How Bats Find New Homes

When planning where to mount your bat house, remember that bats, like us, are creatures of habit. Most North American bats that don’t live in caves search for nursery roost sites by investigating traditional locations: along the trunks of leafless snags, at the broken ends of dead branches high above ground, or along cliff faces. Additionally, a large proportion now have become accustomed to searching the eaves of buildings.
It seems likely, then, that artificial roosts placed in such locations would be the first to be found. We could also hypothesize that, given their apparent preference for height, bats may be less likely to find houses mounted on the lower trunks of live trees, and more likely to find houses high on poles.

To test these hypotheses, we checked 1993 Research Associate reports to see if houses in differing locations also differed in the amount of time required for bats to find them. We compared houses mounted on the sides of buildings, poles, and trees, selecting only those houses that were eventually occupied and that received at least four hours of daily sun. We also omitted houses that were located near where bats had been excluded from buildings and houses that had not been regularly checked for occupancy. The sample was thus restricted to eliminate most biases, especially those that might be introduced if poorly designed houses were included, or if some houses profited from local evictions.

The resulting sample size was 28: 13 houses on buildings, 5 on poles, and 10 on tree trunks. It took bats an average of 71 days to find the houses on buildings and 73 to find those on poles. But it took an average of 255 days to find the houses mounted on tree trunks, meaning that success didn’t come until the second year. Although this sample was small and needs to be corroborated by additional data, it is consistent with the suggestion that the location of houses relative to bat search behavior is probably important.