Bird abundance and biodiversity have declined in response to land conversion and agricultural intensification. Due to this, conservationists have turned their efforts towards agricultural farmlands by calling for wildlife-friendly agroecological practices that promote birds. However, bird conservation in agricultural farmlands can be controversial. While some bird species damage some crops, others can also suppress insect pests but it remains unclear which bird species are beneficial and which are pests. However, the majority of studies investigating the role of birds in agriculture tend to focus on either a service or disservice, with net effects rarely measured. Furthermore, there is thought to be a positive relationship between bird-mediated pest suppression and landscape complexity but only a few studies have looked at this.
In order to measure the net effects of birds in strawberries we conducted an exclusion experiment in six California strawberry farms in 2015. The exclusion experiment consisted of three sets of paired exclusion plots (berries not accessible to birds via netting) and control plots (berries accessible to birds) per farm. Strawberries were harvested from plots on a weekly basis and scored for bird damage and invertebrate damage. Using a bug vacuum, we surveyed for arthropods in the plots before and after setting up the plots. Furthermore, in 2018 we conducted point-count surveys across 20 farms that varied in farm-management and landscape complexity. Additionally, in order to identify which species of birds were eating strawberries and which were eating insects we caught birds via mist-nets across the 20 farm sites. Fecal samples were taken from each mist-netted bird and analyzed using next-generation sequencing in order create diet profiles for each bird. Pairing the point-count data and the fecal sample data will allow us to put bird species into functional guilds of pest or beneficial, allowing us to characterize the bird communities at each farm.
Results/Conclusions
Results from the 2016 exclusion experiment showed that birds suppress ~3.8% damage by insect pests and inflict ~3.2% damage to strawberries meaning that the net effect was neutral. Furthermore, preliminary data from 58 mist-netted birds showed that 19 individuals (30%) consumed Lygus spp., the most economically damaging insect pests to strawberries. These results highlight the need to consider both the costs and benefits of birds when making management decisions.