The collaborators, from the University of California--Santa Cruz and the Smithsonian Conservation Biology Institute, hope to revive the idea of "economic ornithology," a practice encouraged by a branch of the U.S. Bureau of Biological Survey from 1885-1940. This concept recognizes the fact that insectivorous birds can improve agricultural output by reducing arthropod abundances and, therefore, the damage caused to crops by pest insects. Birds have helped raise the incomes of both apple and coffee farmers, for instance, and are poised to help viticulturists, as well. In the U.S., bird-friendly vineyards could become particularly popular in California, where grapes are the second most economically important agricultural crop. In this same region, the conversion of woodland and savanna to agricultural and urban land has reduced the availability of potential breeding habitat for may species of bird. Thus, efforts to promote bird abundances on vineyards might not only help improve grape crops, but could also preserve local bird populations.
(The Bonterra Vineyard, one of two vineyards examined in the current study)
In the current study, the scientists installed nest boxes at two vineyards in order to see whether they could be used to attract insectivorous birds during the breeding season. Each vineyard was divided in half and boxes were distributed only on one side so that the other could be used as a control to evaluate "normal" numbers of birds and associated insectivorous activities. The researchers then performed observations in all 4 areas in order to quantify avian abundance and diversity. They also conducted a "sentinel prey experiment" in which they distributed beet armyworms (Spodoptera exigua) along transects in order to assess whether predation rates were higher in the vineyard halves containing nest boxes.
Areas with an without boxes had similar amounts of avian richness. However, the two box-supplemented halves had more insectivorous birds. These included chipping sparrows (Spizella passerina), tree swallows (Tachycineta bicolor), violet-green swallows (Tachycineta thalassina), and--most notably--western bluebirds (Sialia mexicanus). Bluebirds were the predominant inhabitants of nest boxes, though their neighbors sometimes included the two swallow species and, in one case, ash-throated flycatchers (Myiarchus cinerascens).
(A beet armyworm, Spodoptera exigua, and the damage it can do to foliage.)
At the beginning of the breeding season, avian abundance doubled at the nest box treatment sites, then increased again later in the summer as fledglings emerged from the boxes to forage with their parents. Regardless of time of year, though, box-supplemented areas always contained higher avian abundances than control areas, a pattern that was predominantly driven by the presence of the western bluebirds: In treatment areas, 1.8 individuals were observed every 5 minutes, while only 0.18 individuals were observed every 5 minutes in control areas.
In box-supplemented areas, armyworms were removed 2.4 times more often than in control sites, and were most likely to be taken when distributed along transects placed within 25 m of active nest boxes. In fact, removal rates in treatment areas were always higher when the prey were left near active boxes than when prey were placed randomly throughout the habitat. Thus, though it is impossible to confirm that bluebirds--rather than alternative predators such as squirrels, lizards, and other bird species--were the sole armyworm predator, it does seem likely that the birds were the predominant source of insectivorous activity in box-supplemented areas.
(Western bluebird, Sialia mexicanus)
This is good news for the grape-growers, since a pair of bluebird parents can capture well over 100 g of prey per day during the breeding season. Further, these birds found and occupied nest boxes soon after they were distributed, suggesting that growers could quickly and easily attract this species to their vineyards. All three species of bluebird in the U.S.--western bluebirds, eastern bluebirds (Sialia sialis), and mountain bluebirds (Sialia currucoides)--are willing to use, and in some places are dependent on, human-supplied nest boxes. Therefore, the authors suggest that agriculturalists across the country may be able to form partnerships with these birds in order to receive pest maintenance services in exchange for the provision of shelter. They caution that additional studies will be needed to confirm that bluebird reproductive output on vineyards is as healthy as that found in natural areas. However, bluebirds are known to respond positively to breeding conditions in other anthropogenic areas, suggesting that they should also find vineyards and other farms to be suitable.
Should viticulturists--or other types of farmers--pursue this sort of "ecologically friendly" growing practice, the authors suggest that this could be advertised via special product labels (such as the Bird Friendly stamp used by coffee-growers) or by encouraging stores to group these green products together in a particular section of the store. This will allow conservation-minded buyers to give their dollars to like-minded vendors.
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Jedlicka, J.A., Greenberg, R., and Letourneau, D.K. 2011. Avian conservation practices strengthen ecosystem services in California vineyards. PLoS ONE 6(11):e27347.
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