Team Chiro

The prime objective of Vigie-Chiro is to compute population trends for a maximum of bats in France. These trends are calculated based on recordings collected between 2006 and 2019 on 173 car transects, 110 walk transects and 339 fix points.

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The car and walk transects have been monitored during a mean of 6 years. Fix points, from the most recent protocol, have been monitored during a mean of 3 years. Pipistrellus pipistrellus excluded, theses trends still have a low precision because of the low number of sites monitored during the beginning of the period.

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Methods:

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The data are modelled with models called GLMM (negative binomial family). The variable of interest is the number of bat passes per section/point/night.

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Three types of temporal modelling of the trend were performed: log-linear, quadratic or estimation by year (periods of 2 and 4 years).

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The models take into account meteorological hazards (temperature and wind), the protocol used, the timing of the road transect (fix variables), and the site and the recording material used (random variables).

 

We also controlled the capacity of the models to separate these effects from the population trend using the VIF (Variance Inflation Factor). The model is said to be robust only if the VIF of the trend parameter is inferior to 2. It is robust for Nyctalus leisleri, N. noctula, Pipistrellus pipistrellus, P. kuhlii, P. nathusii and Eptesicus serotinus. Other species for which monitoring increased thanks to the fixed points protocol still display a high VIF because their trend highly depends on the very last years (see at the end of this page) and the temporal trends can still be confused with some meteorological variables.

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A possible degradation of the material is also controlled by the measurement of the bat pass length of Pipistrelles, after having removed all recordings suspected to be deficient. Over the whole period, the loss in bat pass length is of -0.8% (CI +/- 4%). Therefore, declines cannot be attributed to a possible bias in the loss of sensitivity of the material, because it is negligible.