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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 124. No. 2. June 2012 
I ABLE I. Reproductive timing of the White Stork in breeding seasons when early clutch reduction was noted, and in 
two breeding seasons when prolonged incubation occurred (2009-2010). 
Breeding variables 
First arrival date 
Arrival of the first 
partner (= 
permanent nest 
occupation) 
Arrival of the 
second partner 
Incubation period 
Eggs or eggshells 
discarded 
Last visit at nest 
2003 
5 Apr (2 birds, 
short visit) 
14 Apr 
17 Apr 
—22 Apr-25 May 
19 Apr 
23 Aug 
2005 
26 Mar (2 birds, 
short visit) 
15 Apr 
7 May 
~16 May-19 Jun 
7-8 May (2 eggs). 
II Jun 
29 Aug 
2006 
4 Apr 
4 Apr 
17 Apr 
?—-4 Jun 
20 Apr. -26 
Apr (2 eggs) 
25 Aug 
2009 
13 Apr (I bird. 
short visit) 
18 Apr 
19 Apr (18 Apr in 
vicinity) 
~24 Apr-22 Jun 
19 Apr. 21 Apr, 15- 
22 Jun. -28 Jul 
17 Aug 
2010 
29 Mar 
29 Mar 
12 Apr 
— 15 Apr-19 Jun 
21 Apr. 19 Jun 
18 Aug 
the records of prolonged incubation for 18 species 
of diurnal and nocturnal raptors. The range of 
prolonged incubation found was 33-161% in 
excess of the normal period (the average was 
94%j. The length of extended incubation in seven 
nests of the Red-necked Grebe ( Podiceps gri.se- 
gena) was 42-50%’ beyond the average incubation 
period (Kloskowski 1999). Cases of extremely 
long incubations were also recorded for the Eur¬ 
asian Wren ( Troglodytes troglodytes), 219% in 
excess of the normal period: European Robin 
(Erithacus rubeeula ) t 243% (Skutch 1962); and 
Northern Bobwhile {Colinus virginiunus): 326% 
(Hernandez et al. 2006). 
There were behavioral indications suggesting 
the prolonged incubation in 2009 and 2010 was 
caused by laying infertile eggs, rather than 
embryo death. First, there was a distinct lack of 
copulation by the breeding pairs in both breeding 
seasons. 1 did not notice one case of copulating 
storks at the site throughout the entire breeding 
seasons in 2009 and 2010. despite intensive 
observations. The White Stork is a species with 
a high copulation rate. The average number of 
copulations reaches -200 during the breeding 
season with a frequency up to two times/hr 
(Tortosa and Redondo 1992, Bochenski and 
Jerzak 2006). Copulations at this particular nest 
were regularly observed in previous years, 
especially in the pre-incubation period. 
Second, the period between arrival of the 
second partner and beginning of incubation was 
distinctly short in both years. The period amount¬ 
ed to 6 days in 2009, and 3 days in 2010. A 
reduced arrival-breeding interval is believed to 
enhance breeding success (Fulin et al. 2009)- 
however, the recorded values were several times 
shorter than average. It takes 12-13 days on 
average, before the first egg is laid after the arrival 
ol the second partner to the nest in western Poland 
(Tryjanowski el al. 2004. Kosicki 2010). The 
short period between arrival of the second partner 
and beginning of incubation by the observed 
slorks, may indicate that some eggs were formed 
before being fertilized, even if both birds were 
able to breed. However, the lack of copulations in 
2009 and 2010 and rapid initiation of breeding 
imply that at least one bird was immature or 
inlertile. or the pairs were of the same sex. 
Production of eggs could also be influenced by 
food stress (Deeming 2002). Insufficient food and 
low digestive efficiency by the female (Kwie- 
einski and Tryjanowski 2009)may cause breeding 
stress that leads to production of infertile eggs 
Egg losses for White Storks are usually caused 
by fights with intruders trying to take over a nest 
already occupied by a pair (Profus 2006. Tobdlka 
et al. 2011). However. White Stork females may 
occasionally lay infertile eggs before the male's 
arrival and then throw them out (Schulz 1998). 
This happened in 2009; one egg was discarded 
after the second partner arrived at the nest. Similar 
clutch reduction occurred soon after arrival ot the 
Second partner at this nest in previous years. In 
2003. one egg was discarded 2 days after the 
arrival of the second partner. In 2005. two eggs 
were discarded up to I day after the arrival of the 
second partner. In 2006, one egg was discarded 
3 days after the arrival of the second partner 
(Table 1). Regular occurrence of early clutch 
reduction has not been reported for the White 
Stork. This suggests the nest was repeatedly first 
