Birds increase crop yields via consumption of pests in some contexts but disrupt pest control via intraguild predation in others. Landscape complexity acts as an inconsistent mediator, sometimes increasing, decreasing, or not impacting pest control. Here, we examined how landscape context and seasonal variation mediate the impact of birds on arthropod pests and natural enemies, leaf damage, and yields of broccoli (Brassica oleracea) on highly diversified farms that spanned the USA west coast. Our study had two complementary components: a bird exclusion experiment and molecular diet analysis of 357 fecal samples collected from the most commonly captured bird species that also foraged in Brassica fields—American Goldfinch (Spinus tristis), American Robin (Turdus migratorius), Savannah Sparrow (Passerculus sandwichensis), Song Sparrow (Melospiza melodia), and White-crowned Sparrow (Zonotrichia leucophrys). Bird access yielded higher, rather than lower, numbers of pest aphids and increased their parasitism, while no other arthropods examined were consistently impacted. Independent of bird presence, percent natural cover in the landscape sometimes increased and sometimes decreased densities of arthropods in the mid-growth period, with diminishing impacts in the late-growth period. Herbivore feeding damage to broccoli leaves decreased with increasing amounts of natural land cover and in the late-growth period. Molecular diet analysis revealed that Brassica pests and predatory arthropods were relatively uncommon prey for birds. Landscape context did not alter the prey items found in bird diets. Altogether, our bird-exclusion experiment and molecular diet analysis suggested that birds have relatively modest impacts on the arthropods associated with broccoli plantings. More broadly, the limited support in our study for net natural pest control services suggests that financial incentives may be required to encourage the adoption of bird-friendly farming practices in certain cropping systems.
LAY SUMMARY
Birds may contribute pest control services through consumption of arthropod pests or disrupt pest control through predation on arthropod natural enemies. The net effect of birds on crop pests and subsequent yields may be mediated by landscape context.
To better understand the net effects of birds on broccoli, we combined a bird exclusion experiment with DNA-based diet analysis of 5 key bird species.
Both birds and landscape context impacted arthropods on broccoli plants. However, we observed no net effect of birds on broccoli yield regardless of landscape context.
DNA-based diet analysis revealed that while landscape did not alter the likelihood of bird species consuming pests or arthropod natural enemies, some bird species were more likely than others to consume arthropod natural enemies.
The limited impacts of birds on broccoli leaf damage and yields suggested that pest control services by birds may be an insufficient motivator to employ bird-friendly farm practices.
