Nesting Cavity Preference in Carnegiea gigantea
Dr. Kathy Winnett-Murray and Dr. K. Greg Murray
Saguaros (Carnegiea gigantea) are slow growth, softwood cacti found only within the Sonoran Desert. Because of their pulpy interior and protective spines, they make ideal nesting sites for many bird species. Gila Woodpeckers (Melanerpes uropygialis) and Gilded Flickers (Colaptes chrysoides) are the only two species that create nesting holes, but several other bird species also inhabit them. I hypothesized that nest height would be higher on taller Saguaros, since at least part of the advantage of nesting in Saguaros lies in avoiding ground-dwelling predators. Assuming that most species nesting in Saguaros rear chicks in the summer months, I hypothesized that nest entrances would face north in order to ensure a cooler nest microclimate. In order to test these hypotheses, I measured elevation, nest height, lowest arm height, total cactus height, number of arms, number of nests, and nest entrance compass direction on 71 Saguaros near Tucson, in southern Arizona. Results indicated that average nest height increased with cactus height, and that nest and cactus height were proportionally related. Sample sizes at different elevations were not large enough to test the correlation between elevation and nest height or direction. There was also no significant preference for nest entrance orientation. Other research suggests that there is a non-random selection of nests at Organ Pipe (north-facing) during summer (rearing) months (Inouye and Huntly et al. 1980). However, a broader study that was not focused on rearing season found that nests were located at random orientations and that birds preferred large saguaros (Kerpez and Smith et al. 1990). Bird preference in nest orientation is likely based on the rearing season and current weather conditions. Both my results and previous studies suggest a preference for higher nest locations in mature saguaros. Therefore, maintaining a population of mature saguaros is critical to nesting bird reproduction in the Sonoran Desert.
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