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Syntopy and co-utilization of a “roosting area” by the barn owl ( Tyto alba ) and spotted eagle owl ( Bubo africanus ) in Tanzania

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SHORT COMMUNICATION
Syntopy and co-utilization of a roosting areaby the barn owl
(Tyto alba) and spotted eagle owl (Bubo africanus) in Tanzania
Timothy L. Campbell
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Zachary W. Pierce
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Kathleen N. Dollman
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Marion K. Bamford
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Charles M. Musiba
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Cassian Magori
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Department of Anthropology, Texas A&M University, College Station, TX, USA
2
Department of Biological Sciences, Sam Houston State University, Huntsville, TX, USA
3
Evolutionary Studies Institute and School of Geoscience, University of the Witwatersrand, Johannesburg, South Africa
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Department of Anthropology, University of Colorado Denver, Denver, CO, USA
5
Saint Francis University College of Health and Allied Sciences, Ifakara, Tanzania
Correspondence
Timothy L. Campbell
Email: camp8943@tamu.edu
Funding Information
Department of Anthropology, Texas A&M University Milo E. Carlson
Graduate Student Dissertation Award; MSC L.T. Jordan Institute for
International Awareness; TZ RSA/JRP/RG.2013.15; Animal Behavioral
Society; NRF-COSTECH (ISTA 86916); University of Colorado Denver,
Office of Research Service; University of Colorado Office of International
Affairs
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INTRODUCTION
Owls are predators of small animal communities and are useful in
studying these communities as parts of prey items consumed are
preserved in regurgitated pellets (Bunn, Warburton, & Wilson, 1982;
Fry, Keith, & Urban, 1993; Hockey, Dean, & Ryan, 2005; Steyn,
1982). Analyses of prey remains within pellets have long been recog-
nized as contributing valuable information on modern faunal commu-
nity composition (Avenant, 2005; Avery, 1992; Avery, Avery, &
Colahan, 2003; Davis, 1958; Dodson & Wexlar, 1979). Furthermore,
the contribution of owls to the vertebrate fossil record is also well
documented, (Andrews, 1990; Davis, 1959; Dodson & Wexlar, 1979)
and fossilized prey remains are commonly used to reconstruct pale-
oenvironments (e.g. Avery, 2001; Fernandez-Jalvo et al., 1998; Reed,
2007). In many studies, barn owls (Tyto alba Scopoli, 1769) are iden-
tified as the presumed accumulating agent, although spotted eagle
owls (Bubo africanus Temminck, 1821) have also been considered.
Reported roosting habits of these two species vary, with barn owls
preferring closed environments such as caves, crevasses, hollow
trees and building interiors (Bunn et al., 1982; Kemp, 2005a; Reed,
2005; Steyn, 1982; Wilson, Wilson, & Fry, 1993). In contrast,
spotted eagle owls roost in more open areas such as on the ground
in vegetation, along rocky outcrops and in tree crowns (Kemp, 1993,
2005b; Reed, 2005; Steyn, 1982). Although a few studies have com-
pared the diets and roosting habits of these two species living in the
same area (Brain, 1981; Demeter, 1981; Dixon & Perrin, 1994; Men-
delsohn, 1989; Reed, 2005, 2007, 2011; Tilson & LeRoux, 1983;
Vernon, 1980), reported distances between roosts occupied by dif-
ferent owls have generally ranged from several hundred metres to
several kilometres apart. To our knowledge, these two species have
rarely been observed occupying roosts in close proximity (e.g. Brain,
1981; Dean, 1978; Mendelsohn, 1989), and have never been
reported utilizing the same roost where a large accumulation of prey
remains has been found with one exception. Grindley, Siegfried, and
Vernon (1973) report having observed a pair of spotted eagle owls
at a pellet accumulation in South Africa, although they ultimately
inferred that a barn owl produced the assemblage based on the frag-
mentation pattern of prey crania. As such, we report the first obser-
vations of both species associated with a large pellet accumulation
at a canopy roost. Additionally, we also document one spotted eagle
owl and one barn owl each associated with separate cavity roosts,
with all three roosts located within a 100-m transect.
Accepted: 24 August 2017
DOI: 10.1111/aje.12485
Afr J Ecol. 2017;15. wileyonlinelibrary.com/journal/aje ©2017 John Wiley & Sons Ltd
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MATERIALS AND METHODS
Observations were made during JuneJuly of 2015 and June of
2016 at 3.229°S, 35.166°E within the fossil-bearing site of Laetoli in
the Ngorongoro Conservation Area, northern Tanzania (Figure 1).
Located in the Serengeti ecosystem on the western flank of the
Ngorongoro Volcanic Highlands, this area contains habitats ranging
from open grasslands to rolling hills with wooded areas along drai-
nages (Musiba, 1999; Reed, 2011). Supported by these varied habi-
tats are as many as nine different species of owls (Fry et al., 1993).
Additionally, 26 rodent and shrew genera within the size range of
prey taken by both species of owl have been reported within the
Serengeti ecosystem (Reed, 2005, 2011). Located within a 2 km
radius of the site are a variety of different microhabitats, with open
areas utilized for cattle grazing by local Maasai located to the south
and east, and wooded areas associated with the Garusi River and its
tributaries located north and west. The vegetation is a mixed decidu-
ous woodland dominated by Euphorbia candelabrum, Acacia tortilis,
Acacia depanolobium, Boscia albitrunca, Commiphora africana, Acacia
mellifera and Balanites aegyptiaca (in order of abundance). The A. de-
panolobium trees are stunted, and more like shrubs in this area as
the soils are a mixture of well drained volcanic tuffs and very poorly
drained mbuga (black cotton) soils, the latter of which is preferred
by A. drepanolobium. Grass cover is patchy as some of the ground
surface is very hard or the soils are very thin, but comprises Setaria
verticilata under the tress and Aristida adscensionis, Aristida junci-
formis, Digitaria microblephara, Eragrostis cilianensis, Microchloa kunthii
and Eragrostis tenuifolia in the open areas. In the areas protected by
trees or rocky outcrops, there are a number of herbs mixed in with
the grasses, such as Kalanchoe lanceolata and Kleinia kleinioides.
Although the observation reported here was made during the dry
season, in the wet season a greater floral diversity may be apparent.
Owls were visually identified during the day, and all observations
were made along a low ridgeline (~100 m long) supporting a series
of conspicuous candelabras (E. candelabrum) and three large concen-
trations of pellets and/or disaggregated bone detritus designated as
roosts A, B and C (Figure 1). Individual candelabras along the ridge-
line are denoted as C-1 through C-15.
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RESULTS AND DISCUSSION
All three roosts and both owl species were first identified on 26
June 2015. A barn owl was first found within the crown of a large
candelabra (C-1, roost A), underneath which a large accumulation of
pellets and disaggregated bone detritus was located. Upon approach
the owl emerged from the interior, flew overhead for several
passes, and landed on C-7 where it perched for several minutes
before flying off to the northwest. Subsequently, a small cavity in
the side of the cliff face was found ~40 m west of roost A. On
approach a spotted eagle owl took flight and landed on C-9 where
it perched for several minutes before flying off to the east. At the
time, the owl took flight the cavity opening was obscured from
vision by vegetation, and we cannot confirm if the owl was present
inside, or in front of the cavity. We are, however, confident that
the owl was perched within two metres of the entrance. Desig-
nated roost B, an abundance of disaggregated bone detritus, and
two partial pellets were found within and underneath of the cavity.
Following this, a third small cavity was located ~60 m west of roost
B. On approach a barn owl emerged and flew off to the east. Des-
ignated roost C, many fresh and weathered pellets, as well as
microfaunal bone detritus, was found within and underneath this
cavity. On 7 July, the roosting area was revisited, and a barn owl
was again seen in residence at roost C. On this occasion, upon
emerging the owl again headed east, landing on C-12 where it
perched for several minutes before moving on in the same direction
(Figure 2).
In June of 2016, the roosting area was revisited to collect new
prey remains which accumulated over the year. On 17 June, a spot-
ted eagle owl was found perched on the ground on the south side
of the C-1 (i.e. roost A). On approach it took flight, landing on C-7
where it remained for several minutes before continuing on south-
east (Figure 2). At this roost, both fresh and weathered pellets, along
with bone detritus, were recovered. Subsequently, cavity roosts B
and C were visited, although no owls were observed. Camera traps
were placed outside both openings and left until 25 June; however,
no owls were photographed during this time. No new prey remains
were recovered from roost B, although roost C had numerous
FIGURE 1 Map of low ridgeline with
identified owl roosts, placement of fifteen
conspicuous candelabra trees, and insets of
study location in the Ngorongoro
Conservation Area (NCA), Tanzania.
Positions of roosts, trees, and base of
ridgeline georeferenced using a GPSMAP
64s. Roost A is a canopy roost and
corresponds to candelabra 1 while roosts B
and C are cavity roosts found within the
hillside. All roosts were defined by the
presence of large accumulations of owl
pellets, disaggregated pellet remains, and
loose bone detritus
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CAMPBELL ET AL.
weathered pellets and bone detritus indicating that it had been used
during the year.
At many spots along the length of the ridgeline, and under the
candelabras, individual pellets or small concentrations of bone detri-
tus were observed. As such, the entire 100 m transect appears to
function as one large roosting area.Although an analysis of the
prey remains recovered will be the subject of a future paper, a few
field observations are provided. Vertebrate remains recovered from
disaggregated pellets at all three roosts consisted primarily of small
rodents and shrews, with a smaller proportion of avifauna. Rodent
genera present at all three roosts included the dendromurine Den-
dromus, and the murine Mus, both identified based on size and max-
illary first molar morphology (Happold, 2013). Additionally, several
larger murine genera were also present as well as two different
sized gerbils, the larger of which is identified as Gerbilliscus with the
smaller form representing Gerbillus based on occlusal morphology,
size, and modern biogeography (Happold, 2013). For shrews,
although a range of different sizes was examined, all specimens pos-
sessed three maxillary unicusps, and lacked vascular foramina
between the parietals indicating the genus Crocidura (Happold &
Happold, 2013).
Although preliminary results show no differences in mammalian
prey, this is not unexpected. Several studies comparing the diets of
these two owls living in the same area have shown that they gener-
ally select the same mammalian prey (Brain, 1981; Mendelsohn,
1989; Reed, 2011; Tilson & LeRoux, 1983). Moreover, in his analysis
of the diets of these two species in the Serengeti, Reed (2005)
found identical mammalian taxonomic representation, though with a
greater abundance of larger taxa (>60 g) at roosts utilized by spotted
eagle owls. Of greater interest is the nonmammalian prey associated
with the spotted eagle owl at roost B. Here, various herpetofaunal
elements including snake craniodental remains were recovered. Of
these, the latter is significant as spotted eagle owls are known to
prey on snakes (Broadley, 1974; Chapin, 1939; Demeter, 1981;
Grobler, 1980; Sweeney, 1961; Welbourne, 1973) and we know of
no similar reports for barn owls.
A total of 51 complete pellets and 152 partial pellets were
recovered (N=203), of which 35, 87 and 115 had visible avian,
shrew and rodent remains, respectively. Complete pellets found at
roost C (n=36) were primarily dense, compact, and had a glossy
sheen when fresh, typical of those produced by barn owls (Andrews,
1990; Bunn et al., 1982). Even pellets which were heavily weath-
ered, such that the hair and other nonosseous portions were lost,
retained their compact nature. At roost B, a different pellet consis-
tency was observed with the two partial pellets recovered being
loosely consolidated and lacking a dark sheen on the exterior. Finally,
complete pellets found at roost A (n=15) were mixed with both
compact, and glossy pellets recovered, as well as loosely consoli-
dated pellets lacking a dark sheen. Observations on pellet morphol-
ogy, as well as prey composition from disaggregated castings, are
consistent with the specific owls observed at roosts B (B. africanus)
and C (T. alba). Moreover, while not conclusive, they are also not
inconsistent with a mixed assemblage produced by the two different
owl species observed at roost A.
In his analysis of roosting behaviour and tropic habits of barn
owls and spotted eagle owls in the Serengeti, Reed (2005) found
that these two species typically segregate by roost type, and only
one species was ever observed occupying each roost. Of the 19
barn owls Reed observed, 16 were found occupying cavity roosts
while the other three were found at exposed roosts. Alternately, all
13 spotted eagle owls were found at exposed roosts. The observa-
tions reported here provide further examples of a spotted eagle
owl associated with a cavity roost (see also Brain, 1981 and Deme-
ter, 1982) and a barn owl utilizing a tree canopy (see also Mendel-
sohn, 1989). In this case, the tree utilized, a candelabra, may be
amenable to roosting by both species as the outer branches may
represent an open canopy type roost preferred by spotted eagle
owls, while the interior may simulate an enclosed cavity preferred
by barn owls. Further investigations of owl roosting behaviour
should focus on these types of trees to determine if they represent
a roost conducive to barn owl preferences, particularly in areas
where dark, enclosed environments are lacking. Finally, to our
knowledge, the report here is the first account of these two spe-
cies of owls observed at the same roost in which a large accumula-
tion of pellets and prey remains was found. As previous
researchers have suggested that fossil assemblages in caves where
owls are the presumed accumulating agents may result from the
activities of multiple species, particularly when assemblages sample
FIGURE 2 A spotted eagle owl (Bubo
africanusleft) and a barn owl (Tyto alba
right) photographed along ridgeline. The
spotted eagle owl is seen here in
candelabra 7 and photographed during the
summer of 2016 in the same tree where a
barn owl was observed after being flushed
out of candelabra 1roost A the previous
year. The barn owl is seen in candelabra
12 and photographed during the summer
of 2015
CAMPBELL ET AL.
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long periods of time (Tchernov, 1992); the observations reported
here are significant as they potentially represent such a process in
action in an open setting.
ACKNOWLEDGEMENTS
We would like to thank the Ngorongoro Conservation Area Author-
ity, the Department of Antiquities in the Ministry of Natural
Resources and Tourism, and all of our local friends and collaborators
for their help with this project. This research was funded by the Ani-
mal Behavioral Society, the MSC L.T. Jordan Institute, and the Texas
A&M University, Department of Anthropology Carlson Dissertation
Award to TLC; a NRF-COSTECH (ISTA 86916) and TZRSA/JRP/
RG.2013.15 grant to CMM, CM and MKB; the University of Color-
ado Denver, Office of Research Service (Small Grant) to CMM and
the University of Colorado Office of International Affairs. Finally, we
would like to thank Denn
e N. Reed and two anonymous reviewers
for critical readings of this manuscript.
ORCID
Timothy L. Campbell http://orcid.org/0000-0002-3307-6752
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How to cite this article: Campbell TL, Pierce ZW, Dollman
KN, Bamford MK, Musiba CM, Magori C. Syntopy and co-
utilization of a roosting areaby the barn owl (Tyto alba) and
spotted eagle owl (Bubo africanus) in Tanzania. Afr J Ecol.
2017;00:15. https://doi.org/10.1111/aje.12485
CAMPBELL ET AL.
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Article
Owls are important consumers of small vertebrates, and because they regurgitate pellets rich in bone, they may be important potential contributors of the concentrated remains of small vertebrates to the fossil record. Owls of three sizes, the large great horned owl ( Bubo virginianus ), the medium-sized barn owl ( Tyto alba ), and the small screech owl ( Otus asio ), were fed a common diet of mice. The bony contents of the pellets were analyzed to determine the amount of bone loss by digestion, bone completeness, and sites of bone breakage. For all three species, only about half the number of bones ingested were recovered in the pellets. Mandibles and femora were most abundant, and pelves and scapulae were the least abundant. Screech owls broke 80% of the cranial and limb elements, barn owls only 30%. Skulls fared poorly in great horned and screech owl pellets, while barn owls returned 80% of the skulls intact, with only the caudal portion of the cranium damaged; barn owls also returned articulated strings of vertebrae and complete paws. These results provide a baseline for the recognition of owls as agents of accumulation of small bones in the fossil record and suggest that the actions of ancient predators may be revealed by species-specific patterns of bone destruction of an assemblage of fossil prey species.