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2011 SOUTHEASTERN NATURALIST 10(4):713–720
Eastern Bluebirds Choose Nest Boxes Based on Box
Kristen J. Navara1,* and Erin M. Anderson1
Abstract - Substantial declines in populations of Sialia sialis (Eastern Bluebird) have
prompted a large movement throughout North America to construct human-made nest
boxes to facilitate bluebird nesting. While large-scale habitat preferences of nesting
bluebirds have been well-established, the factors that influence nest-box preference
within relatively homogeneous habitats remain unclear. We tested the effects of vegetation
height and nest-box orientation, comparing northeast-, southeast-, southwest-, and
northwest-facing boxes, on nest-box occupancy in a population of Eastern Bluebirds in
Athens, GA. Vegetation height surrounding nest boxes influenced nest-box preferences.
Over 65% of boxes with little to no surrounding vegetation were occupied, as compared
with only 21% of boxes surrounded by high vegetation. Nest-box orientation also infl
uenced box selection. Over 68% of boxes facing northwest were occupied, compared
with less than 34% of boxes facing all other directions. These findings differ from those
in studies conducted at northern latitudes, suggesting that preference for box orientation
may vary with latitude.
Concentrated efforts to provide quality nest sites for Sialia sialis L. (Eastern
Bluebird) have been implemented throughout the United States in attempt
to reverse significant reductions in Eastern Bluebird populations that occurred
during the late 1970s (Sauer and Droege 1990). However, despite decades of
box additions and resulting opportunities for studying the nesting behaviors of
Eastern Bluebirds, the precise factors that influence nest-site selection in this species
remain unclear. Even within small areas of seemingly homogeneous habitat,
there can be considerable variation in nest-box occupancy throughout individual
nesting sites. A better understanding of factors that influence Eastern Bluebird
nest-box preference could help to maximize nest-box occupancy.
A range of box characteristics appear to influence nest-box selection and nesting
success. For example, more young are produced and survive when reared in
open-top boxes compared to standard box designs (Radunzel et al. 1997). Eastern
Bluebirds prefer to nest in boxes with 4.4-cm openings compared to 3.5-cm openings,
but prefer a smaller (10 x 10 cm) floor plan compared to a larger (15.3 x 15.3
cm) floor plan (Lumsden 1986). Internal color of the box can influence nest-site
selection, as Eastern Bluebirds prefer to nest in boxes with white compared to
black interiors (Pitts 1977). Researchers have also demonstrated a preference for
nest boxes constructed in close proximity to a second nest box, but facing in the
opposite direction (Plissner and Gowaty 1995), as well as for boxes containing
old nests (Davis et al. 1994).
1Department of Poultry Science, 203 Poultry Science Building, The University of Georgia,
Athens, GA 30602. *Corresponding author - email@example.com.
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Even when box characteristics are consistent, however, there still can be
variation in occupancy within field sites. Previous studies examining nesting
habits of Eastern Bluebirds have concentrated on indicators of small-scale habitat
preference. Eastern Bluebirds prefer to nest in open areas that are well away from
woodland edges (Parren 1991), and overgrown areas are generally avoided (Conner
and Adkisson 1974). The orientation of nest boxes also can be important. The
direction a box faces can influence the internal microenvironment during critical
breeding periods for cavity-nesting birds, due either to variable exposure to wind
and rain (Goodenough et al. 2008) or variation in internal temperatures resulting
from differential sun exposure (Ardia et al. 2006).
Slight variation in microenvironment can have significant impacts on
growing birds. For example, quality of Parus major L. (Great Tit) nestlings
was significantly lower when boxes were facing south-southwest compared
to all other directions (Goodenough et al. 2008). For this reason, some studies
have addressed whether Eastern Bluebirds prefer to nest in boxes with a specific
directional orientation; however, results of these studies are conflicting.
Nesting guides generally suggest that boxes face east, such that eggs and nestlings
benefit the most from the heat of the morning sun (reviewed in Napper
2005). Indeed, Dhondt and Phillips (2001) showed that more nestlings fledged
from boxes facing northeast as compared to northwest, southeast, and southwest,
in northern populations of Eastern Bluebirds. Pinkowski (1976) showed
that significantly more Eastern Bluebirds nested in boxes facing southeast in
his Michigan population. For an Ohio population, Napper (2005) also suggested
that more birds nested in boxes facing southeast, but this difference
was not statistically significant. Parren (1991), on the other hand, found no
directional preferences for nest boxes by Eastern Bluebirds in Vermont. Thus,
the orientation that is optimal for nesting Eastern Bluebirds remains unclear,
and may be site specific.
Here, we test the effects of nest-box orientation and height of vegetation
immediately surrounding the nest box (within 1 m2) on box occupancy by
nesting Eastern Bluebirds. We predicted a preference for boxes facing in an
easterly direction, similar to preferences shown in many previous studies,
and also that boxes with little to no vegetation would be preferred over boxes
where the immediate area surrounding the nest box contained taller woody or
We conducted our research on five field sites surrounding the University of
Georgia campus in Athens, GA (33.95°N, 83.32°W). Locations included two
athletic fields and a golf course used and maintained by the University of Georgia,
one site of agricultural research, and one local park maintained by Clarke
County, GA. We utilized areas that were considered preferable nesting habitats
for Eastern Bluebirds, consisting of mostly open fields with relatively little
brush. However, across sites, there was variation in landscape maintenance that
2011 K.J. Navara and E.M. Anderson 715
influenced the height of vegetation surrounding the individual nest boxes. Some
boxes were surrounded by very little vegetation (i.e., no vegetation greater than
15 cm in height = the maximum height following routine lawn maintenance).
Vegetation around other boxes was allowed to grow to 15 cm or higher (vegetation
allowed to grow around fences or poles due to difficulty of mowing). As a
result, some boxes were surrounded by little if any vegetation, while others were
surrounded by herbaceous plants that reached to the height of the nest box. None
of the boxes at the agricultural research site had vegetation greater than 15 cm,
maintained athletic fields ranged widely in levels of vegetation (percentages of
boxes with vegetation greater than 15 cm in height was 11% at one athletic site,
32% at the second, and 60% at the third). In the county park, a majority of boxes
had substantial surrounding vegetation (76%).
In April 2010, we constructed 83 nest boxes and distributed them among the
five field sites (ranging from 9–21 boxes per site depending on the amount of suitable
nesting habitat available on the field site). All field sites were within 6 km of
one another, and we placed all boxes within a 7-d period. There was no difference
in box occupancy between boxes placed early and those placed late in the week
(early: 45% occupancy, late: 33% occupancy, χ2 = 0.40, P = 0.53). We based the
design of our boxes on the newer Zeleny (1976) NABS nest box plans, 25.4 cm
high, 13.9 cm wide, and 17.78 cm deep, and with a side opening, and mounted
the boxes at a height of approximately 1.52 m. Upon placement, we oriented nest
boxes randomly, and we then recorded the direction which the box was facing
using a handheld compass pointing from the back to the front of the box. We
recorded direction numerically, from 1° to 360°. We then grouped boxes into the
following orientation categories: northeast (1–90°, n = 18), southeast (91–180°,
n = 21), southwest (181–270°, n = 25) or northwest (271–360°, n = 19). We chose
these orientations (rather than cardinal directions of due north, south, east, west)
by initially plotting the percentages of occupied boxes in 10-degree intervals on
a circular graph. A clear visual pattern of occupancy percentages emerged, as can
be seen in Figure 1.
We characterized the vegetation immediately surrounding each box by measuring
the tallest and shortest plant stems located within 1 m of the box. All
measurements were taken once at the end of the nesting season. We categorized
boxes surrounded by at least 1 m2 of vegetation exceeding 90% coverage and 15
cm in height as “vegetation” and boxes surrounded by very short vegetation
(<15 cm high) as “no vegetation”. We chose a height of 15 cm because this was
the highest vegetation that occurred in areas that were routinely mowed. A total
of 43 boxes were assigned to the “vegetation” category while 40 boxes were assigned
to the “no vegetation” category.
Immediately after placement, we checked nest boxes daily for signs of nest
building and egg laying for approximately 8 weeks from 15 April through 15
June 2010. We defined box occupancy as completion of a full nest in the nest box,
716 Southeastern Naturalist Vol. 10, No. 4
regardless of the number of times a pair nested in that box throughout the season.
We calculated the percentage of box occupancy in a given direction or category
using the number of occupied boxes compared to total boxes available in a given
orientation or category (Fig. 1)
We calculated mean angles of orientation from the recorded compass bearings
for all boxes placed, as well as for both occupied and unoccupied boxes
using circular statistics (Zar 1999). These statistical methods allow us to maintain
the circular nature of the data in the analyses. We calculated the differences
in the mean angles towards which occupied and unoccupied boxes faced using
a Watson-Williams test for two samples (Zar 1999). Next, we used logistic regression
analysis to analyze box occupancy in relation to vegetation height (the
highest vegetation recorded within 1 m2 of each nest box) and box orientation.
We compared box occupation for northwest-facing boxes versus boxes in all
other directions (grouped together), and categorized the response variable as “1”
for occupied and “0” for unoccupied. We also analyzed whether the presence or
absence of vegetation influenced box occupancy (categorized as “1” for occupied
and “0” for unoccupied) using a logistic regression, and used individual logistic
regression analyses with the same categorization to examine whether box occupancy
differed between individual orientations (e.g., southwest versus northwest,
southeast versus northeast, etc.).
A total of 35 nest boxes, or 41%, was occupied by nesting Eastern Bluebirds
in our field sites. In all but 3 boxes in which full nests were built, a full clutch of
eggs was laid. The mean angle towards which occupied nest boxes faced differed
significantly from the mean angle towards which unoccupied nest boxes faced
Figure 1. Visual depiction
to box orientation.
northwest had a signifi
cantly higher occupancy
with all other directions
(χ2 = 8.16,
P = 0.017). A statistically
represented by an
asterisk, and numbers
in the triangles
represent the ratio
of occupied to total
2011 K.J. Navara and E.M. Anderson 717
(Watson-Williams test: 306.5° for occupied, 60.03° for unoccupied, F = 19.43,
P < 0.001). The mean angle of the occupied boxes differed significantly from the
mean angle of all nest boxes overall (Watson-Williams test: 306.5° for occupied,
24.9° for all nestboxes, F = 8.94, P = 0.003), while the mean angle of unoccupied
boxes did not (Watson-Williams test: 60.03° for unoccupied, 24.9° for all
nestboxes, F = 1.27, P = 0.262), suggesting that there was preferential occupancy
based on the directional orientation of the nest boxes. Eastern Bluebirds in our
population occupied nest boxes facing northwest more than any other direction.
More than 68% of boxes facing northwest were occupied, compared with 34%
facing all other directions (NW versus all other directions combined, χ2 = 8.16,
P = 0.017; Fig. 1). Occupancy in any of the other three directions was equally
likely (P > 0.85 for comparisons among all other directions, NE: 36%, SE: 33%,
Nest box occupancy was also affected by the presence or absence of vegetation
surrounding the nest box. Boxes without vegetation had significantly higher
occupancy compared to boxes with vegetation (no vegetation: 65% occupancy,
vegetation: 21% occupancy, χ2 = 16.50, P < 0.0001, Fig. 2). Logistic regression
analysis showed a significant relationship between the height of surrounding
vegetation and occupancy (χ2 = 5.75, P =0.017). When considering box orientation
and the presence of vegetation together, both vegetation height and box
orientation significantly influenced the occupancy percentage (logistic regression,
overall χ2 = 12.81, P = 0.005, orientation: P = 0.043, vegetation height: P =
0.027). Boxes facing northwest and containing no surrounding vegetation were
occupied more than all other combinations together. In fact, 11 of 12 (92%) boxes
that were assigned both to the “no vegetation” category and faced northwest were
occupied. In contrast, all other potential combinations had an average occupancy
of only 51%.
Figure 2. Nest-box occupancy
percentages for Eastern Bluebirds
in relation to presence or
absence of vegetation within
1 m2 of the nest box. Boxes
surrounded by at least 1 m2 of
vegetation at least 15 cm in
height were labeled as “vegetation”,
while all others were
labeled as “no vegetation”.
Boxes in the “vegetation” category
had a significantly lower
occupancy percentage compared
to boxes in the “no vegetation”
category (χ2 = 16.50,
P < 0.0001). Numbers inside
the bars indicate the ratio of
occupied to total nest boxes.
718 Southeastern Naturalist Vol. 10, No. 4
We found that both nest box orientation and the presence of vegetation immediately
surrounding the next box are significant predictors of nest box occupancy.
Eastern Bluebirds preferred nests that were facing in a northwest direction, and
that had little to no vegetation within 1 m of the box. The finding that Eastern
Bluebirds prefer boxes surrounded by little to no vegetation is not surprising; previous
studies showed a preference for boxes surrounded by fewer woody stems
(Parren 1991) and those located in open areas that were not densely overgrown
(Conner and Adkisson 1974). Areas with sparse vegetation may be advantageous
because dropping is the primary feeding mode exhibited by Eastern Bluebirds.
(Dropping is a feeding method whereby a bird identifies prey from a perch and
swoops to the ground to snatch the prey with the bill.) Sparse ground cover is required
for this method of foraging. Further, Eastern Bluebirds prefer to forage in
mowed areas rather than undisturbed areas containing tall vegetation (Pinkowski
1977). Nesting in a box that is not surrounded by vegetation may present an advantage
by increasing foraging efficiency. However, given that bluebirds forage
over areas spanning tens or even hundreds of meters from their nest sites, it seems
unlikely that they would choose boxes solely based on the ability to forage in the
Alternatively, predation risk may play a bigger role in the use of vegetation
for nest box selection. Snakes have been characterized as the most important
nest predators of New World passerine birds (Weatherhead and Blouin-Demers
2004) and account for a large percentage of nest predation in our study area
(E. Anderson, University of Georgia, Athens, GA, unpubl. data). Elaphe obsoleta
obsoleta (Say in James) (Rat Snakes), major predators of Eastern Bluebird eggs
and nestlings, often are found in tall herbaceous vegetation and shrub-dominated
fields (Fitch 1963). Compared to mowed areas, tall grasses surrounding nest
boxes typically make better habitat for snakes and make them less visible to nesting
Eastern Bluebirds. It is important to note that even in the best nesting habitat,
vegetation immediately surrounding the box can vary due to different mowing
practices and the inability to use large mowers at close proximity to poles or
fences on which nest boxes are mounted. Our data suggest that it is important
to keep areas immediately surrounding nest boxes clear of vegetation, in order to
Our finding that Eastern Bluebirds in Georgia prefer to nest in northwest-facing
boxes was surprising given that Eastern Bluebirds in Michigan showed preferences
for southeast-facing boxes (Pinkowski 1976). Southeast-facing boxes are
also recommended by Eastern Bluebird guides (reviewed in Napper 2005). It is
possible that the optimal direction of nest boxes varies with location, particularly
with latitude. Indeed, Dhondt and Phillips (2001) showed that southeast boxes
fledged more young, but that this was only true in populations at northern latitudes,
while the effect disappeared at southern latitudes. Ardia et al. (2006) showed that
nest box temperatures in south-facing boxes in Massachusetts were warmer. They
found that Tachycineta bicolor (Vieillot) (Tree Swallows) preferred these boxes
2011 K.J. Navara and E.M. Anderson 719
earlier in the season, but that both the temperature relationship and preference disappeared
as the season progressed and ambient temperatures increased. Average
temperatures in Georgia from April through June are approximately 23.6 °C (74.4
°F), as compared to average temperatures of 11.9 °C (53.5 °F) in Michigan and
13.1 °C (55.5°F) in Massachusetts (NOAA 2001). Perhaps at southern latitudes,
direct morning sun is not necessary to reach an optimal box temperature, even early
in the season. Additionally, temperatures in Athens, GA routinely exceed 26.7 °C
(80.0 °F) during the months of April through June. Direct sun during this period
may raise egg temperatures above the optimal range for incubation and could heatstress
nestlings during the heat of the day.
Nest boxes are constructed and placed for breeding Eastern Bluebirds throughout
the US and Canada. While it is recommended that boxes are placed in areas
that are relatively free of vegetation, we show that the maintenance of the vegetation
immediately surrounding the box influences occupancy even when boxes are
placed in an open field. In addition, current guides assume that optimal nest-box
characteristics are similar for Eastern Bluebirds at all locations; however, the
fact that Eastern Bluebirds located in Georgia selected boxes facing opposite
orientations compared to birds at northern latitudes indicates that guidelines for
the placement of bluebird nest boxes should depend on the geographic location
of the boxes. Further work should be done to examine the optimal direction of
bluebird nest boxes along a latitudinal gradient.
We thank P. Vaden, S. Jackson, and K. Collins for field assistance. We also thank
the Animal Dairy Sciences Department at the University of Georgia, the University of
Georgia Golf Course, Athens Clarke County Leisure Services, and Southeast Clarke Park
for allowing access to field sites. This work was completed with approval from the UGA
Institutional Animal Care and Use Committee, PRN#A2010 4-060.
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