Provisioning Flexibility and Nestling Growth of Green-Backed Tit Parus monticolus in a Mosaic of Mixed-Oak Forest and Cedar Plantation in Taiwan

Abstract

In heterogeneous landscapes, birds may mistime their breeding attempts relative to local food abundance. How birds cope with this challenge has been little studied. In this study, we compared the provisioning behavior and nestling condition of the Green-backed Tit Parus monticolus nesting in a landscape mosaic comprised of native mixed-oak forests and exotic Japanese Cedar Cryptomeria japonica plantations. Parent birds in the larvae-poor plantations time their breeding as they do in the larvae-rich forests and have clutches of similar size. We found that parent birds were flexible in adjusting their criteria for prey choice on the basis of larval abundance. In early spring, in cedar plantations where larval abundance was low, male parents maintained constant nearby feeding trips to the early broods, just as they did in the mixed-oak forests, but reduced their dietary specialization on larvae, while female birds increased their provisioning frequency. Combining both parents' efforts, the early brood nestlings raised in the cedar plantations received food deliveries more frequently and with a higher proportion of non-larval prey than their counterparts did in the mixed-oak forests. In contrast, during late brood rearing, when larvae were abundant, both parents in the plantations foraged heavily on larvae at the expense of other prey, just as they did in the mixed-oak forests. We found that parent birds reared heavier nestlings as the contribution of large larvae to the diet increased. Moreover, the nestlings during early broods in the plantations had lower body weights than their counterparts in the mixed-oak forests. We suggest that the provisioning flexibility of Green-backed Tits helps them gather food for raising nestlings when their breeding attempts are less synchronized with the larval food supply in their habitat. As a result, they can breed successfully in the exotic cedar plantations.