The impact of bird mortality by collision on windfarms has often been evaluated at the individual level, but rarely at the population level. The Lesser kestrel Falco naumanni is an endangered short-lived migratory raptor, susceptible to collision with wind turbines. We evaluated the impacts of windfarm turbine collisions on the demography of the largest lesser kestrel population in France. Using data from local monitoring of reproduction and windfarm mortality surveys, combined with capture-recapture ringing data at a nearby population, we quantified vital parameters of fecundity and survival in order to parameterize a matrix population model to study the viability of this population. The breeding success was high and varied in synchrony with survival probabilities. Between 2013 and 2020, 43 carcasses were found below wind turbines, and when accounting for carcass detection and persistence rates, the true mortality should approach 154 individuals in that period, i.e. 3% of the studied population was affected by collisions each year. The matrix model showed that the population growth observed was only possible if there was a constant recruitment of 26 immigrant individuals each year into the population. Without the excess mortality by the windfarm, we predict that this population would have 22% more breeding pairs than what was observed in 2020. Simulations over 30 years showed that, under the current immigration rate, the population should decline if the excess mortality exceeds 11%. If immigration ceases, the population would decline above 5% excess mortality per year. It is urgent to monitor and reduce the excess mortality by windfarm collisions that threatens this lesser kestrel population. More generally, we advocate the use of population matrix demographic models in impact assessment studies to avoid placing new windfarms close to rare species that could not sustain additional mortality by collisions.