Summary 1.Recognizing the factor(s) that caused a demographic crash is a prerequisite to the development of a tailored population restoration plan. While habitat destruction leaves little scope for population persistence, cryptic habitat deterioration (for example through reduction of food resources) may similarly render an area totally inhospitable, while it still appears to have a suitable habitat configuration. 2. Most European bat species have undergone dramatic declines over the past decades. Once among the most widespread and abundant bat species of Central Europe, the lesser horseshoe bat, Rhinolophus hipposideros (Bechstein 1800), is today extinct in many regions. Although changes in the agricultural landscape have been suggested as the major cause of decline, recent studies have shown that this bat forages almost exclusively in woodland, a habitat that has increased in area across continental Europe over the past decades. This suggests that habitat eradication per se is unlikely to be the main cause of decline. But could cryptic habitat deterioration play a harmful role? 3. We looked at the abundance of insect prey in woodland in the vicinity of colonial roosts with diverging demographic status (extinct, declining or recovering populations), both in the Swiss lowlands (Swiss Plateau) and in the Alps. We predicted that population size correlates positively with prey abundance. 4. Diet composition mirrored local insect prey abundance, confirming an opportunistic foraging strategy. Prey abundance showed marked seasonal variation, but did not differ between sites harbouring extinct, declining or recovering populations. There was also no difference in food abundance between extinct populations in the lowlands and recovering populations in the Alps. 5. Synthesis and applications. Cryptic habitat deterioration through a reduction in prey abundance is unlikely to preclude recolonization of abandoned areas by presently recovering populations. However, sufficient areas of natural forest should be preserved or created around potential nursery roosts. Moreover, connectivity between forest patches must be ensured (by creation of hedges and tree lines) to prevent any spatial gap in recolonization within semi-open agricultural landscapes. Other threatened European forest bats may also benefit from these measures.