The importance of a main dish: nestling diet and foraging behaviour in Mediterranean blue tits in relation to prey phenology.

摘要

Insectivorous birds rely on a short period ofudfood abundance to feed their young; they must time theirudreproduction to match the timing of Lepidoptera larvae,udtheir main prey. Apart from the net result (i.e. birds areudtimed or mistimed with respect to the food’s peak), anudimportant aspect is the possible influence of other factors,udsuch as the seasonality of the environment or the abundanceudand diversity of species contributing to the caterpillarudpeak, on birds’ phenology and their ability to copeudwell with unpredictable food supplies. In a 2-year study,udwe explored the seasonal variation of nestling diet inudMediterranean blue tits Cyanistes caeruleus and howudreproductive parameters (nestling condition, provisioningudrates) are affected by the phenology and composition ofudfood. We also examined the influence of the synchronyudbetween offspring needs and caterpillar peak in shaping theudcomposition of the nestlings’ diet. We found that the effectudof synchrony on nestling condition varied between yearsudwhich may be partially due to differences in food peakudattributes. The adequacy of birds’ timing in relation to preyudphenology affected foraging decisions; those birds thatudwere not able to correctly adjust their timing were forced toudrely on less preferred prey (tortricids). In this sense, weudfound that relative contribution of tortricids (smaller caterpillarsudbut easier to get) and noctuids (preferred prey butudmore difficult to find) to the diet influenced nestling conditionudand parental provisioning effort; parents performedudfewer feeding events and reared heavier nestlings as theudcontribution of noctuids to the diet increased. The relationshipudbetween the proportion of caterpillars and nestlingudmass was curvilinear, whereas that parameter was negativelyudaffected by the percentage of pupae. Our resultsudshow how changes in diet composition may contribute toudexplain the effect of mismatching on birds’ breedingudperformance.