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Résumé :
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Many temperate-zone birds must track seasonal variation in food availability to time breeding and enhance reproductive success. However, the external cues birds use to fine-tune sexual development and timing of reproduction remain unclear. One possibility is that birds may use chemical substances it, plants and bud tissues as a reliable predictor of leaf flush and the appearance of caterpillars, the preferred prey of many insectivorous birds during the breeding season. However, few data on seasonal shifts in key plant material consumption are available. To test this hypothesis, we examined the gizzard contents of Corsican Blue Tits (Cyanistes caeruleus), a small passerine bird with a breeding schedule tightly linked to the phenology of oak trees. We quantified and compared the consumption of tree buds by birds captured during winter (nonbreeding) and early spring (prebreeding), and found that bud use by Blue Tits was limited and constant through time. Fewer than 30% of birds had buds in their gizzards during the critical prebreeding period, and these items represented less than 1% of gizzard contents. It is unlikely that this limited use of buds allowed birds to track a potential chemical signal at the time of reproductive decision making. Unless more data on the consumption of plant material become available, we suggest that other environmental factors be examined to help identify the cues used by temperate-zone birds to time their reproduction.
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