A diurnal variation in spermatogenic activity has been 
observed in the domestic fowl by McCartney (1942) and Riley (1940). 
McCartney found the peak activity at midnight and Riley found 
the peak activity at 3 am. 
The testes of wild birds are extremely sensitive to 
variations in the diurnal light regimen which must exceed a 
certain minimum. In the Starling (Sturnus vulgaris) 12.5 hours 
of light is required (Burger, 1940). Photoperiodism in birds 
does not in general permit a distinction between the effects of 
a long light period and that of a short, dark period (Kirkpatrick 
and Leopold, 1952). According to Fenner and Engels (1952) a 
night interruption will also induce sexual maturation in juncos 
and White-throated Sparrows (Sonotrichia albicollis). Carson 
et al. (1958) found that red and gold illumination treatment 
significantly suppressed subsequent sperm production of cocks as 
compared with the incandescent lighting used on the controls. 
Evidence for the cyclic pattern of gonad activity seems 
to be associated strictly with the lengthening day if one does 
not consider the exceptions. Chapin (1954) reported evidence 
compiled over a period of eleven years that showed the Sooty 
Terns (Sterna fuscata) on Ascension Island assemble to begin 
nesting at an average interval of 9.6 months. In other words, 
they nest 5 times every 4 years. The reason for this unusual 
cycle of reproduction does not seem to be associated with any 
weather or any light intensity changes on the island. Another 
exception is the male parakeet (Melopsittacus undulatus) for 
Vaugien (1953) found fledgling males isolated in complete dark~ 
ness show a greater increase in gonad weight than birds kept 
under natural light. 
Threadgold (1960) found that in the House Sparrow, 
7 foot-candles of light for one hour daily was more effective 
in stimulating spermatogenesis than the same amount of light for 
7 hours. In both cases, a full testicular cycle was induced, 
but spermatic activity was maintained for the longest period in 
those birds receiving the single hour of light. Using growing 
chicks Moreng et al. (1956a) found a statistically significant 
increase in comb and testes weights in the group grown in a dark 
environment as compared to those grown in intense light (Table I). 
Table [#+# 
Organ Weight of five-week-old S.C. White Leghorn Cockerels 
Comb Adrenal Testes wt. Thyroid wt. (0.1%Thiouracil) 
wt. wte 
(gms.). (mgm) (mgm. ) (mgm. ) 
Light 100W 
(Fluorescent) 1.75 59.0 140.0 375.0 
Dark 73W 2.83 63.0 173.0% 432.0 

**% Moreng et al. 1956. 
