Dietary partitioning corresponds with predation of eared
insects
Moth families that contain species with ears (Miller & Surlykke, 2001)
were associated with the diets of NAIs that hunt visually but not bats
that use echolocation or gleaning (p <0.01; Figure 1a; Table
2). The most abundant eared family, Noctuidae moths, occurred in the
diets of 82% of Common Poorwills, 69% of Flammulated Owls, and 32% of
Common Nighthawks sampled. Long-legged Myotis fecal samples contained
Noctuidae moths 35% of the time. However, just 16% of Long-eared
Myotis, 4% of Big Brown Bats, and no Silver-haired Bats consumed
Noctuid moths. Other eared moth families, including Geometridae,
Sphingidae, and Erebidae, also occurred significantly more often in
visual hunters’ diets, but rarely in bat diets. One other family of
eared moths, Drepanidae, associated with the diets of Flammulated Owls
and Common Poorwills (p < 0.05, sit-and-wait predators, Table
S2), but not Common Nighthawks.
Conversely, the non-eared moth family Tortricidae (mostly spruce
budworm), was the most abundant family consumed more often by
echolocators than by visual hunters (p < 0.001). We found
Tortricidae in the diets of 65% of Long-legged Myotis, 63% of
Long-Eared Myotis, 48% of Big Brown Bats, 23% of Silver-haired Bats,
but only 11% of Common Nighthawks, 5% of Common Poorwills, and 31% of
Flammulated Owls. Ten other insect families were consumed more often by
predators that use echolocation (Table 2, p < 0.02). Insect
families that were recovered more often in diets of NAIs that hunt by
aerial hawking (bats and nighthawks) corresponded with those associated
with echolocation strategies (bats only, Table S2). All insect families
and OTUs consumed significantly more often by each individual NAI
species can be found in Tables S3 and S4.
Dietary Partitioning among
species
In general, we saw low dietary overlap among species (Table 3)
regardless of foraging behavior or prey detection method. The highest
OTU overlap in diets occurred among bats, with the two smallest species,
Long-legged Myotis and Western Long-eared Myotis overlapping the most
(22%). Big Brown Bat diets also overlapped considerably with all other
bat species (18-21%). Among nocturnal birds, the highest overlap
occurred between sit-and-wait predators, Common Poorwills, and
Flammulated Owls (17%). Diet overlap between bats and birds was the
lowest, with Flammulated Owls and Silver-haired Bats overlapping by just
2%.
Controlling for differences between years, Permanova analyses on
presence/absence data indicated that NAI species identity explained the
largest variation in diet (R2 = 0.23, p = 0.001).
Pairwise comparisons indicated that the diets of all NAIs differed from
each other (p < 0.006), except for Big Brown Bats, Long-eared
Myotis, and Long-legged Myotis, which had more similar diets (Figure 1d;
Table S6). Diet composition also varied due to whether NAIs detect prey
visually or with echolocation (R2 = 0.12, p = 0.001)
and whether they use aerial hawking or sit-and-wait strategies
(R2 = 0.12, p = 0.001). Because species was the best
predictor of diet, we included it in the full model with collection
month (R2 = 0.18, p = 0.001) and plant community
(R2 = 0.05, p = 0.001), which were also influential as
main effects. The interaction between species and month was a strong
predictor of composition (R2 = 0.17, p = 0.001),
highlighting the importance of seasonal variation within each NAI diet.
The total variation among insectivore diets explained by the model was
70%. We also performed Permanova analyses on Bray-Curtis distances of
compositional data. Relationships were similar, with species
(R2 = 0.10, p = 0.001), month (R2 =
0.09, p = 0.001) and their interaction (R2 = 0.14, p =
0.001) explaining the most variation among NAI diets (Table S3).
Overall, the model using compositional data explained 55% of the total
variation among NAI diets.
At the order level, Common Nighthawks and Long-legged Myotis consumed
mostly Diptera (true flies, 75% and 43% of diets, respectively).
Common Poorwills and Flammulated Owls consumed mostly Lepidoptera
(moths/butterflies; 63% and 88%, respectively). Silver-haired bats
were the only NAI to mostly consume Ephemeroptera (mayflies, 45%). In
comparison, Big Brown Bats and Western Long-eared Myotis consumed
similar abundances of Lepidoptera (30% and 44%, respectively), and
Diptera (31% and 43%, respectively). The top insect orders consumed by
all NAI species combined were Lepidoptera followed by Diptera,
Ephemeroptera, and Coleoptera (Figure 1b). Except for Long-legged
Myotis, the top insect families consumed for each NAI were consistent
between 2017 and 2018 (Figure 2).
We found craneflies belonging to the genus Tipula in 35% of
samples overall, more frequently than any other insect genera recovered.
The most abundant and common prey OTU matched 100% with the family
Tipulidae (BOLD:ADC2461, crane flies, Table 4). Morphological
examination of specimens associated with this OTU confirmed it asTipula (Lunatipula) splendens Doane 1901 (personal communication,
Dr. Jon K. Gelhaus, June 26th, 2021). This OTU
occurred in 26% of all samples, in all NAI species diets except for
Flammulated Owls, and was one of the most frequent and abundant
prey items consumed by Common Poorwills, Common Nighthawks, Big Brown
Bats, Long-eared Myotis, and Long-legged Myotis (Table 3).
The second most abundant OTU matched locally to Choristoneura
freemani (western spruce budworm; BOLD:ABX5883) and was detected in
16% of all NAI samples. It was one of the top two OTUs consumed by most
bat species but occurred in just one Common Nighthawk sample and two
Common Poorwill samples.
Dietary breadth and
turnover
From the fecal samples of all seven NAI species, we identified 73
arthropod families, 165 genera, and 382 OTUs. Silver-haired bats had the
widest diet breadth at the order (10) and family (36) levels (Table S7),
whereas Common Poorwills consumed the greatest number of insect genera
(75) and OTUs (154). We detected the fewest total OTUs in Common
Nighthawk samples (50). Flammulated Owls and Common Poorwills had the
highest variation or turnover among samples, whereas Silver-haired Bats
and Western Long-eared Myotis had the lowest (Table 1). Long-legged
Myotis had the most OTU-rich diet on average (Figure 1c), consuming more
prey OTUs than Common Nighthawks, Big Brown Bats, and Long-eared Myotis
(p < 0.001; Table S8). NAI species, collection month, year,
and plant community were significant predictors of dietary richness.
Species identity had the greatest influence.