Short Communications 
The Wilson Journal of Ornithology 124(2):362-366. 2012 
Prolonged Incubation and Early Clutch Reduction of White Storks 
(Ciconia ciconia) 
Andrzej Wuczynski 1 
ABSTRACT.-— Two cases of prolonged incubation 
by White Storks I, Ciconia ciconia) were observed in the 
same nest m 2009 and 2010 in southwest Poland. 
Incubation lasted at least 59 and 65 days, respectively; 
i- e - lS4 and 103% longer Lhan the average incubation 
period. Extended incubation was accompanied by other 
abnormal breeding behavior. The luck of observed 
copulation in either breeding season, an extremely short 
arrival-breeding interval, and early clutch reduction 
suggest the eggs were infertile. This is the first record of 
prolonged incubation in the Ciconiidae. and probably 
the first record of repeated prolonged incubation by 
wild birds. Received 12 September 20/1. Accepted 21 
December 2011. 
extended incubation is preceded or accompanied 
by other abnormal behavior which could induct 
prolonged incubation. Moreover, there is no 
information about an individual bird's tendency 
to incubate longer than average. To my best 
knowledge, repeated cases of prolonged incuba¬ 
tion in the same nest or by the same individuals 
have not been previously documented in wild 
birds. 
Prolonged incubation has been recorded for a 
number ol bird species (Holcomb 1970. Sutcliffe 
1982. Margalida et al. 2006), but is extremely 
The incubation period is primarily affected by 
the initial egg mass and speed of embryonic 
development (Deeming 2002). This period varies 
little within the particular species in the case of 
fertile eggs (but see Drent 1975). Occasionally, 
anomalies in the incubation period occur and the 
duration may be extended. Prolonged incubation 
is an instinctive behavior, thought to provide a 
safety margin for eggs that take longer than 
normal to hatch (Skuteh 1962). However, exces¬ 
sively Iona incubation periods relate to infertile 
eggs and the adaptive significance of this behavior 
is questionable (Afik and Ward 1989. Kloskowski 
1999). The duration of incubation should be as 
short as possible due to energy constraints for the 
incubating bird (Reid et al. 2002) and increased 
mortality risk from predators at the nest (e.e.. 
Visserand Lessels 2001, Martin 2002, Weidinger 
2002, Miller et al. 2007). It remains unclear why 
incubating birds fail to recognize infertile eggs. 
It is also unknown what causes termination of 
incubation. The secretive behavior of parental 
birds usually prevents continuous observations, 
and available accounts on prolonged incubation 
are based on intermittent visits to the nest by 
researchers. Thus, it is not known whether 
Institute of Nature Conservation. Polish Academy . 
Sciences. Lower-S.lesian field Station, Podwale 75. SO- 4 , 
Wroclaw. Poland; e-mail; a.wuczynski@pwr.wroc.pl 
rare. This phenomenon occurs regularly only in 
the Proeellariiformes (Boersma and Wheelwright 
1979. Hu in 1997) and occasionally in the 
Podicipediformes (Kloskowski 1999). An exten¬ 
sive literature search did not locate reliable 
records ol prolonged incubation by White Storks 
(f iconiu ciconia) and other Ciconiidae. One case 
ol this behavior was observed in western Poland 
in 2011 but exact length of incubation is unknown 
(Marcin Tobolka. unpubl. data). Possible cases of 
extended incubation should be much easier 10 
document for White Storks than for other wild 
birds because it is one of the most popular and 
charismatic bird species in Europe. The White 
Stork is exceptionally well studied (reviews in 
Schulz 1998. Tryjanowski et al. 2006). it nest-on 
easily observed structures (buildings, chimneys, 
poles), and is subject to restoration projects which 
include captive breeding (Biber et al. 70<).\ 
Olsson 2007). Several dozen stork nests in Europe 
are monitored on-line with a camera providing 
continuous data during the course of the breeding 
season (Dolata 2006). However, in-depth infor¬ 
mation on incubation behavior of this species is 
surprisingly scarce. For example, factors affecting 
the onset of incubation and quantitative contribu¬ 
tions of male and female to incubation period are 
poorly recognized. Little is known on the length 
ol incubation recesses, egg sensitivity to cooling, 
or egg reduction by parental birds. 
The White Stork has biological attributes 
thought to favor prolonged incubation (but not 
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