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Roosting Behaviors of the Townsend’s Big-eared Bat (Corynorhinus townsendii) in Redwood Basal Hollows on the California Central Coast

Marcus Richardson

University of California, Santa Cruz

4/4 intro 2/2 citations

Townsend’s big-eared bat (Corynorhinus townsendii) is a species of special concern due to its low tolerance of human disturbance, and its restricting roosting behaviors. C. townsendii is mainly a cave obligate species, but due to anthropogenic factors affecting their roosts they have also been found roosting in cave analogs, like abandoned buildings, under bridges and tree basal hallows (Sherwin1 et al. 2000). On the central coast of California, there have been reports of C. townsendii, along with other bat species, roosting in redwood (Sequoia sempervirens) basal hollows, which are formed by a combination of fire scarring and healing (Mazurek 2004). This was shown by use of guano traps placed in the redwood hollows (Gellman and Zielinski 1996; Zielinski and Gellman 1999) and by radio-tracking (Fellers and Pierson 2002).

These hollows share similar roosting properties to cave hibernacula, in their ability to trap heat in pockets where the bats roost creating an ideal microclimate (Mazurek 2004). Their displacement from caves due to human disturbance may be the cause for their observed roosting behaviors. Previous studies have shown that human disturbance can cause significant energy and weight loss to bats hibernating and roosting in caves (Johnson 1998; Speakman 1991; Thomas 1995). Bats in caves with high human visitation were shown to have higher percent mean weight loss, than bats in caves with low levels of human visitation (Johnson 1998). Furthermore nontactile human disturbance had a significantly lower effect on energy expenditure than tactile human disturbance (Speakman 1991), yet nontactile human disturbance had a great enough effect on hibernating bats to still be significant (Thomas 1995). Due the effect human disturbance has on the energy expenditure on hibernating or roosting bats undergoing torpor, the problem becomes a cost-benefit analysis. C. townsendii has been observed to prefer cave roost sites (Sherwin1 et al. 2000), yet to what cost is it still beneficial to roost in caves? Fig.1. is a representation of the projected relationship between the amount of human disturbance to the cost of roosting in a cave.

4Redwood basal hollows are also a limited resource due to deforestation of old growth forests (Mazurek 2004). Basal hollows can be used in two ways by bats, either as day roosts or temporary night roosts. Bats use temporary night roosts for resting, digestion, etc., but they are only used for a short time during their nightly foraging (Lacki et al. 2007). In order for displacement by human disturbance in caves to be the factor for C. townsendii’s roosting behaviors, basal hollows would have to be used as day roosts and a certain amount of fidelity to these sites must be present to substantiate that claim. Fidelity is an important factor to determining whether the hollows are being used as a cave alternative/replacement, rather than a short circumstantial visit (Sherwin2 et al. 2000).

The goal of this study is to empirically support that human disturbance in caves is one of the main factors causing C. townsendii’s basal hollow roosting behaviors. This will be done by both observing the level of human disturbance in local caves and the use of redwood basal hollows by C. townsendii, to determine if they are mainly used as day roosts or temporary night roost, along with the amount of fidelity colonies have to a given site. Some generalities of the pattern are: (1) that the pattern observed on the central coast of California is representative of other habitats where both C. townsendii occurs and human disturbance in caves is present; and furthermore (2) roosting behaviors in cave analogs by C. townsendii has been observed in places were either/both caves are disturbed or limited, much like the characteristics seen on California’s central coast.

Based on these assumptions I hypothesize in general that: (1) human disturbance in caves displaces C. townsendii causing them to find new roosts elsewhere, such as redwood basal hollows on California’s central coast and (2) due to displacement, basal hollows would be used as day roosts, like caves. Corresponding to these hypotheses I specifically hypothesize that: (1a) human disturbance in caves occurs on a meaningful and regular basis; (2a) sites used as day roosts would yield a consistent and large amount of guano, much like cave roosts; (2b) day roosts would show high levels of activity in two peaks during the night, the first at dusk leaving the hollow and the second at the time interval approaching dawn; and (2c) fidelity should be defined as specific day roost sites which are used on a regular basis, whether in rotation with other sites in the area or on a nightly basis.

Materials and Methods

-Species description 4/4

C. townsendii is a member of the Vespertilionidae family or evening bats. It is an insectivorous bat catching prey during flight by use of its uropatagium, the skin membrane located between the legs. It is known for it’s large ears which are on average 30 to 39mm compared to it’s body which is on average 90 to 133mm. It’s main distribution is mainly in western North America from British Colombia, Canada to Oaxaca, Mexico, and from the western coast to the middle of the United States of America. On the central coast of California are seen in forested areas, which are mainly composed of redwood. As stated before they are cave roost/analog obligates. They produce a single young per year, which are able to fly at three weeks and weaned at six weeks (Kunz and Martin 1982).

-Site description

3/4This will be conducted in two locations in the Santa Cruz County on California’s central coast. The first is in the reserves on the campus of the University of California, Santa Cruz and the second at a privately owned property in Felton, California known as Roaring Camp. Both of the study sites have redwood (S. sempervirens) communities within the coniferous forest habitat, with more hollows located at the second location due to greater amount of old-growth present there. Each site will have the maximum amount of samples within reason, meaning that all the redwoods with basal hollows that are characteristic of bat roosting sites, i.e. larger internal volumes, will be used as a sample.

-Methods 4/5 per hypo no stats refs

(1) a. human disturbance in caves occurs on a meaningful and regular basis:

In order to test this local caves within the area will be located, three near human communities and three further away. Each site will be set up with a sound activated time recorder, to record the time of day, frequency, and intensity of disturbance. They will be checked on a monthly basis and analyzed on monthly temporal scale. This should yield information on human disturbance.

(2) a. sites used as day roosts would yield a consistent and large amount of guano, much like cave roosts:

Water permeable mesh screens (guano traps) will be placed at the bottom of the basal hollow approximately 1-1.5m from the floor. This will be done by tacking the screen into place with use of nails or hooks. The traps will be checked on a weekly basis and will be analyzed by comparing the amount of guano by mass caught in each hollow in relation to diameter at breast height (dbh) and approximate internal volume. After a few weeks of collecting, hollows will be identified as good candidates for the following tests.

(2) b. day roosts would show high levels of activity in two peaks during the night, the first at dusk leaving the hollow and the second at the time interval approaching dawn:

This test is divided into two subtests, the first using Anabat® detectors record the vocal activity levels of the hollow and the second using motion activated infrared cameras to further differentiate between type of activity, i.e. leaving or returning to the hollow.

First the 15 Anabat® detector, which are microphones that record echolocation calls of bats along with the time at which it was recorded, will be installed in the 15 hollows that were deemed good candidates for being frequently used roost site, regardless of location. The data will be collected on a weekly basis, and analyzed to both differentiate the bat echolocation calls from other ambient noise, in which that information will be further processed to determine nightly patterns in activity levels. After a few weeks, hollows will be further scrutinized for being a good candidate for the next step.

Two cameras will then be installed in the most promising hollows, as in the hollows that are most likely a routinely used roosting site with the largest number of suspected individuals as determined by the amount of guano collected and the amount of vocal activity. The data from the cameras will be collected on a daily basis due to limited space to store the information on the camera. The video collected will be analyzed for the amount and types of activity, this will later be used to make assumptions about the other similar hollows. All tested initiated previous to this will still be on going for the duration of the data collection.

(2) c. fidelity should be defined as specific day roost sites that are used on a regular basis, whether in rotation with other sites in the area or on a nightly basis:

This will be tested by the analyzing the data collected from the guano traps, Anabat® detectors, and cameras in relation to each other. By doing this the data can be further scrutinized to yield a pattern of usage whether there is a pattern of rotational hollow use within a given area or a single site used on routine basis. First by looking at the relationship of amount of guano per hollow in a given area pattern may arise showing either differential or approximately equal usage. The Anabat® detectors can later confirm if the hollows deemed as good candidates due to there greater amount of guano, were in fact being used regularly, which will also be supported by the data collected by the camera.

Potential Results 16/20 did not graph Null nor anyu accept or rejection clauses

(1) a. Fig. 2 shows the graphical representation of the potential results, showing the relationship between the amount of disturbance and proximity to a human community. At first it is a negative exponential relationship where higher amounts of disturbance are expected when closer to humans, which then asymptotes at zero as the cave is further from humans.

(2) a. Fig. 3 shows the graphical representation of the relationship between amount of guano to the suspected number of individuals. The relationship is expected to be linear assuming that all individuals produce the same amount of guano, so the addition of another individual would yield a proportional amount of guano.

(2) b. Fig.4 shows the graphical representation of the analyzed data from the Anabat® detectors. Each behavior, both day and night roosting, are represented in a relationship of amount of activity over the duration of the night. The cameras would help to further differentiate between the type of activity seen.

(2) c. It is suspected that there is certain amount of fidelity to either an individual roost site or area, these potential results are unknown, due to lack of information on the behavior.

Figures

Fig. 1. Expected energetic cost to C. townsendii for a given amount of human disturbance. This represents the suspected reason for the observed pattern.

Fig. 2. Potential results for the amount of human disturbance in cave in relation to it’s proximity to a human community. A suspected larger amount of disturbance the closer the cave is located to humans.

Fig. 3. Potential results for the average amount of guano collected per tree in relation to the suspected amount of individuals inhabiting the hollow. A suspect linear relationship between the two, assuming that each individual will produce the same amount of guano, thus proportionally increasing the amount of guano to the number of individuals.

Fig. 4. Potential results for the average amount of action over the course of the night for each given roosting behavior. These are the action patterns that are suspected to be seen for each given behavior.

Works Cited

Fellers, G.M. and Pierson, E.D. (2002) Habitat Use and Foraging Behavior of Townsend’s Big-Eared Bat (Corynorhinus Townsendii) in Coastal California. Jounal of Mammalogy. Vol 83, No 1, pp. 167-177.

Gellman, S.T. and Zielinski, W.J. (1996) Use by Bats of Old-Growth Redwood Hollows on the North Coast of California. Journal of Mammalogy. Vol 77, No 1, pp. 255-265.

Johnson, S.A. et al. (1998) Overwinter Weight Loss of Indiana Bats (Myotis sodalis) from Hibernacula Subject to Human Visitation. The American Midland Naturalist. Vol 139, No 2, pp. 255-261.

Kunz, T.H. and Martin, R.A. (1982) Plecotus townsendii. Mammalian Species. No 175, pp.1-6.

Lacki, M.J. et al. (2007) Bats in Forests: Conservation and Management. Baltimore, Maryland. John Hopkins University Press.