SHORT COMMUNICATIONS 
363 
supported by empirical data). These features 
include: (1) one breeding attempt per year, i.e.. 
birds which cannot renest are under strong 
reproductive pressure: (2) hatching asynchrony, 
i.e.. naturally wide variation in incubation dura¬ 
tion for viable eggs; and (3) low predation risk, 
i.e.. low potential costs of incubation (Marks 
1983. Margalida et al. 2006). The absence of 
reports on prolonged incubation by White Storks 
and related species, despite the above character¬ 
istics. indicate this phenomenon is extremely rare 
in the Ciconiidae. 
I describe two cases, 1 year apart, of prolonged 
incubation, observed in the same nest of the While 
Stork. I also describe abnormal behavior of the 
breeding pair which may help explain the 
prolonged incubation. 
METHODS 
Study Species.— The While Stork has one brood 
per year and the full breeding cycle lasts 16 weeks. 
The average incubation period is 32 (29-34) days 
and usually starts with the second egg. Both mates 
incubate, but the female contributes more than the 
male. Incubation begins —2 weeks after arrival of 
the second partner at the nest. The second partner, 
usually the female, arrives on average 4.3 days 
later than the first partner (all data after Schulz 
1998. Profits 2006). 
Field Procedures .—The extended incubation 
whs observed in 2009 and 2010 in the village 
of Sieniawka (SW Poland. 50 46' N, 16 46' E). 
The location is inside a study plot used for long¬ 
term (since 1989) monitoring of the White Stork 
population. The local population is characterized 
bv one of the lowest stork densities in Poland 
tl.76 pairs/100 knr in 2010) with a continuous 
decrease in numbers (x = 1.4 breeding pairs/yr. 
1996-2010). and low reproductive success (Wuc- 
zyriski 2006a. h). The nest in which the prolonged 
incubation was observed was on a pole not con¬ 
nected to an electric power line. The first breeding 
attempt at this site was noted in 1994. The nest 
location allowed for consistent and repeated obser¬ 
vations throughout the breeding season. The 
conspicuous breeding habit of the White Stork 
provided a unique opportunity to monitor the 
behavior accompanying prolonged incubation. The 
presence of the birds in the nest and their behavior 
was viewed from the ground several times each 
day. The area under the nest was inspected for 
discarded engs or nestlings. The incubation 
duration was established based on the behavior of 
the breeding pair: intermittent attentive periods, 
turning of the eggs, and parents taking turns 
incubating the eggs. Initiation of incubation was 
difficult to ascertain due to long but intermittent 
periods of sitting on the nest preceding actual 
incubation. An additional sign of approaching incu¬ 
bation was when the birds began to line the nest. 
The end of incubation was unambiguously pro¬ 
nounced when the eggs were thrown out. followed 
by the sudden leaving and long absence of the birds. 
OBSERVATIONS 
The first partner permanently occupied the nest 
starting on 18 April 2009. and both partners were 
at the nest on 19 April (Table 1). One egg was 
discarded (direct observation) immediately after 
the second bird appeared at the nest. The 
minimum clutch size in the 2009 breeding season 
was four eggs, based on the number of discarded 
eggs. The latest accepted date for beginning of 
incubation was 24 April (possibly 1-2 days 
earlier), as this was the date that a tightly sitting 
bird was observed. Continuous incubation lasted 
59 days until 22 June. 
The first arrival date in 2010 was on 29 March 
and coincided with consistent nest occupation 
by the first partner. The second bird appeared 
14 days later. The start of the incubation period 
was assumed to be 15 April. Continuous incuba¬ 
tion lasted 65 days and finished on 19 June. Two 
eggs were found under the nest during the course 
of the breeding season, which was the minimum 
clutch size. 
The contents of the eggs which had been 
discarded after the end of the incubation period 
indicated embryo development did not occur in 
either year. The lack of observed copulations was 
prominent in both years during the pre-incubation 
period. Start of incubation was abrupt, soon after 
the arrival of the second partner. 
The length of incubation in 2009 and 2010 was 
27 and 33 days beyond the normal incubation 
period (84 and 103%, respectively). These values 
could be slightly higher as the precise starting 
point of incubation was not known. The incuba¬ 
tion period was average in the 2001-2008 
breeding seasons, when hatching occurred in this 
nest. The nest was not occupied in 2011. 
DISCUSSION 
The length of the prolonged incubations 
observed is within the range reported for other 
species of birds. Margalida et al. (2006) reviewed 
