BIRDS IN VINEYARDS:
Patterns of Damage,
Attracting Birds to Vineyards,
the Role of Vineyards as Habitat
Emily Heaton
Doctoral Candidate
Dept. of Environmental Science, Policy,
and Management
University of California, Berkeley
The following is a summary of information presented in
a presentation given on April 20, 2005 for the Napa Sustainable Winegrowers
Group.
I primarily focused on two
topics during my presentation. First, I
talked about patterns of bird damage to grapes, as most of the scientific
literature concerning birds in vineyards is devoted to grape depredation. Second I talked about my doctoral research on
songbird boxes (also known as bluebird boxes) in North Coast vineyards. While vineyards provide habitat for a number
of bird species, I did not spend much time on that topic because little
research has been conducted regarding the role of vineyard as habitat, and the
research that has been conducted has come primarily from Europe.
PATTERNS OF BIRD DAMAGE IN VINEYARDS
– a brief summary of findings reported in the scientific literature
The main species that cause
damage to grapes are species that flock near harvest time and that also eat
fruits and berries. In North
America, European Starlings, American Robins, House Finches, and
Cedar Waxwings are the most important pest species, although damage by other
species may be serious in individual vineyards.
Type of damage, pluck or peck, is dependent on the bird species and on
berry size. In general, pluck damage is
caused by larger species (e.g. starlings and robins); while smaller species,
such as House Finches, are the principle species associated with peck damage. Not all bird species that utilize vineyards
cause significant damage to grapes. Some
species, such as Western Bluebirds, will eat grapes but do not occur in large
enough flocks to inflict much damage.
Other species do not eat grapes and solely forage for seeds and/or
insects in vineyards (e.g. Red-winged Blackbirds, goldfinches). In fact, even the species that inflict the
most damage can eat large volumes of insects.
Bird damage can vary from
year to year and from farm to farm. It
is usually localized and unevenly distributed.
A wide variety of factors can influence the extent of damage, including:
local abundance of grape-eating species, composition of the surrounding
landscape, proximity to perches and night roosts, vineyard management
practices, availability of alternative food sources, and weather. While the extent of damage can be
unpredictable at the vineyard scale, several patterns emerge from the grape
damage literature; and these findings provide useful information about avian
use of vineyards.
Several
studies have found levels of damage to be greatest along the edge of vineyard
blocks, with damage decreasing with distance toward the center. In addition, damage is often greatest near landscape
features that provide perches and/or cover (such as electrical lines, trees,
roost trees, bushes). Losses can be
especially heavy in relatively isolated vineyards. One study in Australia examined the influence
of environmental variables on grape damage and found that foliage cover, number
of perch sites close to the crop, size of the vineyard block, and the location
of the control efforts within the block were important.
Vertical
stratification of damage may reflect different foraging behaviors by the
depredating species. In a study in Ontario, Canada,
starlings and Cedar Waxwings fed high up on the vine, while robins fed on
grapes close to the ground. Starlings and
waxwings foraged by making short forays into vineyards from nearby perches
(trees and electrical lines), descending into the vineyards from the air and
removing grapes from high up on the vines.
It has been suggested that finches and several other species can be
effectively deterred from feeding on grapes by maintaining heavy foliage in the
vine canopy. Starlings, on the other
hand, may readily burrow into foliage to feed and often use it for shade on hot
days.
Insects
or seeds, rather than grapes, are often the initial attractor to vineyards for
grape-eating species. For example,
starlings may be attracted to freshly tilled vineyards where invertebrate prey
has been exposed. Insect sources such as
feedlots, alfalfa fields, and permanent pastures may result in higher insect
levels in adjacent vineyards, which can attract birds that eat insects.
One researcher
suggests that with regard to total dissolved solids (TDS), there is a threshold
level of acceptability at which grapes become palatable – extensive damage
usually does not start until the fruit matures to 11-13° Brix, and further increases
in TDS beyond this point do not seem to affect the number of birds visiting a
vineyard or the overall level of bird damage.
There is some evidence that individual bird species may show preferences
for certain varietals.
Birds
may quickly become completely habituated to an area if a plentiful supply of
food is available and birds are allowed to feel comfortable in the vineyard. Thus, it is important that efforts to deter
birds begin before large flocks become established in a particular
vineyard. Scaring devices are most
likely to be effective if they are employed at the early stages of ripening
before birds have developed a habit of visiting the site, and if alternative
attractive feeding sites are available.
Application of bird netting is the most effective method of minimizing
damage (note that no research has been done on the use of falcons as a
deterrent) but is only cost-effective in vineyards that experience significant
levels of damage.
My research on songbird boxes
As part of my graduate
research, I conducted a study to assess the effectiveness of songbird nest
boxes (also known as bluebird boxes) in attracting and providing productive
nest sites for native cavity-nesting birds.
This project was done in collaboration with PRBO Conservation Science
and received assistance from the Southern Sonoma County Resource Conservation
District, the Sonoma Valley Vintners and Growers Alliance, and the Carneros Quality Alliance. Over 250 songbird boxes in 21 vineyards were
monitored during the breeding season in 2002 and 2003. Study vineyards were located in the Sonoma Valley
and the Carneros area.
Seven native species used the
boxes for nesting: Western Bluebird, Tree Swallow, Violet-green Swallow,
Ash-throated Flycatcher, House Wren, Oak Titmouse, and White-breasted Nuthatch.
In addition, Nuttall’s Woodpeckers were found
roosting in songbird boxes on several occasions. The non-native House Sparrow also used the
boxes for nesting in a small number of vineyards. On average, about ¾ (73%) of the boxes that were
monitored were used for nesting in a given year. Bluebirds were the most common occupant of
nest boxes, nesting in about one-half (47%) of the available boxes. Tree Swallows were also common and nested in
one-fifth of the boxes. Preferences for
certain habitat conditions were evident for most species. For example, Tree Swallows primarily nested
in boxes near water, specifically irrigation ponds and wetland. Several species (e.g. House Wren, Oak
Titmouse, and White-breasted Nuthatch) only nested in boxes adjacent to or
within woodland.
A little over one-half of nests
(51% in 2002, 57% in 2003) were successful in fledging at least one young. By comparison, a nest success rate of 50-70%
is typical for nests in natural cavities in trees. Predation (probably by raccoons and/or
domestic cats) was the most common cause of nest failure for vineyard boxes. However, predation rates varied significantly
among vineyards, with some experiencing little to no predation while others had
high predation rates. Because predation
rates varied significantly among vineyard sites, nest success was also highly
variable.
House Sparrows compete
aggressively with native cavity-nesting birds and are known to kill nesting
adults and nestlings of other species. House
Sparrows were generally rare in vineyard boxes but did nest in some boxes. These boxes were almost exclusively located near
cattle stockyards or houses with bird feeders that were not designed to exclude
house sparrows. At these localities, several
adult Western Bluebirds and Tree Swallows were found dead with holes pecked in
their heads, indicating an attack by House Sparrows. For this reason, boxes should be placed at
least 100 yards away from cattle feed lots/stockyards and bird feeders that are
not House Sparrow-proof.
Some recommendations for growers
In order to minimize
predation, songbird boxes should be mounted on free-standing posts about 5 feet
off the ground. Smooth, metal poles are
ideal; or a piece of PVC pipe can be slipped over a T-post or wood post to
prevent climbing by predators. In
addition, designs for a variety of predator guards are available on the
internet (see below). While attaching songbird boxes to deer fences is
convenient, these boxes are vulnerable to predation. A smart raccoon can follow a fence line and wipe
out several nests in one night.
In areas where extreme
afternoon temperatures are common (>100°F), avoid facing the entrance hole to the south. South-facing boxes will likely experience
higher box temperatures, increasing the likelihood of heat-related deaths of
nestlings. If possible, face boxes north
or east. Ventilation slots between the
roof and sides of the box should ¼ -½” wide to reduce maximum temperatures in
the box. Also, while bottom-opening
boxes allow for convenient cleaning of boxes after the nesting season, they are
very difficult to monitor. I would like
to encourage growers to refrain from using boxes that only open from the
bottom. An ideal box design might have a bottom that slides out, in addition to
a top or side that hinges open.
A booklet that addresses use
of songbird boxes, owl boxes, and bat boxes in vineyards is nearing completion
and will hopefully be printed within the next 6 months. This booklet will be published by the University of California, Division of Agriculture and
Natural Resources. Notice will be
distributed widely upon its publication.
Until then, two great sources of information regarding songbird boxes
are the North American Bluebird Society (http://www.nabluebirdsociety.org)
and Cornell University’s The Birdhouse Network (http://www.birds.cornell.edu/birdhouse). These websites contain natural history
information, nest-box designs, ideas for minimizing predation, and many other
helpful tips.