We investigated whether visual cues of a nest predator at the nest site prior to the completion of nest building would induce Eastern Bluebirds (Sialia sialis) to abandon their nesting attempt and switch to another box 10 m away. Upon detecting the onset of nest building in one of the paired boxes, we presented the birds with either a visual predator cue (a rubber black rat snake (Pantherophis obsoletus) or a visual control cue (a model Northern Cardinal (Cardinalis cardinalis) on the active nest box for 15 min and recorded the responses of the resident pair of bluebirds. We returned 1 week later to determine whether the presentation of the snake or cardinal resulted in no change, a switch to the other box, or abandonment of the box pair. Although the bluebirds seemed to respond more negatively to the model snake, we found no significant difference in their propensity to abandon the box pair (vs. remain in one of the paired boxes). We found a similar majority of bluebirds continued to nest in the same box (vs. abandon the effort) in both treatments. Even when we restricted our analysis to the box pair, we found no difference in the propensity to stay vs. switch. Although a larger proportion of bluebirds responded to the snake vs. cardinal model with mobbing, the difference did not reach significance. Bluebirds that mobbed the model were no more likely to abandon the box pair than those that did not mob the model. Although more mobbers abandoned their nest start than did non-mobbers, this difference was not significant. We conclude that the costs of finding a new nest cavity and the close proximity of the boxes provided limited the options of these secondary cavity nesters, even those that engaged in mobbing.
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Vol. 130 • No. 2