SHORT COMMUNICATIONS 
365 
occupied by females, which is also unusual in this 
species (Bocheriski and Jerzak 2006). These obser¬ 
vations indicate that early laying of infertile eggs 
by the White Stork may occur more regularly than 
believed It is possible that in some years laying 
infertile eggs may initiate extended incubation. 
It is of interest, that prolonged incubation in the 
same nest occurred in two consecutive years. It is 
probable that in both cases, one or both of the 
birds were the same as the previous pair, in view 
of the high return rale of While Storks to previous 
nest-sites (Chernetsov el al. 2006, Vergara et al. 
2006). This could not be confirmed as the 
observed storks were unmarked. 
Several phenomenon believed to be rare in 
birds were documented. To my knowledge, this is 
the first record of prolonged incubation in the 
Ciconiidae, and probably the first record of 
repeated prolonged incubation for wild birds in 
the same nest. Prolonged incubation was accom¬ 
panied by other behavioral abnormalities, based 
on continuous nest observations. The breeding 
behavior of the parental birds was atypical and 
finished with nest failures. This suggests pro¬ 
longed incubation in the observed White Storks 
resulted from a set of factors, related to individual 
attributes of the parents which reduced reproduc¬ 
tive possibilities. Thus, cases of incubation well 
beyond the expected hatch date for many species 
of birds may be associated with abnormality in 
breeding. Behavior accompanying prolonged in¬ 
cubation is difficult to detect in other bird species, 
unlike in the White Stork, and may be unnoticed, 
further investigation is necessary to understand 
the regulation of incubation behavior of birds, 
especially prolonged incubation. 
ACKNOWLEDGMENTS 
I am grateful to Piotr Profus for help in searching the 
stork literature, and for contributing to our discussions on 
the issue. I thank C. E. Braun and two anonymous reviewers 
for valuable comments on the initial version ol the 
manuscript. 
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