2017 Northeastern Naturalist Notes Vol. 24, No. 2
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G.J. Foley and L.S. Wszola
Observation of Common Nighthawks (Chordeiles minor) and
Bats (Chiroptera) Feeding Concurrently
Gabriel J. Foley1,* and Lyndsie S. Wszola2
Abstract- Records of bats and birds concurrently exploiting the same food source are rare in the
literature. We observed an instance of bats and Chordeiles minor (Common Nighthawk) foraging
in artificial light around the Washington Monument. Our observation corroborates earlier evidence
that bats and Common Nighthawks both exploit the foraging opportunity created by artificial lights.
Because the monument provided spatial perspective, we were also able to observe that bats and Common
Nighthawks foraged at different heights, suggesting that they partitioned the available foraging
space vertically.
Introduction. Chordeiles minor (J.R. Forster) (Common Nighthawk, hereafter Nighthawk)
and most species of North American bats are aerial insectivores which occupy a
similar foraging niche (Brigham 1990) and therefore may compete for prey resources
(Hardin 1960). While bats are primarily nocturnal, Nighthawks are crepuscular, and these
insectivores likely partition prey resources temporally, as a function of light levels. Artificial
lights have the potential to disrupt the sensory cues distinguishing Nighthawks’
foraging time from bats’ foraging time by increasing ambient light levels at night, thereby
increasing the temporal and spatial overlap of Nighthawk and bat foraging opportunities.
Phototactic insects are commonly attracted to artificial lights, creating a dense localized
food source for aerial insectivores in general (Longcore and Rich 2004) and bats in
particular (Hickey et al. 1996). Nighthawks, which detect insects using vision (Aldridge
and Brigham 1991), have been previously documented foraging beyond twilight around artificial
lights such as street lamps or sports stadiums (G.J. Foley, pers. observ.; Shields and
Bildstein 1979). Because bats use echolocation to orient, they do not require a minimum
level of ambient light to forage, as Nighthawks do (Hickey et al. 1996; Hickey and Fenton,
1990, 1996). Bats and Nighthawks have been observed to share foraging space briefly
around twilight in the absence of artificial light (Brigham 1990), and for longer periods in
artificial light (Shields and Bildstein 1979), with the latter scenario leading to agonistic
interactions between bats and Nighthawks. We are reporting the following observation
because bats and Nighthawks are cryptic, poorly understood taxa experiencing population
declines. Our observation, though brief, demonstrates how 2 ecologically similar taxa may
interact in a modified landscape.
Observations. On 20 August 2016 from 21:45 to 22:15, we observed 3 Nighthawks and
5 bats of an unknown species foraging concurrently around the Washington Monument in
Washington, DC. The Washington Monument is a 169-m white marble obelisk lit at night by
4 lighting fixtures. Each fixture is ~200 m from 1 of the Monument’s 4 corners, mounted on
7-m-high poles, and consists of three 2000-W lights. Using mirrors within the mounted fixtures,
~80% of the light produced strikes the obelisk’s surface. In addition to the taller light
fixtures, there are also sixty-six 150-W lights at the base of the Monument (Federal Energy
Management Program 2008). We made our observations without optical enhancements,
1University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2, Canada. 2Nebraska Cooperative
Fish and Wildlife Research Unit, University of Nebraska–Lincoln, Lincoln, NE 68583-0984. *Corresponding
author - gabriel.j.foley@gmail.com.
Manuscript Editor: Jacques Veilleux
Notes of the Northeastern Naturalist, Issue 24/2, 2017
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2017 Northeastern Naturalist Notes Vol. 24, No. 2
G.J. Foley and L.S. Wszola
~30 m from the base of the monument, but the area in question was well-lit, allowing us to
confidently distinguish between bats and Nighthawks.
When we arrived at the Monument at 21:45, forty-two minutes after nautical twilight,
3 Nighthawks were already feeding, but only a single bat was present. By ~22:00, at least
5 individual bats were also foraging around the Monument. All 3 Nighthawks ceased foraging
around the Monument between 22:00 and 22:15. The Nighthawks foraged in close
proximity to the Monument, with circular flight paths extending >100 m horizontally from
the Monument. The bats foraged in tighter circles around the Monument, usually within
30 m, although we frequently lost sight of the foraging bats. We estimated foraging height
by comparing both taxa to their placement in space relative to the Monument. The Nighthawks
consistently flew 85–150 m above the ground, while the bats foraged 45–75 m above
the ground. This observation of Nighthawks foraging higher than the bats is consistent with
Shields and Bildstein’s (1979) observations.
Throughout the 30 minutes of observation, we did not observe any interactions between
bats and Nighthawks. The absence of agonistic interactions in our observation is potentially
due to a much shorter sampling period. Shields and Bildstein (1979) spent 90.5 hours on
40 separate nights observing bats and Nighthawks foraging together. They observed 81
antagonistic interactions between bats and Nighthawks, and 96% of those interactions were
initiated by bats. This averages 1.1 interactions per hour, which, assuming the same rate,
means we had a 55% chance of witnessing an antagonistic observa tion.
Our observation of Nighthawks and bats feeding together is consistent with the few
previous observations of the 2 taxa (Brigham 1990, Shields and Bildstein 1979). In all 3
studies, Nighthawks arrived before bats and ceased foraging within 90 minutes of nautical
twilight. In each location, Nighthawks were observed foraging higher than the bats and
arrived earlier and left earlier. Despite both taxa having nearly identical insectivorous foraging
niches (Brigham 1990), they appear to partition the foraging resources both spatially
and temporally. We observed bats and Nighthawks exploiting the same foraging opportunity
created by artificial lights, raising the question of how modified landscapes influence interactions
between taxa whose niches have historically been partitioned by temporal, spatial,
or sensory cues.
Acknowledgments. We thank the organizers of the sixth North American Ornithological Conference
for facilitating the authors’ acquaintance and creating the opportunity for this observation. We
also thank Mark Brigham and T.J. Fontaine for their support and editorial advice.
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