Nest Site Selection in the Eastern Bluebird:
The Role of Parasite Avoidance

Anne Ford, Katie Lannon, Jasmina Radeva, Ned Robinson, Ellen Rockwell, Kelly Westbrook
Dr. Mark Stanback (Faculty Mentor)




Nest site limitation often forces secondary cavity nesters such as eastern bluebirds (Sialia sialis) to reuse cavities in which nest ectoparasites are abundant. By presenting bluebirds with different pairwise choices of boxes in which to build their second nest of the season, we assessed the relative strength of parasite avoidance in our population. Although bluebirds preferred to reuse successful cavities, their aversion to soiled nests was even stronger. However, additional choice experiments involving boxes of a less preferred type showed that parasite avoidance is relative to the quality of alternate cavities: bluebirds willingly assumed the costs of parasitism in order to avoid an undesirable box type.





Because of the constant threat of nest predation, safe sites for breeding are highly prized by cavity nesting birds. Indeed, successful cavities are often reused repeatedly, even among species that can excavate their own cavities. However, successful nest sites also harbor substantial populations of ectoparasites (fleas, mites, lice) that are known to have detrimental effects on the growth and health of nestlings. Multiple-brooded secondary cavity nesters could thus minimize parasite loads on subsequent broods by avoiding recently used cavities. By presenting bluebirds with different pairwise choices of boxes in which to build their second nest of the season, we addressed the following questions: 1) Do bluebirds avoid soiled nests? 2) Do bluebirds prefer successful cavities? and 3) How does the quality of alternative cavities affect these decisions?





o 200 pairs of bluebird boxes were erected near Davidson, NC. Half of the pairs consisted of one Schwegler® "woodcrete" (cement) box and one wooden box; half consisted of two cement boxes.


o Paired boxes were pole-mounted 1 m apart. All boxes were 1.5 m above ground level and were cleaned prior to the start of the breeding season.


o After the first nesting, successful boxes were randomly assigned to two treatments: control (old nest left in) and experimental (old nest removed). Subsequent nesting choices were then recorded.



Each bulleted piont links to a respective graphic.


o Parasite avoidance is dependent on the quality of alternate nest sites.


o Bluebirds avoid soiled nests (when an alternate box of equal quality is available).


o Bluebirds prefer to reuse successful cavities (when soiled nests are removed).


o The aversion to low-quality nest sites (wood boxes) is stronger than the aversion to soiled nests.





Thanks to Janet Beamer, Ela Burton, Robert Dickerson, and Dawn Houseton (our Hughes/LOL research interns) for their assistance in the field.