Non-eared prey partitioning among bats and birds
Though these results show a clear link between the ultrasonic hearing of moths and their absence in bat diets compared to nocturnal birds, the partitioning of non-eared insects is less clear. Moths in the family Tortricidae lack hearing organs (Fullard & Napoleone, 2001). This may explain why bats consumed Tortricidae in such high amounts. The most commonly consumed Tortricidae moths, spruce budworm, tend to fly near treetops (Soutar & Fullard, 2004). Common Nighthawks that hunt high above the ground and Flammulated Owls that hawk from tree perches would still encounter these insects. Indeed, 11% of nighthawks and 31% of Flammulated Owls consumed Tortricidae in this study. However, for Common Poorwills that generally hunt only up to three meters above ground (Brigham and Barclay 1992), spruce budworm may often be out of range. This would explain why Common Poorwills preyed on Tortricidae moths less often than all the other NAIs.
Previous studies suggest that variation in echolocation calls leads sympatric bat species to detect different prey resources, enabling coexistence (Razgour et al., 2011). However, such diet partitioning has not been shown empirically among the assemblage of bats in our study. Although overall diet composition did not differ significantly among Big Brown Bats, Long-legged Myotis and Long-eared Myotis (Permanova), we observed low overlap in the insect taxa consumed (18-22%), suggesting some specialization. This pattern indicates that although these bats consume high abundances of the same species (i.e. spruce budworm), coexistence may be promoted due to differences in species consumed at lower frequencies. This hypothesis was also supported by stronger differences among species when analyzing presence/absence data compared to relative abundances, which is less sensitive to rare species. Additionally, minor differences in foraging locations may enable coexistence among sympatric species with similar foraging behaviors (Kent & Sherry, 2020), or resources like spruce budworm may be abundant enough to render partitioning unnecessary. Still, greater sampling efforts in the future may reveal finer-scale diet partitioning among the bats in this study that we were unable to detect here.