When birds have a sweet tooth for grapes

The delightful and the hungry

A shorter version of this article is available in the December 2021 issue of The Grape Press. To read this, and other issues, please click here.

Whether it’s the melodic song of a Blackbird on a misty autumnal morning, the laugh of a Green Spotted Woodpecker that just burst into a fast flight, or a breath-taking Starling murmuration over a strikingly orange and pink sky, birds certainly bring us lots of joy and help us connect to nature. However, if you’re a winegrower, there may come a time, when birds cause you to lose sleep, as it seemed to be the case in 2021.

Going for the low hanging bunches

Perhaps going less for the alcohol-content and more for the sugar, birds like us seem quite fond of grapes. Across all the grape-growing regions of the world, birds can cause severe economic losses by inflicting severe grape damage. Following the recent UK harvest, I’ve heard from some winegrowers who saw their entire crop of Reichensteiner and 80% of Phoenix end up in the stomachs of these feathery pests rather than in a press. Meanwhile other winegrowers shared their joy of seeing birds in their vineyards and admitted to happily sharing a few percent of the crop with their visitors.

Bird damage to grapes is spatially variable. Numerous studies from the key wine growing regions found greatest bird damage around field edges and being higher near hedges or lone trees. These habitat features act as safety blankets for birds, allowing them to perch quietly and look out for predators before making the short journey to nick a grape or two. This is why it’s been suggested that small and isolated fields that are surrounded by non-crop habitat, such as a woodland, are most susceptible to grape damage from birds.

Birds can shift their dietary preferences throughout the harvest season, making grape damage temporally variable. In South Africa, omnivorous species typically fed on the early ripening grape varieties before switching to insects when these became available. Reports from UK vineyards indicated a similar switch in the diet, as birds were known to suddenly take an apparent dislike to grapes over other food sources. This shift may be linked to changes in the availability of alternative food sources such as wild berries, grass and cereal seeds and insects, or it may indeed be linked to the grape varieties available, as birds prefer red and sweeter castes.

The 2021 harvest season was unusual and challenging, as the limited rays of sunshine made ripening slow, and gave more opportunity for damage as some grapes weren’t harvested until November. This made for an anxious wait as winegrowers vigilantly monitored sugar levels and with each Brix degree up, came more birds which always seemed quicker to know when the sugar levels were right. One winegrower even jokingly said that he doesn’t need to bother using his refractometer when birds were around! This preference of birds for grapes with high sugar content was indeed experimentally demonstrated and taken together with the shift in birds’ dietary preferences in response to changing food availability sheds some light into why the timing of bird damage can be so variable and unpredictable year-on-year.

The culprits

Vineyards can be bursting with bird life so pinning down the key grape thieves can be difficult, though a few species tend to regularly come up in conversations, namely Starlings, Blackbirds, corvids (e.g. Carrion Crows and Magpies), pigeons and Pheasants. These species are definitely common across UK vineyards, which makes them likely candidates, though caution should be taken blaming the most apparent species as bird losses are not always directly linked to birds’ abundance and relatively-rare species with a sweet tooth have been known to inflict a lot of damage.

A novel approach to identifying the culprits was developed and trialled by scientists in Portuguese vineyards, where motion-triggered cameras were used. The main damage-inflicting species were local species of pigeons, blackbird and House Sparrows, which were most active in the morning and evening. Pigeons and blackbirds, which are larger, were observed taking whole grapes, whilst sparrows pecked holes in grapes, leaving the skin behind.

The location of damage is likely to depend on the species of bird that’s inflicting it. Studies found high occurrence of damage at the highest grape bunches, which are exposed to birds that perch on the vine to take a bite. The species most likely taking this approach are medium to large-sized, such as blackbirds, thrushes and pigeons, meanwhile smaller birds such as finches, tend to take refuge among the leaves and perch on the trellis wire for their snack. During my visits to vineyards, I noticed that many low-hanging bunches take a hard beating, with many having been plucked clean. The most likely species responsible for this is the Pheasant, which a generalist species that readily feeds on grapes, often jumping up to reach the berries. Damage by Pheasants does seem to be closely linked to their abundance as vineyards with neighbouring pheasant release pens tended to be badly hit.

Starlings are interesting because unlike other species, they comfortably feed in the centre of fields, as opposed to the edges, and can quickly eat the whole crop of a small field within hours. This is likely down to their grouping behaviour as Starlings travel in groups that can vary between a dozen to several dozens in size. Safety in numbers means that starlings readily perch in the open on utility wires, directly above grape fields and go down as a group to feed on the grapes. Having said that, some vineyards in the UK have been enjoying murmurations and high yields, meaning that Starlings don’t always mean bad news for your wine.

Control methods

Bird damage can lead to losses reaching hundreds of Pounds per hectare, thus many methods of controlling bird damage and trying to minimise it have been devised and trialled. There are many control methods, which range from visual (e.g. hawk-kites), auditory (alarm call playbacks) and direct exclusion with netting. All these methods are readily used in UK vineyards, though all come with drawbacks. The simple methods such as eye-spot balloons and hawk-kites have the advantage of being cheap and simple to deploy, but they often lose effectiveness as birds become habituated to them. Exclusion netting is more effective, as it directly prevents birds from accessing the fruits, but it can be costly and laboursome to deploy and collect prior to harvest, especially across large vineyards.

Recent advances in technology have led to the development of more complicated multi-modal devices, such as unmanned aerial vehicles (UAV, also known as drones). Bird psychology has been exploited to build sophisticated and targeted UAV-scarers that are equipped with a loudspeaker emitting distress calls of the main bird pests, and with a taxidermy crow attached beneath to mimic the sight of a bird of prey with a prey item. These UAVs can also be set to mimic flight patterns of birds in attempt to increase their resemblance of real predators. Some studies found these UAVs to be effective for extended periods of time at reducing bird damage caused by larger birds, such as corvids, though they were less effective at deterring smaller passerines.

There’s growing preference for natural farming practices among consumers and pressure to farm with nature in mind. Natural methods of attracting predatory species of birds may do the trick here, as provision of perches for birds of prey in Australian vineyards resulted in 50% reduction in grape damage thanks to lowered foraging activity of birds in the vineyard. Encouraging birds of prey back to UK vineyards may be a nature-friendly and a cost-effective solution that warrants further thought.

A positive outlook

Birds are natural parts of our landscapes and UK vineyards, and a part of me does admire their adaptability to readily adopt grapes into their diets. Of course, it is saddening to see the fruits of one’s labour disappear over a few hours, when there are arguably alternative food resources about. The decision to invest in bird control methods will undoubtedly depend on the degree of damage, which will vary between sites and years. My research across UK vineyards may help to identify some predictors of bird damage and name the key species responsible, which should help to inform targeted and optimised management. In the meantime, I hope that UK winegrowers and birds find a middle ground where everyone can enjoy the beauty of vineyards, and maybe just a few grapes.

 

About the author:

My name’s Natalia Zielonka, and I’m a PhD candidate at the University of East Anglia. I’m supported by the UKRI BBSRC Norwich Research Park Biosciences Doctoral Training Partnership, and my supervisors are Dr Lynn Dicks (based at the University of Cambridge) and Dr Simon Butler (UEA). If you have any questions then please don’t hesitate to contact me on [email protected] and via Twitter: @Nat_B_Zielonka.

Photo credit: Natalia Zielonka

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