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.