Dietary studies suggest that amphibians are opportunistic predators. However, there is little information on the ability of individuals to change their feeding strategy in time because most studies do not evaluate prey availability and its effect on individual behaviour. To better understand how variation in prey availability may affect the feeding strategy of newts, we studied the Alpine newt, Ichthyosaura alpestris, during April and June in 2015, when we monitored prey availability and the species dietary habits. In April at low prey diversity, the newts were generalists, i.e., their diet overlapped almost completely with prey availability. In June when prey diversity was high, the newts became specialists. At the individual level, 9 out of 15 recaptured newts shifted from a generalist to a specialist feeding strategy from April to June, suggesting a rapid behavioural change in response to increasing prey diversity, in accordance with optimal foraging theory. These results stress the importance of sampling the same individuals several times during an extended period of time to better understand the patterns of diet variation in amphibians.