Bluebirds Prefer Successful Cavities - If They Are Clean

[Evelyn Cooper]

In recent decades, a contentious discussion has taken place in the literature as a result of two opposing assumptions about nest-site reuse—that birds either prefer or avoid used cavities.

Birds may prefer to reuse successful cavities either because construction of a new nest may constitute a significant time and energy cost, because successful cavities are more valuable than untested sites, or simply because suitable nest cavities are rare. Indeed, a variety of avian species have been shown to exhibit a preference for soiled nests or at least lack of aversion to them.

Conversely, birds may avoid used nests due to the ectoparasites they contain. Both observational and experimental research has demonstrated that nest ectoparasites can reduce reproductive success. Not surprisingly, some birds have been shown sensitive to costs associated with parasites. Some species have been shown to discriminate between high and low infestation levels in used nests and choose accordingly.

The results suggest that secondary cavity nesters raising more than one brood per season should take steps to minimize parasitism costs associated with being multibrooded. To assess relative importance of nest success versus presence of soiled nests in the nest-site reuse decisions of Eastern Bluebirds, we performed a controlled experiment addressing those two conflicting variables simultaneously.

We manipulated nest box choices in Eastern Bluebirds to assess first whether the presence of a previously used (and presumably parasite-ridden) nest cavity increases or decreases the likelihood of within-season nest-box reuse. Second, we wanted to determine whether birds prefer previously successful cavities.

One hundred pairs of identical bluebird nest boxes were erected in suitable habitat near Davidson, Meckenburg County, North Carolina. The box pairs consisted of two Schwegler woodcrete boxes. Woodcrete is a mixture of sawdust and cement. All box pairs were pole-mounted four feet apart, 60 inches above ground level, and both boxes within each pair faced the same direction.
Between breeding seasons every box was cleaned. Consequently, at the beginning of each breeding season, bluebird pairs at a particular location were choosing between two identical clean boxes for their first nesting. We considered a nest box chosen when at least one egg was laid in it and a nesting successful if it fledged at least one chick. We omitted renests following failures from our analyses because bluebirds and other species are more likely to change breeding sites if the previous attempt fails.

After the first nesting, we randomly assigned box pairs containing a successful box to one of two treatments. For half of those box pairs, we removed all old nest material (and presumably most of the active parasites) from the used box within one week of the first brood’s fledging. Those experimental boxes are hereafter referred to as cleaned. The clean unused boxes in those experimental pairs are hereinafter referred to as unused. For control box pairs, the boxes were visited, but the old nests were not removed. These boxes are hereinafter referred to as soiled and the alternative boxes, unused. Subsequent nesting choices in both experimental and control pairs were then recorded.

When adults were forced to chose between a soiled but successful nest box and unused nest site of equal quality, 71 percent of bluebirds chose to move to the unused box (of 45 pairs, 32 pairs switched to the unused box; 13 reused the soiled nest).

Thus given a choice between a soiled and an unused box, bluebirds chose the unused but parasite-free cavity significantly more often. Together, these results suggest three things:
• Bluebirds recognize a cost of within-season nest reuse and are willing to switch nest sites to minimize parasitism.
• Bluebirds prefer successful cavities, but only if they are clean.
• In our population, in which cost of nest switching was minimized, the aversion to parasites was stronger than the preference for successful cavities.

Under natural conditions, renesting bluebirds have limited options. Alternate cavities may be scarce, distant, defended, suboptimal, of unknown quality, or themselves soiled. If ectoparasitism costs are typically less than the costs of within-season nest-site changes, one might expect nest-site-limited species to reused successful nest sites, regardless of their cleanliness. Such a rule of thumb is not apparent in our population.

If costs of parasitism typically outweigh costs of nest-site switching, one might expect bluebirds to avoid recently used cavities, regardless of their cleanliness. Again, we find no evidence of such a rule of thumb. Instead, bluebirds made situation dependent assessments. Given a choice between a soiled and an unused box, bluebirds were very willing to switch to an “untested”, but parasite-free cavity located in the immediate vicinity. Presented with a cleaned successful box and an identical unused one, bluebirds opted to reuse the former.

Faced with a novel situation, specifically a clean successful cavity, bluebirds responded apparently optimally. However, the generality of our results may be limited to within-season nest-site reuse in secondary cavity nesters. Despite recent interest in effects of parasites, little effort has been made in the literature to distinguish within-verses between-season nest-site reuse. In fact, with the exception of Gowaty and Plissner (1997), most published studies of nest-site reuse focus on between-season patterns. With respect to ectoparasites, those two types of nest reuse are very different.

First, within a season, there is a much greater probability that both members of the pair are present and cognizant of the parasite loads within a particular successful nest cavity. Second, the parasitic species that take advantage of sequential nests within a season often differ from those that overwinter in nest cavities. That may explain why our results differ from those of Davis et al (1994) in which Eastern Bluebirds breeding in Kentucky showed a significant preference for boxes containing successful nests from the prior year.

Perhaps the number and variety of ectoparasites that overwinter in bluebird nests is low enough that the success-signaling function of nests from the previous year out-weighs the parasitism costs of their reuse.

By providing all bluebird pairs with an alternative nest site in the immediate vicinity (four feet away), our study controlled for the potentially confounding variables of nest-site quality and availability. Our results indicate that bluebirds operate under two conflicting rules of thumb (“reuse successful cavities: and ”avoid soiled cavities”) These in turn are the basis of the two conflicting hypotheses found in the literature concerning nest-site reuse. In our study, aversion to parasites out-weighed preference for successful cavities.

However, it would be inappropriate to conclude that avoidance of parasites will always outweigh the preference for successful cavities. We purposefully minimized cost of nest-site switching by providing two boxes side by side. Gowaty and Plissner’s (1997) data clearly demonstrate that by altering the cost of nest-site switching (breeding dispersal), one can later the nest-site choice eventually made by the birds. Indeed, if the quality of alternate cavities is low enough, one would expect birds to preferentially reuse soiled cavities. Nest-site preferences are thus best considered to relative rather than absolute.

By: MarkT. Stanback and Anne A. Ford
Journal of the North Americal Bluebird Society
Spring 2002, Vol. 24, No. 2