The birds and the bees: pollination of fruit-bearing hedgerow plants and consequences for birds

Abstract

Hedgerow fruits provide a food resource for several UK farmland bird species from late summer, through winter and into spring. This project aims to develop the understanding of the interactions between fruit-bearing hedgerow flowers, their pollinators, hedgerow fruits and frugivorous birds. Experiments revealed that flowers of blackthorn, hawthorn and ivy all benefited from insect visits in order to develop fruit. The flowers of bramble and dog rose showed little requirement for insect pollination, and produced fruit when insects were excluded. There was evidence that for the hedges under study, the pollination service provided by insects to blackthorn and hawthorn flowers was inadequate since the flowers of these plants were pollen limited. The relative abundance of different insect groups foraging on blackthorn flowers was highly variable between hedges, suggesting that the contribution of a particular insect group to blackthorn pollination may vary according to their local density. Bumblebees, bristly flies and solitary bees were considered to have the greatest value for pollinating blackthorn flowers, based on foraging attributes (bumblebees and solitary bees), and abundance (bristly flies), but their activity did not correlate with the proportion of flowers that set fruit. Solitary bee activity correlated with hawthorn pollination, and there was strong evidence that social wasps were the best pollinators of ivy flowers on the hedges studied. Environmental factors such as hedge aspect did not significantly affect the activity of most pollinators (with the exception of solitary bees) or the proportion of blackthorn flowers that set fruit. Equally, the presence of the mass-flowering, attractive forage source, oilseed rape in fields adjacent to hedgerows, did not significantly influence the activity of most pollinators or the proportion of hawthorn flowers that set fruit. The abundance of some frugivorous birds, in particular the migratory thrushes (redwings and fieldfares) was positively related to the yield of fruits, including sloes and haws in hedges. So the evidence suggests that on these farms, pollinator communities are important for ensuring some hedgerow shrubs provide copious fruit, which may be vital for birds during winter months when invertebrate food is scarce. These links between flowers, pollinators, fruits and birds are discussed, alongside suggestions for safeguarding the fruit supply for farmland birds in the future.