by André A. Dhondt and Tina Phillips
Does the orientation of
box affect the breeding success of cavity-nesting
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.
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.
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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