Generalist passerine birds perform a functional role as pollinators in temperate Europe

Many flowering plants require pollinators, and their interactions with animals are crucial to ecosystem function. The role of interactions involving specialised partners is generally accepted, but the function of interactions between non‐specialised partners has been less readily recognised.

Although insects are considered primary pollinators in Europe, there are numerous records of passerine birds feeding at flowers and carrying pollen. Whether birds play a functional role as pollinators of native European plants outside the Mediterranean region, however, has not been determined.

Here we explore the contribution of generalist passerine birds to pollination of native woody plants with floral rewards during spring–summer in the United Kingdom (UK), at a protected site with diverse fauna and flora.

We scored pollen on 29 bird species and found pollen transport across the community (89% of individuals carried pollen), with birds being recurrent flower visitors. Nine bird species (31%) regularly carried significant pollen loads, most often for Salix caprea , Prunus spinosa , Crataegus monogyna and Rhamnus cathartica . Although most of the species involved lacked specialised morphological characteristics associated with bird pollination, other features facilitating pollination were evident.

We experimentally assessed the functional outcome of pollen transport by birds for plant reproduction. When birds were excluded from flowers, fruit‐set was reduced for half of the seven plant species tested (early seed parasitism meant fruit‐set data were not available for one species). This positive effect of bird access was greatest for dioecious or self‐incompatible species, and those flowering earlier in spring when temperatures are cool and variable.

Synthesis . Passerine birds in temperate Europe can fill a role as pollinators, and this may be most functional when the availability of insects, as both pollinators and food, is least reliable. Factors such as phenological overlap, flower aggregation and accessibility, and plant reproductive strategy are important predictors of plant–pollinator interactions, and the morphological ‘fit’ prescribed by flower‐visitor syndromes is context‐dependent. Birds and flowers that are apparently generalists may be effective mutualists, and this interaction deserves attention, given the central role they play in communities.