Prolonged Incubation and Early Clutch Reduction of White Storks (Ciconia ciconia)

—Two cases of prolonged incubation by White Storks (Ciconia ciconia) were observed in the same nest in 2009 and 2010 in southwest Poland. Incubation lasted at least 59 and 65 days, respectively; i.e., 84 and 103% longer than the average incubation period. Extended incubation was accompanied by other abnormal breeding behavior. The lack 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 2011. Accepted 21 December 2011. 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 (Skutch 1962). However, excessively long 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.g., Visser and 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 extended incubation is preceded or accompanied by other abnormal behavior which could induce 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 incubation 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 of bird species (Holcomb 1970, Sutcliffe 1982, Margalida et al. 2006), but is extremely rare. This phenomenon occurs regularly only in the Procellariiformes (Boersma and Wheelwright 1979, Huin 1997) and occasionally in the Podicipediformes (Kloskowski 1999). An extensive literature search did not locate reliable records of prolonged incubation by White Storks (Ciconia ciconia) and other Ciconiidae. One case of this behavior was observed in western Poland in 2011 but exact length of incubation is unknown (Marcin Tobółka, unpubl. data). Possible cases of extended incubation should be much easier to 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 nests on easily observed structures (buildings, chimneys, poles), and is subject to restoration projects which include captive breeding (Biber et al. 2003, 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 information on incubation behavior of this species is surprisingly scarce. For example, factors affecting the onset of incubation and quantitative contributions of male and female to incubation period are poorly recognized. Little is known on the length of 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 1 Institute of Nature Conservation, Polish Academy of Sciences, Lower-Silesian Field Station, Podwale 75, 50-449 Wrocław, Poland; e-mail: [email protected]