19 April 2005 Varmints in the Vineyard – Bird Damage to Grapes

 

 

This problem is common in New Zealand vineyards and throughout the world.

 

Dr Valerie Saxton from the Food and Wine Group at Lincoln University has undertaken research into the damage done to grapes by marauding birds.

 

"Before I started this work I was surprised to find that such a huge problem that impacts on the wine industry had so little scientific research undertaken. There were no answers to questions such as: what birds are doing the damage, when and how they do it, and what attracts or repels them from attacking grapes," said Dr Saxton.

 

"My work has provided some baseline data about birds and their habits and preferences that the wine industry can build on to develop more effective and ecologically sound methods to deter birds from attacking grapes."

 

Dr Saxton studied the grape damaging habits of blackbirds and silvereyes in their natural environment. She chose these particular species because they are both common in New Zealand vineyards but have completely different lifestyles and habits.

 

Because of the difficulties in using natural grapes Dr Saxton developed an artificial grape that she presented on trays that mimicked the two level style of grape trellising used in New Zealand.

 

She investigated what it was in grapes that attracted the birds by presenting these birds with artificial grapes of different colours and containing a range of different levels of sugar, acid and tannin. In order not to disturb the birds she took videos of them as they came to the tables and ate the grapes.

 

As grape growers are well aware the two species attack the grapes differently, blackbirds, who are a solitary species, eat the whole grapes, while silvereyes, who travel in flocks, peck at several grapes without removing them from the vines. Her results showed that the species had different preferences for colour, and levels of sugar, acid, colour and tannin. These preferences are probably governed by differences in digestion and seasonal metabolism.

 

One reason for this problem is that New Zealand's temperate climate results in a long two month ripening season for grapes that allows them to be particularly vulnerable to bird attack, while mild winters mean increasing numbers of birds survive to nest in the spring, which means even more birds.

 

"Despite considerable efforts being put into reducing bird damage of grapes by grape growers, bird control methods remain primitive, are ineffective in the long term, and can involve methods that reduce environmental quality," said Dr Saxton.

 

Further research will determine more accurately the actual costs of bird damage to the wine industry. Apart from capital costs of netting, other devices and labour, there is a flow on cost of lost wine to export markets.

 

Dr Saxton's work was funded by Lincoln University and New Zealand Winegrowers.