birds with highly developed brood patches find relief from 
"neripheral irritation" by sitting on eggs. 
Baily (1952) working with California Gulls and several 
passerines, and Saeke and Tambe (1954), working with pigeons, 
showed a relatively high prolactin content of the pituitary 
as long as nest building continued but low content during 
laying. Daily intermuscular injections of fifteen I.U. of 
prolactin induced full broodiness when, but only when, birds 
were molting. Lehrman (1961) suggested that progesterone is 
directly concarned with incubation behavior but once the be- 
havior 1s induced, prolactin secretion begins. Blakely et al. 
(1951) using doses of 200 mg. of diethylstilbesterol were ef- 
fective in breaking up broodiness in turkeys. Eisner (1961) 
noted that dosage levels are critical, for small doses of pro- 
gesterone induce ovulation, while larger doses inhibit it. 
Kosin (1948) injected 10 mg. testosterone propionate and 
failed to break up already broody turkeys although in laying 
birds it seemed to prevent broodiness. Bailey (1952) reported 
that androgens had no effect on the brood patch. Laying, 
incubation, and molt usually occur in sequence but are not 
necessarily action-and-reaction processes. Some of these 
processes occur simultaneously, such as incubation and molt 
in some species. Broody birds renest if the nest is destroyed 
or they can adopt a new mate if pair-bond is broken; in both 
cases necessitating a turning-back of the physiological clock. 
Once incubation has begun, it must continue until the eggs are 
hatched. It is obvious that eggs are the stimulation that is 
needed for if the eggs are removed, incubation behavior ceases. 
Weidmann (1956) induced incubation in gulls by placing dummy 
eggs in the nest just before laying and thereby prevented 
laying. Tinbergen (1954) also working on gulls removed the 
eggs a short distance from the nest, and showed that both eggs 
and nest stimulated incubation behavior for the bird incubated 
first the eggs, and then the nest. The gulls did not, however, 
attend to only the nest itself when no eggs were on the 
territory. Tinbergen also showed that one bird which lost its mate 
abandoned the eggs which indicates that perhaps a mate is needed 
for the completion of the cycle in gulls. Patel (1936) working 
with pigeons came to the same conclusion. Patel experimentally 
separated a male from its mate but both sexes were kept visible 
to each other. Both progressed normally, developing the 
necessary “mtlk" at hatching time. When the mates were removed 
from visual contact, however, incubation behavior was lost in 
the male. 
Sturkie (1954) reviewed how lightestimulation and 
other mechanisms affect the action of the pituitary. For pur- 
poses of this paper, I will discuss the actual visual stimulation 
which causes these external effects. We have seen that environ- 
mental stimulation helps to regulate the commencement of the 
nesting cycle. These mechanisms through natural selection assure 
the maximum success from an environmental point of view for the 
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