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Bat Movements and Foraging

Movements To and Among Roosts

We determined movements of bats from foraging areas to roosts, and between roosts using miniature radiotransmitters. We also documented movements between selected colony sites with PIT tag readers installed at roost entrances. Movements between roosts are of potential importance to the transmission of rabies. If there is considerable mixing of bats among colony sites throughout the city, the potential for spread of rabies virus may be higher.

In general, bats were faithful to colonies in series of nearby buildings but did not mix freely among colonies throughout the city. Radiotagged bats and bats with PIT tags shifted between roosts in adjacent or nearby buildings frequently, and also shifted daily locations within separate parts of larger buildings.

This "roost-switching" is known to be common in bats, but factors associated with such movements are poorly known. Two common explanations are that bats leave roosts due to ectoparasite loads or due to microclimate changes. We evaluated these alternatives by calculating movement transition probabilities and evaluating AIC-based models that incorporated covariates of ectoparasite loads (Steatonyssus mite counts on individual bats) and daily temperature changes. Although ectoparasites were common on bats that roosted in buildings , we found that bats moved between roosting locations based primarily on daily temperature shifts.

Bat caught in mist net for tagging Harp trap suspended near roost entrance at emergence Capturing bats for tagging using a funnel trap Miniature radiotransmitter

Foraging Behavior

We radio tracked big brown bats from different roosts in Fort Collins during summer 2004 to determine what areas were used for foraging. Bats from widely scattered buildings favored the riparian areas along the river corridor. This suggests another possibility for rabies transmission between bats from different colonies if bats become aggressive in foraging areas and biting by rabid bats occurs on the wing.

Figure 1. Roost sites and main foraging area of 16 big brown bats (Eptesicus fuscus) followed during the summer of 2004 in Fort Collins. Yellow symbols represent sites where bats roosted during the day. More than 80 % of the radio signals detected from foraging bats originated from the area circumscribed by the red polygon. Natural Areas are at the core of this polygon.

USGS biologist Ernest Valdez analyzed dietary items in fecal pellets collected weekly at roosts of big brown bats in buildings in Fort Collins. Preliminary results indicate that these bats shift from a diet that consisted largely of moths (Lepidoptera) and flies (Diptera) in mid-May to a variety of hard-bodied insects including seed bugs (Lygaeidae), weevils (Curculionidae), and ground beetles (Carabidae) in mid-July. These hard-bodied prey items were then replaced by soft-bodied prey items in September. Weevils include agricultural pests likely taken over alfalfa fields on the outskirts of the city.