clutches. They showed in hens that 4eegg cycle birds had 
eel pa thyroids and higher secretion rates than did 2-egg cycle 
rase 
2. Inhibition of egg production. 
For reviews of inhibition prior to 1950, the following 
works are helpful: Sturkie Avian Physiology, 1954; Cole and 
Cupps agproduet ion in Domestic Animals, 1959; Romanoff and 
Romanoff The Avian Egg, 1949 and Romanoff The Avian Embryo, 1960, 
The two basic measurements of the success of oogenesis 
inhibition are per cent egg production and per cent hatchability 
resulting from a treatment as compared with a reference con- 
trol. It is appropriate to first consider a paper by Titus 
(1953) on the size differences in these two measurements nec- 
essary for significant difference, Titus ran a "uniformity 
trial" with 600 Rhode Island Red pullets which had not been pre- 
tested or selected for uniformity. He found the coefficient of 
variation of the number of eggs layed to be between 17 per cent 
and 25 per cent with a modal value of 20 per cent. For hatch« 
ability, the coefficient of variation was 12 per cent to 350 per 
cent with a 20 per cent modal value. Therefore, he estimated 
the number of birds needed to show a difference in egg pro- 
duction of 10 eggs when average egg production was 200 eggs and 
a difference in hatchability of 5 per cent when the average was 
70 per cent (at the 5 per cent level of significance using the 
modal coefficients of variation) would have to be not less than 
123 birds for egg production and 60 for per cent hatchability. 
The use of the smallest coefficient of variation requires not 
less than 94 birds for egg production and not less than 24 for 
per cent hatchability. On the basis of these criteria, the 
conclusions of some of the following papers might well be 
questioned. 
Egg production as a physiological process is influenced 
by a number of external environmental factors. The literature 
on these, particularly on light, is voluminous enough to be a 
seminar in itself and, therefore, I shall not dwell on these, 
other than to mention them. They are not generally adapted to 
manipulation as far as wild birds are concerned. A review by 
Marshall (1959) covers a number of these. 
As the work of numerous workers has shown, light 
stimulates egg production; lack of it or too much of it may have 
an inhibitory effect. However, some species particularly those 
in dry areas and the tropical regions may not be directly af-~ 
fected by light regulation. 
Temperature and rainfall are two important factors for 
some bird species. Sudden drops in temperature or extremely high 
54 
