raising of the brood. Paynter (1949) and Lack (1954) showed 
that late and second broods in gulls and swifts were less suc~- 
cessful elther through asynchronization in the colonies, 
dwindling food supply, or lack of strong attentiveness on the 
part of the parent. 
Clutch size may or may not be regulated by visual 
or tactile response. To quote David Lack (1954), "The restriction 
of clutch size is due not to a negative, the inability to pro- 
duce more eggs, but to a positive act, the cessation of laying, 
which often involves the reabsorption of further eggs already 
formed internally, but not yet laid." 
Eisner (1961) coneluded that the most obvious 
physiological control of termination of laying is a rise in the 
level of prolactin or progesterone. In different species, there 
are three categories of clutch size. The first category is 
the fixed clutch size: gulls, petrels, terns and related species, 
lay a specific number of eggs, the number is not significantly 
affected by environmental factors (Lack, 1954). Tinbergen (1960) 
showed that the removal or addition of eggs to Herring Gulls' 
nests did not induce the gulls to lay more than or less than 
three eggs. Either a determined number of follicles begin to 
mature, or there is an accumulation of progesterone, with a 
given number of ovulations, or an inhibitory prolactin level is 
reached during this period, thus laying is inhibited without 
dependence on external stimuli. Weidmann (1956) showed that 
more than three follicles began maturation in gulls. If the 
gulls incubated the first clutch, other follicles did not 
develop. Although the clutch is three in theory, there are 
many two-egg nests. Darling (1958) relates this to density 
dependent variations. The more gulls there are nesting, the 
greater the stimulation per pair. Synchronization is better 
in large colonies than in small ones, Social stimulation speeds 
up the process and shortens the total nesting period, Palmer 
(1941) found this to be true in Common ferns (Sterna hirundo) 
also but cautions that the advantages gained from increased 
stinulation may be offset by overcrowded colonies that there 
is probably an optimum colony size. 
The second category of clutch size determination is a 
governing of the number of eggs layed by an attempt of the 
laying bird to seemingly reach a minimal quota. Lack (1954) 
reports that a Flicker (Colapter aurcitus) was induced to lay 
71 eggs; she laid an egg a day for 71 days, to replace the egg 
a day which was removed from her nest. Wetherbee and Wetherbee 
(1961), however, could not corroborate this often cited ex- 
periment. The domestic hen is an excellent example of this 
category for the hens lay for months when the eggs are removed 
from the nest. 
A third category of clutch size determination is de- 
pendent upon environment. In raptorial birds and storks, the 
clutch size goes up in correlation with the food supply (Lack, 
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