Data from: Double mutualism: Dual rewards or redundancy? Insights from the interactions between mistletoes and their avian partners in the tropical hotspot of Southwest China

Mistletoes, recognized as keystone species, supply plentiful floral and
fruit resources for birds, thereby facilitating mutualistic interactions
in pollination and seed dispersal. Some avian species are “double
mutualists”, engaging in both functions. However, their precise roles and
effectiveness as mutualists remain inadequately understood. In this study,
we conducted 690 hours of video-recorded observations of avian pollinators
and dispersers of the mistletoe species Dendrophthoe pentandra
(Loranthaceae) across three sites in the seasonal tropical forests of
Xishuangbanna, a biodiversity hotspot in Southwest China. We also
performed a phylogenetic analysis of avian traits, including beak length,
body length, and body mass. Our findings documented a total of 833 visits
by 13 bird species from 8 families, with 11 species probing flowers (528
visits) and 7 species consuming fruits (43 visits). Three species –
Dicaeum minullum, D. ignipectus, and Zosterops simplex – were identified
as double mutualists, as they visited the plants during both the flowering
and fruiting phases. Notably, double-mutualist birds accounted for 87.5%
of flower visits and 95.5% of fruit visits. The generalist frugivore Z.
simplex was the most frequent floral visitor, while the
mistletoe-specialist frugivore D. minullum emerged as the primary
pollinator and seed disperser across all sites, demonstrating the highest
efficiency. All double-mutualist species exhibited high mobility and
specialized morphology, characterized by small to medium-sized, slightly
curved beaks and lower body mass, which contributed to greater
effectiveness in pollination and seed dispersal, confirming their roles as
“double mutualists” of this plant. Phylogenetic analysis indicated that
ancestral mistletoe-associated birds typically possessed short to medium
beaks and medium body masses, whereas double-mutualist birds appear to
have evolved shorter beaks and smaller bodies. Our findings show that
mistletoe-associated birds that perform both pollination and dispersal are
morphologically adapted to the floral structure of mistletoes, fostering
closer mutualistic relationships with host plants and enhancing fitness of
both plants and their avian partners.