In October 2001, the world population of Raso Lark Alauda razae, confined to the uninhabited, arid islet of Raso in the Cape Verde Islands, was estimated at between 128 and 138 birds, of which 61–66% were males. The biased sex ratio was confirmed by standardized observations of randomly selected birds. The male-dominated sex ratio may result from differences in bill morphology and feeding methods. The males spent much time digging for the bulbs of the nutsedge Cyperus bulbosus, whereas females were seen to dig far less frequently. These differences may have been due to significant sexual differences in bill size. Both sexes also took a range of invertebrate prey, particularly lepidopteran larvae. Unpaired males showed differences in behaviour to paired males, particularly with respect to song flight length. Measurement of museum skins suggested significant seasonal changes in bill size, possibly related to changes in diet or feeding methods. Bill sizes in both sexes were highly variable, but significantly more variable in males. Nest predation, almost certainly by a near-endemic gecko, was extremely high, Mayfield estimates suggesting a nest survival rate of less than 5% between the onset of laying and the end of incubation. Cats and dogs were thought to be absent, although analysis of droppings shows that both have been present in the recent past. Analysis of historical data shows a strong correlation between population size and rainfall, and numbers of birds have fallen to extremely low levels during droughts. The conservation of the species is discussed in the light of these findings.