Generalist passerine pollination of a winter-flowering fruit tree in central China

Abstract

Background and aims: Winter-flowering plants outside the tropics may experience a shortage of pollinator service, given that insect activity is largely limited by low temperature. Birds can be alternative pollinators for these plants, but experimental evidence for the pollination role of birds in winter-flowering plants is scarce.

Methods: Pollinator visitation to the loquat, Eriobotrya japonica (Rosaceae), was observed across the flowering season from November to January for two years in central China. Self- and cross-hand pollination was conducted in the field to investigate self-compatibility and pollen limitation. In addition, inflorescences were covered by bird cages and nylon mesh nets to exclude birds and all animal pollinators, respectively, to investigate the pollination role of birds in seed production.

Results: Self-fertilization in the loquat yielded few seeds. In early winter insect visit frequency was relatively higher, while in late winter insect pollinators were absent and two passerine birds (Pycnonotus sinensis and Zosterops japonicus) became the major floral visitors. However, seed-set of open-pollinated flowers did not differ between early and late winter. Exclusion of bird visitation greatly reduced seed-set, indicating that passerine birds were important pollinators for the loquat in late winter. The whitish perigynous flowers reward passerines with relatively large volumes of dilute nectar. Our observation on the loquat and other Rosaceae species suggested that perigyny might be related to bird pollination but the association needs further study.

Conclusions: These findings suggest that floral traits and phenology would be favoured to attract bird pollinators in cold weather, in which insect activity is limited.

Fig. 1.

Inflorescences, flower visitors and infructescences of Eriobotrya japonica. (A) A flowering inflorescence covered by snow; (B) a section of the flower showing nectar droplets; (C) a honey-bee visiting caged flowers from which birds were excluded; (D) Pycnonotus sinensis visiting flowers; (E) Zosterops japonicus visiting flowers; and (F) fruits in April.

Fig. 2.

Visit frequency (mean ± s.e.) of insects and birds during observation periods in winter 2009 and 2010.

Fig. 3.

Seed set (mean ± s.e.) in Eriobotrya japonica under three pollination treatments: cages excluding birds; mesh excluding birds and insects; and unprotected controls. Different letters above the boxes indicate significant differences between treatments (one-way ANOVA, Tukey HSD, cage vs. mesh, P = 0·36; cage vs. control, P < 0·01; mesh vs. control, P < 0·01). Numbers within the boxes indicate treated inflorescences.