1. Methods Results Conclusions
Do Natterer’s bats show foraging site fidelity? Simone Mordue1, James Aegerter 2, Steven Rushton1, Aileen Mill School of Biology, Newcastle University, Newcastle Upon Tyne, UK. NE1 7RU 2National Wildlife Management Centre, Animal and Plant Health Agency, Sand Hutton, York, UK. YO41 1LZ.
Studies on foraging behaviour of bats help prioritise conservation areas and species management plans. The effectiveness of the protected area depends on how long the animal stays there. GPS tracking on large animals, such as Elephant seals, provides long term foraging data and has indicated that these animals show a high degree of foraging site fidelity returning to the same foraging sites each year.
For UK bats, most of which cannot bear the weight of GPS tags, radio tracking is often the only option to infer their use of space. Radio tracking a bat from dusk until dawn is costly in terms of equipment and time. Little evidence is available to help researchers decide how many nights of tracking data are needed to describe the bats typical use of space and most bat foraging data come from individuals sampled in only one season or year. Foraging fidelity in bats has been reported for a few species, however few studies have tested whether foraging behaviour is consistent to the same individual in the following seasons/years, within the same time period or in the same environment.
One of the hypotheses regarding the communal roosting of bats is that they follow the optimal foraging theory whereby they choose to roost close to their foraging grounds. An understanding of why bats roost together and where they forage will be beneficial not only to inform Favourable Conservation Status but also to inform the epidemiology of bat diseases.
Methods
Natterer’s bats were caught by mist net, static hand net and harp traps at Low Catton during 2003 and Wallington between 2013 and 2015 (fig. 1,2)
Bats biometrics were recorded and selected bats were fitted with Pip AG317 radio transmitters (fig. 3)
Bats were radio-tracked by single workers using the close approach method on foot and in a vehicle (fig 4). One bat was tracked per night. Bats were tracked from their emergence from roosts until their return, with their locations recorded at 10 minute intervals
Each night of radio tracking was treated as an independent variable for the purposes of data analysis. Nights of tracking data with continuous gaps of more than 20 minutes were excluded from the analysis. Analysis was carried out in R and ArcMap.
Polygon observations were digitised, transformed into clouds of points and then used to create utilised distributions. 50% core foraging areas were determined for each bat tracked. Pairwise overlap measures between repeatedly tracked and different bats were calculated.
Fig. 1. The locations of the two study sites in Northern England
Fig.2. A Natterer’s bat caught in a mist net
Fig 3. A natterer’s bat with forearm ring and radio transmitter attached
Aims: We aim to test whether Natterer’s bats, Myotis nattereri, show foraging site fidelity on a nightly basis and over different years to determine if a single full night of radio tracking is sufficient to infer there typical behaviour and to accept or reject the hypothesis of optimal foraging.
Hypothesis: There will be more overlap between repeatedly tracked bats than different bats
Fig 4. Radio tracking using a Telonics receiver and flexible Yagi antenna at Wallington, Northumberland
Results
The core area for each bat was compared between nights and between years and the proportion of area overlap was calculated for each. For each bat this was compared to the proportion of overlap with every other bats core foraging area.
There was 35% more overlap for the same individual within a year than for different individuals over years. There was also 8% more overlap between the same bats tracked over different years than for different individuals tracked over years (fig.5,6). We therefore accept the hypothesis that Natterer’s bats exhibit foraging site fidelity (Permutation test, reps, p<0.001).
Fig 5. The proportion of overlap of 50% core areas for different bats (red), same bat different years (green) and same bat, same year (blue) from both colonies. Scores close to 1 indicate 100% overlap of core foraging areas whilst scores of zero indicate independent use of space and no overlap with core foraging areas.
Fig 6. An example from Low Catton of 50% core areas generated using utilised distributions. Each colour represents a different bat, solid lines show the core area from the first night of tracking, dashed lines show the second night.
Conclusions
The spatial organisation of foraging areas in the two colonies of Natterer’s bats, strongly indicates that bats partition the foraging area of the entire colony and indicates that there may be individual specialisation (foraging site fidelity), or a preference of habitats used or insects caught. This suggests that. a single complete night of radio tracking should be sufficient to describe an individuals typical behaviour.
The degree of foraging fidelity was much higher during the same year of tracking than it was inter annually, suggesting that Natterer’s bats may alter their foraging area on an annual basis. This could be due to a number of factors including landscape changes, prey availability or a change in social status of the individual. Further work could look at annual landscape level changes and habitat composition of foraging areas to assess if this remains constant over time.
Our knowledge of the dynamism of regular roost changing but fidelity to foraging grounds does not support the theory of optimal foraging, but suggests other drivers for communal roosting such as social cohesion. Further work is needed to better understand the roosting habits of bats to inform disease epidemiology.
Our results could help inform Favourable Conservation Status for UK bats as animals with static home ranges are likely to be vulnerable to any habitat level changes within their home range.