Eared moths are eaten more often by nocturnal birds than bats
In this study, we observed previously unreported dietary partitioning
among co-occurring nocturnal aerial insectivorous birds and bats.
Variation in NAI diets correlated with prey detection method based on
both presence/absence and compositional data. This trend was driven
mainly by birds successfully preying on eared moths more often than
bats. Previous studies of bat diets predominately used visual
examinations of feces, which inhibits investigations of moth’s auditory
abilities and often resulted in order level taxonomic designations of
prey. As such, this may be the first evidence that multiple families of
eared moths largely avoid predation by a suite of bat species in their
natural environment.
Eared moths can detect bat echolocation calls from farther away than
bats can detect moths, approximately ten times farther in the case of
Noctuid moths (Surlykke et al., 1999). As a result, moth adaptations to
avoid bats (Hofstede & Ratcliffe, 2016; Waters, 2003) leave open niche
space for nocturnal insectivorous birds that hunt visually.
Complementary to visual detection methods, both Common Poorwills and
Common Nighthawks have a velvety coating on wing and tail feathers
adapted for quiet flight (Clark, LePiane, and Liu 2020), which may make
them difficult for eared moths to detect. Indeed, Eared moths,
especially Noctuid moths, made up a large portion of Common Poorwill and
Common Nighthawk diets, demonstrating the success of quiet flight
adaptations. Flammulated Owls also fly quietly and possess relatively
long wings that allow them to move quickly (though perhaps without much
agility) throughout the forest canopy (Johnson, 1997). Rather than
aerial hawking, Flammulated Owls, like Common Poorwills, primarily use a
sit-and-wait hunting strategy. This consists of flying from a perch
inside the tree crown to capture insects resting in other areas of the
same crown or adjacent trees (Reynolds & Linkhart, 1987). Together,
these results indicate that birds that can ambush prey, rather than
alert them with echolocation calls, can initiate successful attacks on
eared insects at closer ranges.
The lower occurrence of eared moths in bat diets demonstrates the
effectiveness of moth adaptations to bat predation (Hofstede &
Ratcliffe, 2016). Still, Long-legged and Long-eared Myotis tended to
consume eared moths at slightly higher rates than the other bats in this
study. Long-legged Myotis makes echolocation calls at higher frequencies
and detects prey at greater distances than Big Brown Bats and other
myotis species, which may give it an advantage (Fenton & Bell, 1979;
Saunders & Barclay, 1992). Alternatively, Long-eared Myotis uses
passive hearing and low-amplitude calls while gleaning, which are
undetectable by some eared moths (Faure, Fullard, and Barclay 1990).
Gleaning by Myotis species evolved subsequent to echolocation strategies
(Morales et al., 2019) and may be a counteradaptation to reduce
detection by eared prey (Razak, 2018). However, gleaning may also have
evolved as a general adaptation to hunting in cluttered areas (Brinkløv
et al., 2010). An obvious counterstrategy to eared prey would be for
bats to use a sit-and-wait hunting strategy. However, the physiology of
most bats precludes them from leaping into flight (Schutt et al., 1997).
The decreased ability of bats to capture eared moths may result in more
specialized diets compared to nocturnal birds. For example, Common
Poorwills and Flammulated Owls had greater diet variation among
individuals than other NAI species. This, together with their small
foraging ranges, may indicate that they are relatively generalist
consumers. Although these birds consumed a larger proportion of eared
moths, they may opportunistically sally after any large insect they see
from their perch. Previous investigations found that Common Poorwills
only consumed prey >5mm in length, despite a higher
abundance of smaller insects, potentially due to visual constraints
(Bayne & Brigham, 1995). We did not find any evidence contradicting
this, however since we used DNA instead of morphology to indicate prey,
we were unable to definitively determine prey size.
In addition to moths, ultrasonic hearing via tympanal organs has evolved
independently within at least eight other insect orders, including
Orthoptera, Mantodea, Blattodea, Hemiptera, Hymenoptera, Coleoptera,
Neuroptera, and Diptera (Göpfert & Hennig, 2016; Hoy & Robert, 1996).
Besides serving to detect and avoid predators, insect hearing has also
evolved as a means of communication (Hoy & Robert, 1996). In
Neuroptera, green lacewings can detect ultrasonic frequencies and avoid
predation by bats (Miller, 1975), and a recent study indicates a similar
ability in Myrmeleontidae of the Neuroptera (antlions) (Holderied et
al., 2018). However, no insect family with known tympanal hearing
abilities were significantly associated with bat diets in this study.
Other insect families have evolved different mechanisms of hearing (e.g.
Culicidae; Hoy & Robert, 1996), however these insects did not appear to
avoid detection by bats more than birds.