Shortening day length as a previously unrecognized selective pressure for early breeding in a bird with long parental care
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2014-11-23Autor
Podlaszczuk, Marcin
Wojciechowski, Zbigniew
Minias, Piotr
Janiszewski, Tomasz
Wojciechowska, Agnieszka
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Several different selective pressures have been
suggested to explain an intense competition for early return
to breeding grounds in birds. In this study we hypothesized
that shortening day length during summer months may
constitute additional selective force acting towards early
breeding in avian species with long parental care. To test
this hypothesis, we studied time budget and foraging
activities of early-nesting and late-nesting white storks
Ciconia ciconia from the Central-European population. We
found that duration and distance of foraging trips increased
significantly over the course of the reproductive season.
The relative frequency of foraging trips increased at the
expense of other activities, such as resting, plumage
maintenance, and nest maintenance. Mean daily foraging
duration increased with increasing day length in the early
part of the season, with 0.68 h of foraging per individual
per 13.16 h of day length in mid-April increasing to 7.42 h
of foraging per individual during solstice (16.61 h of day
length). Afterwards, mean foraging duration continued
increasing in spite of decreasing day length, reaching
11.63 h of foraging per individual per 14.92 h of day
length at the end of the season in mid-August, when storks
were forced to continue foraging after sunset in order to
meet energy requirements of fledglings. The results suggest
that shortening day length during summer months may constitute a serious time constraint on food delivery rates to offspring for late-breeding pairs of white stork
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