INTRODUCTION

Aerial insectivores like birds and bats are decreasing at alarming rates across North America (Spiller & Dettmers, 2019), in part due to simultaneous declines of aerial insects (Sánchez-Bayo & Wyckhuys, 2019). Niche theory predicts that in resource-limited environments, species that occupy the same guild will partition dietary resources to avoid competitive exclusion (MacArthur & Levins, 1964). Such partitioning is often underpinned by variations in morphology or behavior that allow species to exploit different resources (Schoener, 1974). Dietary niche partitioning related to prey size (Vesterinen et al., 2018), predator morphology, and echolocation behavior (Emrich et al., 2014) is evident among many sympatric bat species. If and how dietary partitioning occurs among co-occurring nocturnal insectivorous birds and bats is less clear, but by identifying the processes that promote the coexistence of aerial insectivores, we can better predict future community dynamics.
Interactions between bats and moths provide a model system for studying the evolution of predator-prey relationships (Waters 2003; Hofstede and Ratcliffe 2016). Prey capture by bats is often dependent on echolocation behavior and how insects respond (Fenton and Fullard 1979). Moths with ultrasound-sensitive ears can hear echolocation calls at distances up to 100 meters (e.g., noctuids; Miller and Surlykke 2001) and avoid predation through evasive maneuvers or sounds (Dunning and Kruger, 1996). This adaptation arose independently in moths at least six times (Hofstede & Ratcliffe, 2016). In turn, some bats echolocate at low intensities or high enough frequencies to go undetected by moths (Faure, Fullard, and Barclay 1990; Hofstede and Ratcliffe 2016). Yet, how evolutionary interactions between moths and bats may extend to dietary resource partitioning between bats and nocturnal insectivorous birds is unknown (Yack et al., 2020). Nocturnal birds often use visual cues and possess adaptions for silent flight that enable them to evade detection by insects (Clark et al., 2020). These adaptations may allow them to exploit resources that bats cannot. For example, eared moths can only detect the cyclic wingbeats of approaching birds within 2.5 meters (Hofstede and Ratcliffe 2016; Waters 2003), perhaps making moths more vulnerable to predation by visually-oriented insectivores. The distributions of bats and nocturnal insectivorous birds suggests that they interact. However, little research exists on if, or to what extent they may partition prey resources, or the underlying mechanisms (Fenton and Fleming 1976).
We used fecal DNA barcoding to analyze the diets of seven co-occurring nocturnal aerial insectivores (hereafter NAIs). We compared the diet composition and richness of three nocturnal birds: Chordeiles minor (Common Nighthawks), Phalaenoptilus nuttallii (Common Poorwills), Psiloscops flammeolus (Flammulated Owls), and four bat species: Eptesicus fuscus (Big Brown Bats),Lasionycteris noctivagans (Silver-haired Bats), Myotis Volans (Long-legged Myotis), and Myotis evotis (Western Long-eared Myotis). Despite differences in prey detection methods used by these insectivores (Table 1), previous studies using microscopy of fecal samples have reported broad similarities in the insects they consume, primarily moths and beetles (Agosta, 2002; Csada et al., 1992; Ober & Hayes, 2008; Reynolds & Linkhart, 1987; Todd et al., 1998; Whitaker, 1995). However, NAI diets vary across regions and over time, hampering cross-study comparisons. Additionally, traditional methods of prey analysis in feces primarily result in prey identification to only the order or family level, which masks resource partitioning at finer taxonomic resolutions.
As with differences in prey detection methods, NAIs in this study also display different foraging behaviors. For example, Flammulated Owls (Goggans 1985) and Common Poorwills (Brigham and Barclay 1992) are sit-and-wait predators (Table 1). Both use their legs to launch after prey from the ground or perches, a foraging behavior not found in insectivorous bats. Modifications of the pelvis that allow bats to hang from perches and fly prevent bats from jumping into flight (Schutt et al., 1997). Instead, the bats in this study hunt by foraging insects while in flight, termed “aerial hawking” (Saunders & Barclay, 1992), or, as in Long-eared Myotis, sometimes by gleaning insects from the ground and foliage (Faure and Barclay 1994). Like bats, Common Nighthawks are also aerial hawkers and prey on insects at a wide range of heights above ground and over great distances in a single foraging bout (Clark et al., 2020).
It is not always clear if or to what extent differences in prey detection and foraging behavior translate to differences in diet. Insectivores with different foraging behaviors may still target the same prey (e.g., Brigham and Fenton 1991; Kent and Sherry 2020). Prey movement may also overlap with the foraging range of more than one type of predator. Still, foraging behaviors and prey detection methods that do correspond to dietary differences may decrease interspecific competition among NAIs.
To our knowledge, this is the first study to use fecal DNA barcoding to investigate the diets of multiple, distantly related, co-occurring NAIs. Our objectives were two-fold. First, we developed a reference barcode database from 56,191 locally collected arthropod specimens to provide more accurate taxonomic assignments of potential prey items than possible in previous studies. We then used DNA barcoding of fecal samples to match prey DNA to our insect barcodes and determine the degree to which NAI diets differ. We expected that differences in diet would depend on NAI species identity and correspond with 1. prey detection methods (i.e., echolocation or visual hunting) and 2. differences in foraging behavior (i.e., aerial hawking or sit-and-wait predators). We found evidence of dietary partitioning among all species. Additionally, eared moths were consumed significantly more often by birds than by echolocating bats, suggesting that evolutionary interactions between bats and moths may enhance resource partitioning between bats and birds.