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Eastern Bluebirds Choose Nest Boxes Based on Box Orientation
Kristen J. Navara and Erin M. Anderson

Southeastern Naturalist, Volume 10, Issue 4 (2011): 713–720

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2011 SOUTHEASTERN NATURALIST 10(4):713–720 Eastern Bluebirds Choose Nest Boxes Based on Box Orientation 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. Introduction 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 - knavara@uga.edu. 714 Southeastern Naturalist Vol. 10, No. 4 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 herbaceous vegetation. Field-site Description 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%). Methods 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.). Results 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 of nest-box occupancy according to box orientation. Boxes facing northwest had a signifi cantly higher occupancy compared with all other directions (χ2 = 8.16, P = 0.017). A statistically significant difference is represented by an asterisk, and numbers in the triangles represent the ratio of occupied to total nest boxes. 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%, SW: 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 Discussion 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 immediate vicinity. 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 maximize occupancy. 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. Acknowledgments 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. Literature Cited Ardia, D.R., J.H. Pérez, and E.D. Clotfelter. 2006. Nest-box orientation affects internal temperature and nest-site selection by Tree Swallows. Journal of Field Ornithology 77:339–344. Conner, R.N., and C.S. Adkisson. 1974. Eastern Bluebirds nesting in clearcuts. Journal of Wildlife Management 38:934–935. Davis, W.H., P.J. Kalisz, and R.J. Wells. 1994. Eastern Bluebirds prefer boxes containing old nests. Journal of Field Ornithology 65:250–253. Dhondt, A.A., and T. Phillips. 2001. A question of preference: Nest-box orientation and success of cavity-nesting birds. In Birdscope. Cornell Lab of Ornithology. Available online at http://www.birds.cornell.edu/Publications/Birdscope/Spring2001/preference. html. Accessed 5 July 2011. Fitch, H.S. 1963. Natural history of the Black Rat Snake. Copeia 1963:649–658. Goodenough, A.E., A.G. Hart, and S.L. Elliot. 2008. Variation in offspring quality with cavity orientation in the Great Tit. Ethology Ecology and Evolution 20:375–389. Lumsden, H.G. 1986. Choice of nest boxes by Tree Swallows, Tachycineta bicolor, House Wrens, Troglodytes aedon, Eastern Bluebirds, Sialia sialis, and European Starlings, Sturnus vulgaris. Canadian Field-Naturalist 100:343–349. 720 Southeastern Naturalist Vol. 10, No. 4 Napper, K.M. 2005. Effects of multiple factors on nest-site selection and nesting success in Eastern Bluebirds (Sialia sialis). University of Akron, Akron, OH. 52 pp. National Oceanic and Atmospheric Administration (NOAA). 2001. Average mean temperature index by month: Climatology by state based on climate division data: 1971–2000. Available online at http://www.esrl.noaa.gov/psd/data/usclimate/tmp. state.19712000.climo. Accessed 5 July 2011. Parren, S.G. 1991. Evaluation of nest-box sites selected by Eastern Bluebirds, Tree Swallows, and House Wrens. Wildlife Society Bulletin 19:270–277. Pinkowski, B.C. 1976. Use of tree cavities by nesting Eastern Bluebirds. Journal of Wildlife Management 10:556–563. Pinkowski, B.C. 1977. Foraging behavior of the Eastern Bluebird. Wilson Bulletin 89:404–414. Pitts, D. 1977. Do Eastern Bluebirds and House Sparrows prefer nest boxes with white or black interiors? Bird-banding 48:75–76. Plissner, J.H., and P.A. Gowaty. 1995. Eastern Bluebirds are attracted to two-box sites. Wilson Bulletin 107:289–295. Radunzel, L.A., D.M. Muschitz, V.M. Bauldry, and P. Arcese. 1997. A long-term study of the breeding success of Eastern Bluebirds by year and cavity type. Journal of Field Ornithology 68:7–18. Sauer, J.R., and S. Droege. 1990. Recent population trends of the Eastern Bluebird. Wilson Bulletin 102:239–252. Weatherhead, P.J., and G. Blouin-Demers. 2004. Understanding avian nest predation: Why ornithologists should study snakes. Journal of Avian Biology 35:185–190. Zar, J.H. 1999. Biostatistical Analysis. Prentice Hall, Upper Saddle River, NJ. 960 pp. Zeleny, L. 1976. The Bluebird: How You Can Help Its Fight For Survival. Indiana University Press, Bloomington, IN. 192 pp.