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VOLUME 12, ISSUE 2, ARTICLE 5
Davis, C. M., J. A. Heath, and C. J. W. McClure. 2017. Nest box use by American Kestrels and other cavity-nesting birds during the nonbreeding
season. Avian Conservation and Ecology 12(2):5. https://doi.org/10.5751/ACE-01044-120205
Copyright © 2017 by the author(s). Published here under license by the Resilience Alliance.
Short Communication
Nest box use by American Kestrels and other cavity-nesting birds
during the nonbreeding season
Caitlin M. Davis 1, Julie A. Heath 1 and Christopher J. W. McClure 2
1Boise State University Department of Biological Sciences and Raptor Research Center, 2The Peregrine Fund
ABSTRACT. Nest boxes are posted to provide breeding sites for cavity-nesting birds but less is known about their function in the
nonbreeding season, when nest boxes may become important roost sites. In winter months, we surveyed 79 nest boxes before dawn for
roosting American Kestrels (Falco sparverius) and other cavity-nesting birds in southwestern Idaho and we reviewed camera recordings
from the entire nonbreeding season at a nest box within the study site to better understand nest box use in the nonbreeding season.
During surveys we found seven American Kestrels roosting in six nest boxes, Northern Flickers (Colaptes auratus) in 16 nest boxes and
a European Starling (Sturnus vulgaris) in one nest box. Video recordings revealed inter- and intra-specific conflicts within the nest box
as well as a positive relationship between the length of night and the time spent roosting in the box. These results suggest that cavity-
nesting birds in our study area are likely to seek out and compete for nest boxes to use as roost sites in the nonbreeding season and the
effects of nest boxes on the nonbreeding season ecology of birds should be considered.
Utilisation de nichoirs par les Crécerelles d'Amérique et d'autres nicheurs cavicoles en dehors de la
saison de nidification
RÉSUMÉ. Les nichoirs sont installés afin de fournir des sites de nidification aux oiseaux nicheurs cavicoles; or, on en connait peu sur
leur fonction en dehors de la saison de nidification, quand ils servent alors peut-être de dortoirs. Pendant les mois d'hiver, nous avons
examiné 79 nichoirs avant l'aube à la recherche de Crécerelles d'Amérique (Falco sparverius) et d'autres nicheurs cavicoles dans le sud-
ouest de l'Idaho. Nous avons aussi visionné les vidéos prises à un nichoir dans l'aire d'étude durant l'entièreté de la saison hors nidification
afin de mieux comprendre l'utilisation des nichoirs à cette saison. Nous avons trouvé sept crécerelles dormant dans six nichoirs, des
Pics flamboyants (Colaptes auratus) dans 16 nichoirs et un Étourneau sansonnet (Sturnus vulgaris) dans un autre nichoir. Les bandes
vidéos ont révélé des conflits inter et intra-spécifiques dans le nichoir ainsi qu'une relation positive entre la longueur de la nuit et le
temps passé à dormir dans le nichoir. Nos résultats indiquent que les nicheurs cavicoles de notre aire d'étude recherchent
vraisemblablement les nichoirs et se font concurrence pour ceux-ci afin de les utiliser comme dortoir en dehors de la saison de nidification;
nous pensons que les effets des nichoirs sur l'écologie des oiseaux hors saison de nidification devraient être pris en compte.
Key Words: cavity nester; Colaptes auratus; competition; Falco sparverius; nest box; roost
INTRODUCTION
Protected roost sites, e.g., dense vegetation or cavities, are critical
to the survival of many birds wintering in temperate climates
because they can alleviate metabolic pressures brought on by
lower temperatures, extreme weather, and limited food (Walsberg
1986, Cooper 1999, McCafferty et al. 2001). Roosting in closed
sites such as cavities in winter provides more thermal benefits
(Kendeigh 1961, Cooper 1999) and better protection from
predators (Sunde et al. 2003, Bock et al. 2013) than roosting in
open sites such as tree limbs. This is most likely why cavity-nesting
species will use cavities as roosts in the nonbreeding season and
preferentially choose them during winter months (Duguay et al.
1997, Mainwaring 2011, Bock et al. 2013). In human-altered
landscapes where natural tree cavities may be limited, some
secondary cavity nesting species use other available closed roost
sites such as human-made structures, e.g. barns, sheds, or
abandoned buildings, and nest boxes as roosts in the nonbreeding
season (Mainwaring 2011). Therefore, although nest boxes are
typically posted to increase breeding productivity, nest boxes may
also create important roost sites that lead to improved
overwintering survival and beneficial population-level effects.
The effectiveness of nest box programs as a management tool
typically requires that nest boxes either decrease nest site
limitation or create improved productivity relative to natural
cavities (McClure et al. 2017a). Without careful placement, nest
boxes may become ecological traps (Strasser and Heath 2013).
For example, nest boxes provide similar thermal benefits as
natural cavities when temperatures are above freezing (Kendeigh
1961, Grüebler et al. 2014). However, when temperatures drop
below freezing, natural cavities may better buffer extreme ambient
temperatures and have significantly greater thermal benefits
compared to artificial cavities (McComb and Noble 1981,
Grüebler et al. 2014). The suitability of nest boxes as winter roosts
is therefore unclear. Thus, it is important to explore the use of
nest boxes in the nonbreeding season to reveal whether cavity-
nesting birds use nest boxes as roosts.
Address of Correspondent: Caitlin M. Davis, 1910 University Drive, Department of Biological Sciences, Raptor Research Center, Boise State
University, Boise, ID, USA, caitlindavis@u.boisestate.edu
Avian Conservation and Ecology 12(2): 5
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American Kestrels (Falco sparverius) are small, cavity-nesting
falcons that are widespread across North and South America.
Kestrels readily use nest boxes for breeding and nest box programs
are common throughout their range (Smallwood and Bird 2002).
In response to observed declines in populations across much of
North America, McClure et al. (2017b) recently called for research
into the nonbreeding ecology of American Kestrels. Kestrel
roosting behavior during the nonbreeding season seems to be
variable across the species’ range and their use of nest boxes as
roosts is not well known. In winter, without the presence of nest
boxes, Kestrels have been documented roosting in human-made
structures in Ohio (Mills 1975) and Louisiana (Doody 1994), and
in one communal tree roost in California (Miller and Lanzone
2015). Additionally, Kestrels used spruce trees as roosts while
migrating through Saskatchewan (Bortolotti and Wiebe 1993). In
Pennsylvania, where established nest boxes were available,
Kestrels were not observed using boxes to roost and used other
human-made structures (Ardia 2001). However, during one
winter in Missouri, female Kestrels roosted in all 50 nest boxes in
the study area (Toland and Elder 1987), which is, to our
knowledge, the only known documentation of this behavior.
These behavioral differences raise questions about the suitability
of nest boxes as winter roosts and why Kestrels used them in one
location, but not the other. Here, we examine the use of nest boxes
by Kestrels during the nonbreeding season in relation to
seasonality, day length, and heterospecifics in our study site in
southwestern Idaho. We predicted that Kestrels will be more likely
to roost in nest boxes during periods of low minimum
temperatures, and will remain in roosts longer as day length
increases. Additionally, we predicted that there is inter- and intra-
specific competition for nest boxes as roosts during the
nonbreeding season.
METHODS
Breeding American Kestrels in southwestern Idaho (43°N 116°
W) have been closely monitored through the use of established
nest boxes for over 25 years (Steenhof and Peterson 2009) yet no
studies have examined their roosting behavior during the
nonbreeding season. At this study site, Kestrel pairs initiate egg-
laying from mid-March through mid-July, so we considered the
nonbreeding season to be mid-July through mid-March. Our
study area is over 1000 km² and in a human-altered landscape
consisting of suburban, agricultural, and shrub-steppe cover-
types. There is an abundance of human-made structures as well
as interspersed patches of trees available for roosting. In January
2015, we blocked the entrance to the nest box and visually
inspected 79 nest boxes in the study area between 05:00 and
sunrise. If a Kestrel was inside the nest box, we caught the bird
by hand and read the band or marked it with an aluminum U.S.
Geological Service band and an alphanumeric color band, and
returned it to the nest box. If another species was in the box, the
bird was left in the box with the lid closed and the plug removed.
All methods and protocols described above were approved by the
Boise State University IACUC review board (IACUC Approval
Number 006-AC14-024).
We also video-recorded the activity in a nest box installed on the
roof of the research library at the campus of the World Center
for Birds of Prey in Boise, Idaho that was within the study site.
We recorded videos using the KestrelCam (BOSCH Flexidome
NDI-50022-V3, McClure et al. 2015). The nest box was modified
to include an opaque window on one side to improve lighting for
the KestrelCam. The KestrelCam recorded 20 seconds of video
anytime it sensed movement between 22 July 2014 and 11 March
2015. The KestrelCam was equipped with an infra-red light and
had the ability to sense movement and record at night. We
reviewed all videos and recorded the species, the activity of
individuals, and the duration of the nest box visit. We considered
visits that lasted > 8 hours and overlapped the dark part of the
day to be roosting events. For each roosting event, we recorded
the entry and exit times.
We obtained the nightly minimum temperature at the weather
station located at the Boise Airport (5.6 km from the KestrelCam
nest box) from the U.S. National Oceanic and Atmospheric
Administration’s website (https://www.ncdc.noaa.gov/). We then
examined whether time spent in the nest box per night was
correlated with the length of the night using linear regression, and
whether the probability of a Kestrel roosting in the box was
correlated with minimum temperature using logistic regression.
All analyses were performed using R (R Development Core Team
2016).
RESULTS
We found seven American Kestrels roosting in six of the 79 boxes,
including 4 lone females, 1 lone male, and 1 male and female pair.
In 16 of the nest boxes we found lone roosting Northern Flickers
(Colaptes auratus) and in 1 box, a single European Starling
(Sturnus vulgaris). The remaining 56 boxes were empty.
The KestrelCam captured 722 videos of American Kestrels, 122
videos of Northern Flickers, six of Black-billed Magpies (Pica
hudsonia), and two of European Starlings. Birds entered the nest
box during the day and during the night. At 17:22 on 22 October
2014, we recorded an aggressive encounter between a male
American Kestrel and a Black-billed Magpie with the Kestrel in
the box and the magpie outside pecking inwards. We also recorded
three aggressive encounters between Northern Flickers and
American Kestrels, the most aggressive of which (see Videos)
occurred at 12:40 on 11 October involving a male Kestrel jumping
on the back of a male Flicker with the flicker eventually escaping
the box. Another encounter between a Flicker and a Kestrel
occurred at 18:45 on 21 October and involved a female Kestrel
initially perched outside of the entrance but leaving after being
pecked by a male flicker inside the box. And, at 8:27 on 21
February a male Kestrel inside the box displaced a male Flicker
that was perched outside the entrance.
There were 55 videos showing two Kestrels inside of the box, all
of which where the sex of both birds could be determined involved
a male and a female. Though females were observed entering the
KestrelCam nest box for short periods of time, they never
remained in the box for the duration of the night. A notable
sequence of videos occurred beginning at 17:22 on 31 December,
which showed periods of peace and aggression between two
Kestrels, with the male mostly being the aggressor (see Videos).
Fifteen of 17 (88%) days in which there were videos of males and
females together occurred after 1 February and those videos
mostly showed a male performing a courtship display to a female
within the nest box.
Avian Conservation and Ecology 12(2): 5
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Another interesting event occurred 18 November beginning at
05:02 and again at 07:47 (see Videos) when primary and rectricial
feathers of a male Kestrel were frozen to the floor of the
KestrelCam nest box. The bird can be seen struggling to free his
feathers, eventually loosening them from the nest box, and
ultimately surviving the ordeal. The low temperature on that night
was -16 degrees Celsius, lower than typical minimum
temperatures for this region.
American Kestrels visited the KestrelCam box on 58% of the days
during the recording period, whereas Northern Flickers visited
the box on 8% (Fig. 1). American Kestrels appeared to visit the
KestrelCam box mostly during two periods: late October through
early January and February through the end of the survey period,
corresponding with the courtship period (Fig. 1). Although
several species visited the KestrelCam nest box over the course of
the study, only male American Kestrels were observed roosting
in the box overnight. We recorded American Kestrels roosting on
48 occasions between 23 October and 2 February (Fig. 1). There
were four roost occasions for which we could not determine entry
time. On average, Kestrels entered the box to roost 2.37 (SD =
27.46) minutes after sunset and exited the box 4.83 (SD = 75.10)
minutes after sunrise. The length of a roosting occasion increased
over the course of the study in concert with the length of night
(R² = 0.27; Fig. 2). Additionally, from 233 days of observations
we found that the probability of a Kestrel roosting in the
KestrelCam nest box increased significantly (β = -0.01, SE =
0.002, p < 0.001) as daily minimum temperature decreased.
Fig. 1. Timeline indicating days when (A) American Kestrels
(Falco sparverius) and (B) Northern Flickers (Colaptes auratus)
were observed visiting in daylight (black bars) or roosting
overnight (gray bars) in a nest box at the World Center for
Birds of Prey in Boise, Idaho.
Fig. 2. Number of hours spent roosting per night by American
Kestrels (Falco sparverius) in a nest box at the World Center for
Birds of Prey in Boise, Idaho and the number of hours in a
given night.
DISCUSSION
We found that American Kestrels and other cavity-nesting birds
used nest boxes during the nonbreeding season. Further,
conspecific interactions within nest boxes may be a part of pair
formation or, perhaps, a strategy for thermoregulation when
roosting. Interspecific interactions recorded at one nest box over
the course of the nonbreeding season suggest that there may be
competition for at least some nest boxes. Although thought of as
a habitat enhancement for breeding, nest boxes clearly provide
important resources for roosting and early breeding season
displays.
Daytime surveys of Kestrels in the study area suggest that Kestrel
density (~0.5 Kestrels per km²) is much higher than the rate of
occupied nest boxes (Heath, unpublished data) and some nest
boxes were not used as roosts. These results are similar to the
findings of Ardia (2001), where nest boxes were available in
Kestrel territories during winter but not used by Kestrels.
However, we only surveyed nest boxes during a short period (20
days) within one season and data, though limited, from the
KestrelCam suggested that the probability of occupancy
increased as minimum temperatures decreased. Thus, patterns of
roosting can vary within a season. Alternatively, Kestrels may not
have used some boxes because other roosts within occupied
territories provided more thermal benefits. Though nest boxes
provide thermoregulatory savings in winter, their low buffering
capacity at low ambient temperatures might be less desirable to
wintering birds (Grüebler et al. 2014). In addition, differences in
the structural characteristics and location of nest boxes can
greatly alter microclimate (Mainwaring 2011). The boxes in the
area are positioned and designed to provide a favorable
microclimate during the breeding season but some may not
provide ample benefits during winter months. Indeed, the video
recorded by the KestrelCam of the male Kestrel with its feathers
apparently frozen to the floor of the nest box suggests a unique
hazard to birds during the winter (see Videos). In addition,
following some relatively cold winters, we have found American
Kestrels that appeared to die of exposure during early March nest
box visits (Heath, unpublished data). Thus, whether nest boxes
Avian Conservation and Ecology 12(2): 5
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may act as an ecological trap by attracting cavity nesters to roosts
that do not have the thermoregulatory benefits of natural cavities
warrants further investigation.
The observation of a male and female roosting together in a nest
box is contrary to the general description of behavior of American
Kestrels in the nonbreeding season, when they are known to be
territorial toward conspecifics (Cade 1955, Mills 1976). However,
in areas with resident pairs, male and female Kestrels may retain
pair bonds throughout the year (Heath, unpublished data) or pairs
may have tolerated each other when food resources were
particularly high. That resource levels might mediate competition
between conspecifics is supported by documentation that Kestrels
will sometimes tolerate familiar conspecifics during the day
(Doody 1994) and will occupy a communal roost in a grove of
trees when it is beneficial in the local habitat (Miller and Lanzone
2015). Communal roosting may also have been observed because
increased thermal benefits were sought. Roosting together in a
cavity provides substantial thermal benefits for both individuals
because of metabolic heating (Cooper 1999, Grüebler et al. 2014).
Pairs roosting together in cavities may be more common than we
previously thought, however, the aggressive encounter between a
male and female within the KestrelCam nest box suggests that
attempts to share roost sites are not always without conflict.
Our results also suggest that Kestrels will remain in roosts for
longer periods as the length of night increases (Fig. 2) and that
Kestrels are more likely to use nest boxes on colder nights. Night-
length is known to affect weight and metabolism of birds
(Lehikoinen 1987, Haftorn 1989) and it is intuitive that Kestrels
would remain roosting during night when temperatures are low.
That Kestrels generally enter and leave the roost in concert with
sunset and sunrise, respectively, suggests that roosting behavior
of Kestrels might not severely hamper the interpretation of data
from geolocators, as has been demonstrated in other species (Gow
2016). It would be interesting to compare roosting behavior of
Kestrels to other sites with warmer minimum temperatures or
communities of cavity-nesting birds to compare the relative
importance of nest boxes for nonbreeding season resources.
More information is needed on the seasonal and year-to-year
variation in roosting behavior across the range of American
Kestrels to understand the mechanisms involved in nonbreeding
season roost selection. If nest boxes are introduced in efforts to
boost breeding populations of a particular species, consideration
of nonbreeding season use may be important to encourage extra
attention to the construction and maintenance of nest boxes over
the whole year.
Responses to this article can be read online at:
http://www.ace-eco.org/issues/responses.php/1044
Acknowledgments:
We would like to thank Michael Henderson and Hannah Brown for
assisting with nighttime surveys. This research was supported by an
award from the National Science Foundation (DEB 1145552), a
Boise State Provost’s Office Work Study Award, Boise State
University’s Department of Biological Sciences and Raptor
Research Center, and American Kestrel Adopt-a-box sponsors. We
also thank Ms. Judith King and Lynn and Jack Loacker for funding.
We thank Bosch and Matt Thomas of New/Era Sales, Inc. for
donating the KestrelCam. We also thank The Peregrine Fund’s
research library for help in obtaining literature, Delora Hilleary for
reviewing videos from the KestrelCam, and Amy Siedenstrang for
help in making figures.
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Editor-in-Chief: Keith A.Hobson
Subject Editor: Jean-Pierre L.Savard
