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.