Wesołowski T., Rowiński P. 2014. Do Blue Tits Cyanistes caeruleus synchronize reproduction with caterpillar peaks in a primeval forest? Bird Study 61: 231-245.

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Capsule Folivorous caterpillars constituted the majority of nestlings’ food in a primeval forest. Blue Tit broods only partially matched the caterpillar peak, and the mismatch did not affect food composition or nesting success.

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Aims To describe factors influencing the timing of reproduction in Blue Tits under primeval conditions (Białowieża National Park, Poland) and to check whether they schedule breeding so as to synchronize broods with a seasonal caterpillar peak.

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Methods We gathered information on phenology of leaf development, seasonal availability of folivorous caterpillars (frass collection), timing of Blue Tit breeding, composition of its nestling food, and nest fate over a three-year period.

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Results Caterpillars constituted c. 74% of nestling diet, but only 17–65% of broods matched the caterpillar peak in any season. Neither total nest loss, nor frequency of brood reduction depended on the level of mismatch. Caterpillar availability was probably adequate every year, regardless of the amount of mismatch, and no selective advantage of precise matching was detectable. Phenological events at all trophic levels occurred earlier in warmer springs. Egg-laying coincided with tree bud burst and appearance of caterpillars, but was not critically dependent on their timing.

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Conclusion The observations are consistent with the view that Blue Tits under primeval conditions in Białowieża National Park, Poland, breed as early as possible, rather than synchronizing their breeding with the caterpillar peak later in the season.