Bird behaviour in vineyards: A review

Abstract

Cost-effective management of bird pests is a major challenge for wine grape producers throughout New Zealand. The present situation is that considerable effort and money is being spent to reduce bird damage, yet few of the techniques employed are either reliable or sustainable. Bird control methods of the future will need to be bird species-specific, and based on a better understanding of bird foraging behaviour. This review, prepared for the Marlborough District Council by Lincoln University's Centre for Viticulture and Oenology, provides a summary of research to date on bird behaviour and management in vineyards. It aims to assist current management practices, and underpin future research on bird behaviour and management. The chemical and physical changes associated with ripening of fleshy grapes have evolved in part as a mechanism to attract the birds that then disperse the plants' seeds - this is the biological challenge that we face when we try to deter birds from eating grapes. The cues that birds use to identify the ripeness of grapes include colour, aroma, taste and tactile cues. Of these, aroma is potentially the least important as birds do not have well-developed olfactory systems. Birds can readily discern colour, but little is known about the role of colour in their fruit foraging behavior. Birds typically sample newly-encountered grapes and thus use taste and tactile cues to help to determine when the crop is palatable. Birds also observe, and readily learn from, the foraging behaviour of other nearby birds. As grapes ripen they reach a threshold at which they become markedly more attractive to birds. This threshold is associated with increasing sugar, softening of the grape, and changes in many aroma and flavour factors including the amount and type of sugars, acids and phenolics. Increase in sugar is frequently cited as the key factor determining the palatability of grapes to birds, but to date there is negligible research evidence to confirm this - the relative importance of changes in the other fruit components remains unclear. Bird damage levels vary markedly between vineyards, between years, and within a vineyard in a single season. At present, much of this variation is difficult to predict as the causal environmental factors are not properly understood. Damage is typically greatest at the periphery of a vineyard, so vineyard shape and area will influence damage levels. Shelterbelts and woodlots provide roost sites and encourage birds to forage in adjacent vines. Human settlements provide habitat that encourages higher densities of birds to develop, which can result in conflict between commercial vineyards and local residents. For species such as starlings, roost sites may be remote to the vineyard and thus difficult for growers to manage. Varietal differences in susceptibility to bird damage are apparent, with red/black varieties generally more vulnerable than green (although this is a bird-species dependent effect). Foraging behavior of the different bird species also varies. For example, blackbirds and thrushes are largely territorial, and approach fruit from low in the canopy, whereas starlings flock into a vineyard from above, having sent 'scouts' ahead to identify hazards and opportunities to feed. These different feeding habits are one reason why species-specific strategies for bird control are needed. - At present in New Zealand, no chemical bird repellants can be directly used on grape crops. Birds habituate quickly to most of the visual and auditory control devices currently in use. Methods that help reduce habituation include moving the devices frequently, using multiple deterrent methods in a coordinated manner, and removing devices as soon as habituation becomes evident. However, such measures remain relatively ineffective in most vineyards, so new research into innovative, cost effective and environmentally-friendly control measures is needed. For bird control measures to become more effective in New Zealand vineyards, further research under local conditions is needed into three main aspects of bird behaviour and management: • what cues attract the birds to vineyards, and to the fruit therein? • how do different species of birds behave and feed, and are there particular aspects of each species' behaviour that can be targeted by control? • how can a suite of different control measures best be integrated to achieve sustained deterrence of a mixed-species assemblage of pest birds?