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A Question of Preference
by André A. Dhondt and Tina Phillips

Does the orientation of a nest box affect the breeding success of cavity-nesting birds?

Cavity-nesting birds clearly choose some nest boxes more often than others. Researchers have carried out numerous experiments to determine whether birds prefer to nest in boxes with round or oval holes, flat or slanted roofs (like the Peterson boxes), and with large or small dimensions. Another factor is the direction in which the box faces. The orientation of the box may determine the microhabit of the nest box and thereby influence nest-site choices and, utimately, the birds' breeding success. In her book Bluebird TrailsA Guide to Success, Dorene Scriven notes that bluebirds choose, in decreasing order of preference, east-, north-, south-, and west-facing boxes, perhaps to avoid afternoon sun in the entrance hole or the prevailing direction of storms.

Questions of nest-box preference have two components. First, if we could identify the ideal box, the ideal site, and the ideal orientation, would more nest boxes be occupied and successful? The second question is simply, why do the birds have these preferences? If birds have a preference, there must be a reason. A preference would have come about through natural selection, which implies that birds breeding in preferred boxes raise more young and thus pass more of their genetic material to future generations.

 As part of the Lab of Ornithology's study of cavity-nesting birds, The Birdhouse Network (TBN) requested information on the orientation of nest boxes from participants. Although we did not ask participants to set up groups of nest boxes facing different directions to address the question of preference directly, our data do permit us to explore whether nest-box orientation has any effect on breeding success.

TBN participants reported 4,208 Eastern Bluebird breeding attempts in 1998, 1999, and 2000. Nestlings were seen and the nest-box orientation reported for 2,935 of these breeding attempts. Although we could only use a subset of the reported breeding attempts in the analysis, the sample was still quite large. In this report we explore whether breeding success varies with nest-box orientation and speculate on why the direction a box faces may be a factor.

For the 2,935 Eastern Bluebirds nests, the mean clutch size was 4.46 eggs, the mean number of young fledged was 3.47, and the mean fledging success (number of young fledged divided by the number of eggs laid) was 0.78. In Table 1, we report these data by orientation. Where-as the mean clutch size only varied between 4.51 (northeast) and 4.41 (northwest), a difference of only 2.2 percent, the variation in the mean number of young fledged was 13.7 percent, with 3.8 young fledging on average from boxes facing northeast and only 3.28 young fledging from boxes facing west.

According to Birdhouse Network data, Eastern Bluebird pairs fledge more young on average from east-facing nest boxes.

photo by Marie Read
Overall, it is clear that nest boxes facing in easterly directions fledged on average more young than boxes facing in other directions. To test the extent to which these differences are statistically significant, we compared adjacent orientations two by two (that's how they are ranked in the table). We also included latitude and egg-laying date as variables in the analysis.

None of the clutch-size comparisons was significant. This means that clutch size does not vary with orientation. For nesting success and mean number of fledglings, however, two of the eight comparisons were significant. These were the comparisons between boxes facing northeast and the adjacent directions north and east. (In other words, north by northeast and east by northeast). Comparing all eight orientations in a single analysis confirms that statistically significant differences exist between the mean number of young fledged in nest boxes facing different directions. This is not the result of birds laying clutches of different sizes in the different boxes; rather, it is caused by varying proportions of the eggs producing fledglings.

Table 1

TBN Eastern Bluebird results (1998­2000 where n=2,935). Mean number of eggs laid (black) and mean number of young fledged (red) in nest boxes facing in different directions. Clutch size does not vary with orientation, whereas boxes facing northeast fledge more young. This graph does not include the lay-date interaction.

Now that we have identified an ideal orientation, we can speculate on why these differences exist. It could be that birds lay earlier in more successful boxes or that older and more experienced females use these boxes. Or it could be that boxes facing different directions differ in microclimate. Our data do not allow us to explore many of these hypotheses, but some comments are possible. The fact that mean clutch size does not vary with orientation weakens the egg-laying-date hypothesis. Simple calculation of lay dates (Table 1) shows that eggs are laid earliest in boxes that face northwest, leading us to reject that hypothesis completely.

What about microclimate? Some researchers suggest that boxes facing east warm up earlier in the morning, giving a small temperature advantage to birds nesting in boxes facing that direction. And, as we suggested, bluebirds may choose east-facing boxes to avoid the hot afternoon sun. Because we assume that differences in breeding success are the underlying cause of a preference, we must ask whether differences in nesting success between east- and west-facing boxes vary with latitude. Note that the two hypotheses yield different predictions. If west-facing boxes overheat, the difference in nesting success between east- and west-facing boxes should be larger in the south than the north (see inset B in Figure 2). If east-facing boxes boost survival rates because they warm up earlier in the morning, that advantage should decrease as we move south. We repeated our analysis by adding the interaction between latitude and orientation in the statistical model (see Figure 2; for explanations about interaction terms in statistical analyses see the Spring 2000 issue of Birdscope). Much to our surprise, we did find such an effect. The mean number of nestlings increases with latitude, but a difference exists in the rate of increase between west- and east-facing boxes. In statistical terms, there is a significant latitude-by-orientation interaction. As a result of that difference, orientation of the nest box has no discernable effect on nesting success at southern latitudes, but it does farther north. Our data, therefore, suggest there is a benefit to breeding in east-facing nest boxes at northern latitudes, where night temperatures tend to be colder. No benefit, however, could be detected farther south. These data tend to support the "early-morning-warming-up hypothesis."

This analysis not only supports the theory that the orientation of nest boxes affects Eastern Bluebird nesting success in the North, it actually yields a hypothesis as to why they prefer to raise young in east-facing boxes. Many studies on nest-box orientation have been conducted, but most have a relatively small sample size in a geographically isolated region. This highlights the beauty and power of geospatially driven citizen-science data: we not only have a large sample size, we have latitude and longitude information spanning the continent.

A word of caution, however; orientation is only one factor that influences the nest-box selection and breeding success of Eastern Bluebirds. Many other factors, such as vegetation, climate, and predation, also have an effect. So we should not expect to see drastic changes in breeding success because we orient our nest boxes to the northeast. In addition, to cite Dorene Scriven again, "bluebirds will use any direction." Also, this analysis was conducted only on Eastern Bluebirds, a species for which we have abundant data. We cannot infer from this study that the breeding success of all cavity nesters will be boosted significantly by facing their nest boxes toward the east. But as we collect more data, we hope to conduct similar analyses for other cavity-nesting species.

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