We examined the possible contributions of food availability and the energetic and nutritional contents of seeds on the diets of adult and juvenile Serins (Serinus serinus). We found that Serins are specialized granivores during the breeding season, with a single plant species (Diplotaxis virgata) comprising the majority of the diets' of adults and offspring. Food availability did not account for this diet specialization. Rather, the high protein and energy content of this plant appears to account for this strong preference and may explain why Serins, unlike almost all other granivorous bird species, do not provision their nestlings with insects. Because Serins and many other species often breed in modern agrosystems, knowledge of feeding adaptations may be invaluable for habitat management.