The diet of the insectivorous Hawaiian hoary bat (Lasiurus cinereus semotus) in an open and a cluttered habitat

The Hawaiian hoary bat, Lasiurus cinereus semotus, foraged in both an open and a cluttered habitat. In the cluttered habitat it used slow, manoeuvring flight. In the open habitat it used rapid, less manoeuvring flight with echolocation calls of higher frequency than in the cluttered habitat. The combination of faster flight with higher frequency echolocation calls means that it should detect insect prey too late to manoeuvre for capture unless it preyed on relatively larger insects in the open habitat. The mean size of dietary items should therefore be greater in the open habitat than in the cluttered habitat. There should also be a greater preponderance of larger insects in the open than in the cluttered habitat to ensure that the use of rapid flight with higher frequency echolocation calls is efficient. I tested these predictions by comparing the diets and insect fauna in one open site with those in one closed site on the island of Hawai'i. As predicted, the dietary items were larger (t = 60.9, df = 259, p < 0.00001) in the open habitat (length = 18.9 ± 1.3 mm (mean ± SD); range 14-24 mm) than in the cluttered habitat (6.5 ± 1.6 mm; range 4-15 mm). In the cluttered habitat 86% of the insects fell into the three smallest length categories, while 52% of the insects in the open habitat fell into the two largest length categories. The preponderance of larger insects in the open habitat, with the consequent increase in detection range, enabled the bats to increase their flight speed despite their use of higher frequency echolocation calls. Rapid flight increases the rate at which insect prey are encountered and improves the foraging efficiency of bats. This provides evidence that bats are capable of altering their foraging strategy in response to prey characteristics and not just to the physical environment.