The study compares ecological requirements of two related (congeneric) butterflies, Polyommatus damon and P. thersites, both of them reaching their northern distribution margins in Central Europe, where they co-occur on xeric grasslands, utilising identical larval host plants. Despite these similarities, one of them is substantially more endangered than the other. We describe their egg-laying behaviour and egg placement patterns, and analyse their distribution in a model landscape, showing that minute life history details affect differing species' fates in human-dominated landscapes.